xref: /openbmc/linux/tools/include/nolibc/nolibc.h (revision 30ca2051)

/* SPDX-License-Identifier: LGPL-2.1 OR MIT */
/* nolibc.h
 * Copyright (C) 2017-2018 Willy Tarreau <w@1wt.eu>
 */

/*
 * This file is designed to be used as a libc alternative for minimal programs
 * with very limited requirements. It consists of a small number of syscall and
 * type definitions, and the minimal startup code needed to call main().
 * All syscalls are declared as static functions so that they can be optimized
 * away by the compiler when not used.
 *
 * Syscalls are split into 3 levels:
 *   - The lower level is the arch-specific syscall() definition, consisting in
 *     assembly code in compound expressions. These are called my_syscall0() to
 *     my_syscall6() depending on the number of arguments. The MIPS
 *     implementation is limited to 5 arguments. All input arguments are cast
 *     to a long stored in a register. These expressions always return the
 *     syscall's return value as a signed long value which is often either a
 *     pointer or the negated errno value.
 *
 *   - The second level is mostly architecture-independent. It is made of
 *     static functions called sys_<name>() which rely on my_syscallN()
 *     depending on the syscall definition. These functions are responsible
 *     for exposing the appropriate types for the syscall arguments (int,
 *     pointers, etc) and for setting the appropriate return type (often int).
 *     A few of them are architecture-specific because the syscalls are not all
 *     mapped exactly the same among architectures. For example, some archs do
 *     not implement select() and need pselect6() instead, so the sys_select()
 *     function will have to abstract this.
 *
 *   - The third level is the libc call definition. It exposes the lower raw
 *     sys_<name>() calls in a way that looks like what a libc usually does,
 *     takes care of specific input values, and of setting errno upon error.
 *     There can be minor variations compared to standard libc calls. For
 *     example the open() call always takes 3 args here.
 *
 * The errno variable is declared static and unused. This way it can be
 * optimized away if not used. However this means that a program made of
 * multiple C files may observe different errno values (one per C file). For
 * the type of programs this project targets it usually is not a problem. The
 * resulting program may even be reduced by defining the NOLIBC_IGNORE_ERRNO
 * macro, in which case the errno value will never be assigned.
 *
 * Some stdint-like integer types are defined. These are valid on all currently
 * supported architectures, because signs are enforced, ints are assumed to be
 * 32 bits, longs the size of a pointer and long long 64 bits. If more
 * architectures have to be supported, this may need to be adapted.
 *
 * Some macro definitions like the O_* values passed to open(), and some
 * structures like the sys_stat struct depend on the architecture.
 *
 * The definitions start with the architecture-specific parts, which are picked
 * based on what the compiler knows about the target architecture, and are
 * completed with the generic code. Since it is the compiler which sets the
 * target architecture, cross-compiling normally works out of the box without
 * having to specify anything.
 *
 * Finally some very common libc-level functions are provided. It is the case
 * for a few functions usually found in string.h, ctype.h, or stdlib.h. Nothing
 * is currently provided regarding stdio emulation.
 *
 * The macro NOLIBC is always defined, so that it is possible for a program to
 * check this macro to know if it is being built against and decide to disable
 * some features or simply not to include some standard libc files.
 *
 * Ideally this file should be split in multiple files for easier long term
 * maintenance, but provided as a single file as it is now, it's quite
 * convenient to use. Maybe some variations involving a set of includes at the
 * top could work.
 *
 * A simple static executable may be built this way :
 *      $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \
 *            -static -include nolibc.h -lgcc -o hello hello.c
 *
 * A very useful calling convention table may be found here :
 *      http://man7.org/linux/man-pages/man2/syscall.2.html
 *
 * This doc is quite convenient though not necessarily up to date :
 *      https://w3challs.com/syscalls/
 *
 */

/* Some archs (at least aarch64) don't expose the regular syscalls anymore by
 * default, either because they have an "_at" replacement, or because there are
 * more modern alternatives. For now we'd rather still use them.
 */
#define __ARCH_WANT_SYSCALL_NO_AT
#define __ARCH_WANT_SYSCALL_NO_FLAGS
#define __ARCH_WANT_SYSCALL_DEPRECATED

#include <asm/unistd.h>
#include <asm/ioctls.h>
#include <asm/errno.h>
#include <linux/fs.h>
#include <linux/loop.h>

#define NOLIBC

/* this way it will be removed if unused */
static int errno;

#ifndef NOLIBC_IGNORE_ERRNO
#define SET_ERRNO(v) do { errno = (v); } while (0)
#else
#define SET_ERRNO(v) do { } while (0)
#endif

/* errno codes all ensure that they will not conflict with a valid pointer
 * because they all correspond to the highest addressable memry page.
 */
#define MAX_ERRNO 4095

/* Declare a few quite common macros and types that usually are in stdlib.h,
 * stdint.h, ctype.h, unistd.h and a few other common locations.
 */

#define NULL ((void *)0)

/* stdint types */
typedef unsigned char       uint8_t;
typedef   signed char        int8_t;
typedef unsigned short     uint16_t;
typedef   signed short      int16_t;
typedef unsigned int       uint32_t;
typedef   signed int        int32_t;
typedef unsigned long long uint64_t;
typedef   signed long long  int64_t;
typedef unsigned long        size_t;
typedef   signed long       ssize_t;
typedef unsigned long     uintptr_t;
typedef   signed long      intptr_t;
typedef   signed long     ptrdiff_t;

/* for stat() */
typedef unsigned int          dev_t;
typedef unsigned long         ino_t;
typedef unsigned int         mode_t;
typedef   signed int          pid_t;
typedef unsigned int          uid_t;
typedef unsigned int          gid_t;
typedef unsigned long       nlink_t;
typedef   signed long         off_t;
typedef   signed long     blksize_t;
typedef   signed long      blkcnt_t;
typedef   signed long        time_t;

/* for poll() */
struct pollfd {
	int fd;
	short int events;
	short int revents;
};

/* for select() */
struct timeval {
	long    tv_sec;
	long    tv_usec;
};

/* for pselect() */
struct timespec {
	long    tv_sec;
	long    tv_nsec;
};

/* for gettimeofday() */
struct timezone {
	int tz_minuteswest;
	int tz_dsttime;
};

/* for getdents64() */
struct linux_dirent64 {
	uint64_t       d_ino;
	int64_t        d_off;
	unsigned short d_reclen;
	unsigned char  d_type;
	char           d_name[];
};

/* commonly an fd_set represents 256 FDs */
#define FD_SETSIZE 256
typedef struct { uint32_t fd32[FD_SETSIZE/32]; } fd_set;

/* needed by wait4() */
struct rusage {
	struct timeval ru_utime;
	struct timeval ru_stime;
	long   ru_maxrss;
	long   ru_ixrss;
	long   ru_idrss;
	long   ru_isrss;
	long   ru_minflt;
	long   ru_majflt;
	long   ru_nswap;
	long   ru_inblock;
	long   ru_oublock;
	long   ru_msgsnd;
	long   ru_msgrcv;
	long   ru_nsignals;
	long   ru_nvcsw;
	long   ru_nivcsw;
};

/* stat flags (WARNING, octal here) */
#define S_IFDIR       0040000
#define S_IFCHR       0020000
#define S_IFBLK       0060000
#define S_IFREG       0100000
#define S_IFIFO       0010000
#define S_IFLNK       0120000
#define S_IFSOCK      0140000
#define S_IFMT        0170000

#define S_ISDIR(mode)  (((mode) & S_IFDIR) == S_IFDIR)
#define S_ISCHR(mode)  (((mode) & S_IFCHR) == S_IFCHR)
#define S_ISBLK(mode)  (((mode) & S_IFBLK) == S_IFBLK)
#define S_ISREG(mode)  (((mode) & S_IFREG) == S_IFREG)
#define S_ISFIFO(mode) (((mode) & S_IFIFO) == S_IFIFO)
#define S_ISLNK(mode)  (((mode) & S_IFLNK) == S_IFLNK)
#define S_ISSOCK(mode) (((mode) & S_IFSOCK) == S_IFSOCK)

#define DT_UNKNOWN 0
#define DT_FIFO    1
#define DT_CHR     2
#define DT_DIR     4
#define DT_BLK     6
#define DT_REG     8
#define DT_LNK    10
#define DT_SOCK   12

/* all the *at functions */
#ifndef AT_FDWCD
#define AT_FDCWD             -100
#endif

/* lseek */
#define SEEK_SET        0
#define SEEK_CUR        1
#define SEEK_END        2

/* reboot */
#define LINUX_REBOOT_MAGIC1         0xfee1dead
#define LINUX_REBOOT_MAGIC2         0x28121969
#define LINUX_REBOOT_CMD_HALT       0xcdef0123
#define LINUX_REBOOT_CMD_POWER_OFF  0x4321fedc
#define LINUX_REBOOT_CMD_RESTART    0x01234567
#define LINUX_REBOOT_CMD_SW_SUSPEND 0xd000fce2


/* The format of the struct as returned by the libc to the application, which
 * significantly differs from the format returned by the stat() syscall flavours.
 */
struct stat {
	dev_t     st_dev;     /* ID of device containing file */
	ino_t     st_ino;     /* inode number */
	mode_t    st_mode;    /* protection */
	nlink_t   st_nlink;   /* number of hard links */
	uid_t     st_uid;     /* user ID of owner */
	gid_t     st_gid;     /* group ID of owner */
	dev_t     st_rdev;    /* device ID (if special file) */
	off_t     st_size;    /* total size, in bytes */
	blksize_t st_blksize; /* blocksize for file system I/O */
	blkcnt_t  st_blocks;  /* number of 512B blocks allocated */
	time_t    st_atime;   /* time of last access */
	time_t    st_mtime;   /* time of last modification */
	time_t    st_ctime;   /* time of last status change */
};

#define WEXITSTATUS(status)   (((status) & 0xff00) >> 8)
#define WIFEXITED(status)     (((status) & 0x7f) == 0)


/* Below comes the architecture-specific code. For each architecture, we have
 * the syscall declarations and the _start code definition. This is the only
 * global part. On all architectures the kernel puts everything in the stack
 * before jumping to _start just above us, without any return address (_start
 * is not a function but an entry pint). So at the stack pointer we find argc.
 * Then argv[] begins, and ends at the first NULL. Then we have envp which
 * starts and ends with a NULL as well. So envp=argv+argc+1.
 */

#if defined(__x86_64__)
/* Syscalls for x86_64 :
 *   - registers are 64-bit
 *   - syscall number is passed in rax
 *   - arguments are in rdi, rsi, rdx, r10, r8, r9 respectively
 *   - the system call is performed by calling the syscall instruction
 *   - syscall return comes in rax
 *   - rcx and r8..r11 may be clobbered, others are preserved.
 *   - the arguments are cast to long and assigned into the target registers
 *     which are then simply passed as registers to the asm code, so that we
 *     don't have to experience issues with register constraints.
 *   - the syscall number is always specified last in order to allow to force
 *     some registers before (gcc refuses a %-register at the last position).
 */

#define my_syscall0(num)                                                      \
({                                                                            \
	long _ret;                                                            \
	register long _num  asm("rax") = (num);                               \
									      \
	asm volatile (                                                        \
		"syscall\n"                                                   \
		: "=a" (_ret)                                                 \
		: "0"(_num)                                                   \
		: "rcx", "r8", "r9", "r10", "r11", "memory", "cc"             \
	);                                                                    \
	_ret;                                                                 \
})

#define my_syscall1(num, arg1)                                                \
({                                                                            \
	long _ret;                                                            \
	register long _num  asm("rax") = (num);                               \
	register long _arg1 asm("rdi") = (long)(arg1);                        \
									      \
	asm volatile (                                                        \
		"syscall\n"                                                   \
		: "=a" (_ret)                                                 \
		: "r"(_arg1),                                                 \
		  "0"(_num)                                                   \
		: "rcx", "r8", "r9", "r10", "r11", "memory", "cc"             \
	);                                                                    \
	_ret;                                                                 \
})

#define my_syscall2(num, arg1, arg2)                                          \
({                                                                            \
	long _ret;                                                            \
	register long _num  asm("rax") = (num);                               \
	register long _arg1 asm("rdi") = (long)(arg1);                        \
	register long _arg2 asm("rsi") = (long)(arg2);                        \
									      \
	asm volatile (                                                        \
		"syscall\n"                                                   \
		: "=a" (_ret)                                                 \
		: "r"(_arg1), "r"(_arg2),                                     \
		  "0"(_num)                                                   \
		: "rcx", "r8", "r9", "r10", "r11", "memory", "cc"             \
	);                                                                    \
	_ret;                                                                 \
})

#define my_syscall3(num, arg1, arg2, arg3)                                    \
({                                                                            \
	long _ret;                                                            \
	register long _num  asm("rax") = (num);                               \
	register long _arg1 asm("rdi") = (long)(arg1);                        \
	register long _arg2 asm("rsi") = (long)(arg2);                        \
	register long _arg3 asm("rdx") = (long)(arg3);                        \
									      \
	asm volatile (                                                        \
		"syscall\n"                                                   \
		: "=a" (_ret)                                                 \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3),                         \
		  "0"(_num)                                                   \
		: "rcx", "r8", "r9", "r10", "r11", "memory", "cc"             \
	);                                                                    \
	_ret;                                                                 \
})

#define my_syscall4(num, arg1, arg2, arg3, arg4)                              \
({                                                                            \
	long _ret;                                                            \
	register long _num  asm("rax") = (num);                               \
	register long _arg1 asm("rdi") = (long)(arg1);                        \
	register long _arg2 asm("rsi") = (long)(arg2);                        \
	register long _arg3 asm("rdx") = (long)(arg3);                        \
	register long _arg4 asm("r10") = (long)(arg4);                        \
									      \
	asm volatile (                                                        \
		"syscall\n"                                                   \
		: "=a" (_ret), "=r"(_arg4)                                    \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4),             \
		  "0"(_num)                                                   \
		: "rcx", "r8", "r9", "r11", "memory", "cc"                    \
	);                                                                    \
	_ret;                                                                 \
})

#define my_syscall5(num, arg1, arg2, arg3, arg4, arg5)                        \
({                                                                            \
	long _ret;                                                            \
	register long _num  asm("rax") = (num);                               \
	register long _arg1 asm("rdi") = (long)(arg1);                        \
	register long _arg2 asm("rsi") = (long)(arg2);                        \
	register long _arg3 asm("rdx") = (long)(arg3);                        \
	register long _arg4 asm("r10") = (long)(arg4);                        \
	register long _arg5 asm("r8")  = (long)(arg5);                        \
									      \
	asm volatile (                                                        \
		"syscall\n"                                                   \
		: "=a" (_ret), "=r"(_arg4), "=r"(_arg5)                       \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4), "r"(_arg5), \
		  "0"(_num)                                                   \
		: "rcx", "r9", "r11", "memory", "cc"                          \
	);                                                                    \
	_ret;                                                                 \
})

#define my_syscall6(num, arg1, arg2, arg3, arg4, arg5, arg6)                  \
({                                                                            \
	long _ret;                                                            \
	register long _num  asm("rax") = (num);                               \
	register long _arg1 asm("rdi") = (long)(arg1);                        \
	register long _arg2 asm("rsi") = (long)(arg2);                        \
	register long _arg3 asm("rdx") = (long)(arg3);                        \
	register long _arg4 asm("r10") = (long)(arg4);                        \
	register long _arg5 asm("r8")  = (long)(arg5);                        \
	register long _arg6 asm("r9")  = (long)(arg6);                        \
									      \
	asm volatile (                                                        \
		"syscall\n"                                                   \
		: "=a" (_ret), "=r"(_arg4), "=r"(_arg5)                       \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4), "r"(_arg5), \
		  "r"(_arg6), "0"(_num)                                       \
		: "rcx", "r11", "memory", "cc"                                \
	);                                                                    \
	_ret;                                                                 \
})

/* startup code */
asm(".section .text\n"
    ".global _start\n"
    "_start:\n"
    "pop %rdi\n"                // argc   (first arg, %rdi)
    "mov %rsp, %rsi\n"          // argv[] (second arg, %rsi)
    "lea 8(%rsi,%rdi,8),%rdx\n" // then a NULL then envp (third arg, %rdx)
    "and $-16, %rsp\n"          // x86 ABI : esp must be 16-byte aligned when
    "sub $8, %rsp\n"            // entering the callee
    "call main\n"               // main() returns the status code, we'll exit with it.
    "movzb %al, %rdi\n"         // retrieve exit code from 8 lower bits
    "mov $60, %rax\n"           // NR_exit == 60
    "syscall\n"                 // really exit
    "hlt\n"                     // ensure it does not return
    "");

/* fcntl / open */
#define O_RDONLY            0
#define O_WRONLY            1
#define O_RDWR              2
#define O_CREAT          0x40
#define O_EXCL           0x80
#define O_NOCTTY        0x100
#define O_TRUNC         0x200
#define O_APPEND        0x400
#define O_NONBLOCK      0x800
#define O_DIRECTORY   0x10000

/* The struct returned by the stat() syscall, equivalent to stat64(). The
 * syscall returns 116 bytes and stops in the middle of __unused.
 */
struct sys_stat_struct {
	unsigned long st_dev;
	unsigned long st_ino;
	unsigned long st_nlink;
	unsigned int  st_mode;
	unsigned int  st_uid;

	unsigned int  st_gid;
	unsigned int  __pad0;
	unsigned long st_rdev;
	long          st_size;
	long          st_blksize;

	long          st_blocks;
	unsigned long st_atime;
	unsigned long st_atime_nsec;
	unsigned long st_mtime;

	unsigned long st_mtime_nsec;
	unsigned long st_ctime;
	unsigned long st_ctime_nsec;
	long          __unused[3];
};

#elif defined(__i386__) || defined(__i486__) || defined(__i586__) || defined(__i686__)
/* Syscalls for i386 :
 *   - mostly similar to x86_64
 *   - registers are 32-bit
 *   - syscall number is passed in eax
 *   - arguments are in ebx, ecx, edx, esi, edi, ebp respectively
 *   - all registers are preserved (except eax of course)
 *   - the system call is performed by calling int $0x80
 *   - syscall return comes in eax
 *   - the arguments are cast to long and assigned into the target registers
 *     which are then simply passed as registers to the asm code, so that we
 *     don't have to experience issues with register constraints.
 *   - the syscall number is always specified last in order to allow to force
 *     some registers before (gcc refuses a %-register at the last position).
 *
 * Also, i386 supports the old_select syscall if newselect is not available
 */
#define __ARCH_WANT_SYS_OLD_SELECT

#define my_syscall0(num)                                                      \
({                                                                            \
	long _ret;                                                            \
	register long _num asm("eax") = (num);                                \
									      \
	asm volatile (                                                        \
		"int $0x80\n"                                                 \
		: "=a" (_ret)                                                 \
		: "0"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_ret;                                                                 \
})

#define my_syscall1(num, arg1)                                                \
({                                                                            \
	long _ret;                                                            \
	register long _num asm("eax") = (num);                                \
	register long _arg1 asm("ebx") = (long)(arg1);                        \
									      \
	asm volatile (                                                        \
		"int $0x80\n"                                                 \
		: "=a" (_ret)                                                 \
		: "r"(_arg1),                                                 \
		  "0"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_ret;                                                                 \
})

#define my_syscall2(num, arg1, arg2)                                          \
({                                                                            \
	long _ret;                                                            \
	register long _num asm("eax") = (num);                                \
	register long _arg1 asm("ebx") = (long)(arg1);                        \
	register long _arg2 asm("ecx") = (long)(arg2);                        \
									      \
	asm volatile (                                                        \
		"int $0x80\n"                                                 \
		: "=a" (_ret)                                                 \
		: "r"(_arg1), "r"(_arg2),                                     \
		  "0"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_ret;                                                                 \
})

#define my_syscall3(num, arg1, arg2, arg3)                                    \
({                                                                            \
	long _ret;                                                            \
	register long _num asm("eax") = (num);                                \
	register long _arg1 asm("ebx") = (long)(arg1);                        \
	register long _arg2 asm("ecx") = (long)(arg2);                        \
	register long _arg3 asm("edx") = (long)(arg3);                        \
									      \
	asm volatile (                                                        \
		"int $0x80\n"                                                 \
		: "=a" (_ret)                                                 \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3),                         \
		  "0"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_ret;                                                                 \
})

#define my_syscall4(num, arg1, arg2, arg3, arg4)                              \
({                                                                            \
	long _ret;                                                            \
	register long _num asm("eax") = (num);                                \
	register long _arg1 asm("ebx") = (long)(arg1);                        \
	register long _arg2 asm("ecx") = (long)(arg2);                        \
	register long _arg3 asm("edx") = (long)(arg3);                        \
	register long _arg4 asm("esi") = (long)(arg4);                        \
									      \
	asm volatile (                                                        \
		"int $0x80\n"                                                 \
		: "=a" (_ret)                                                 \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4),             \
		  "0"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_ret;                                                                 \
})

#define my_syscall5(num, arg1, arg2, arg3, arg4, arg5)                        \
({                                                                            \
	long _ret;                                                            \
	register long _num asm("eax") = (num);                                \
	register long _arg1 asm("ebx") = (long)(arg1);                        \
	register long _arg2 asm("ecx") = (long)(arg2);                        \
	register long _arg3 asm("edx") = (long)(arg3);                        \
	register long _arg4 asm("esi") = (long)(arg4);                        \
	register long _arg5 asm("edi") = (long)(arg5);                        \
									      \
	asm volatile (                                                        \
		"int $0x80\n"                                                 \
		: "=a" (_ret)                                                 \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4), "r"(_arg5), \
		  "0"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_ret;                                                                 \
})

/* startup code */
asm(".section .text\n"
    ".global _start\n"
    "_start:\n"
    "pop %eax\n"                // argc   (first arg, %eax)
    "mov %esp, %ebx\n"          // argv[] (second arg, %ebx)
    "lea 4(%ebx,%eax,4),%ecx\n" // then a NULL then envp (third arg, %ecx)
    "and $-16, %esp\n"          // x86 ABI : esp must be 16-byte aligned when
    "push %ecx\n"               // push all registers on the stack so that we
    "push %ebx\n"               // support both regparm and plain stack modes
    "push %eax\n"
    "call main\n"               // main() returns the status code in %eax
    "movzbl %al, %ebx\n"        // retrieve exit code from lower 8 bits
    "movl   $1, %eax\n"         // NR_exit == 1
    "int    $0x80\n"            // exit now
    "hlt\n"                     // ensure it does not
    "");

/* fcntl / open */
#define O_RDONLY            0
#define O_WRONLY            1
#define O_RDWR              2
#define O_CREAT          0x40
#define O_EXCL           0x80
#define O_NOCTTY        0x100
#define O_TRUNC         0x200
#define O_APPEND        0x400
#define O_NONBLOCK      0x800
#define O_DIRECTORY   0x10000

/* The struct returned by the stat() syscall, 32-bit only, the syscall returns
 * exactly 56 bytes (stops before the unused array).
 */
struct sys_stat_struct {
	unsigned long  st_dev;
	unsigned long  st_ino;
	unsigned short st_mode;
	unsigned short st_nlink;
	unsigned short st_uid;
	unsigned short st_gid;

	unsigned long  st_rdev;
	unsigned long  st_size;
	unsigned long  st_blksize;
	unsigned long  st_blocks;

	unsigned long  st_atime;
	unsigned long  st_atime_nsec;
	unsigned long  st_mtime;
	unsigned long  st_mtime_nsec;

	unsigned long  st_ctime;
	unsigned long  st_ctime_nsec;
	unsigned long  __unused[2];
};

#elif defined(__ARM_EABI__)
/* Syscalls for ARM in ARM or Thumb modes :
 *   - registers are 32-bit
 *   - stack is 8-byte aligned
 *     ( http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.faqs/ka4127.html)
 *   - syscall number is passed in r7
 *   - arguments are in r0, r1, r2, r3, r4, r5
 *   - the system call is performed by calling svc #0
 *   - syscall return comes in r0.
 *   - only lr is clobbered.
 *   - the arguments are cast to long and assigned into the target registers
 *     which are then simply passed as registers to the asm code, so that we
 *     don't have to experience issues with register constraints.
 *   - the syscall number is always specified last in order to allow to force
 *     some registers before (gcc refuses a %-register at the last position).
 *
 * Also, ARM supports the old_select syscall if newselect is not available
 */
#define __ARCH_WANT_SYS_OLD_SELECT

#define my_syscall0(num)                                                      \
({                                                                            \
	register long _num asm("r7") = (num);                                 \
	register long _arg1 asm("r0");                                        \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r"(_arg1)                                                 \
		: "r"(_num)                                                   \
		: "memory", "cc", "lr"                                        \
	);                                                                    \
	_arg1;                                                                \
})

#define my_syscall1(num, arg1)                                                \
({                                                                            \
	register long _num asm("r7") = (num);                                 \
	register long _arg1 asm("r0") = (long)(arg1);                         \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r"(_arg1)                                                 \
		: "r"(_arg1),                                                 \
		  "r"(_num)                                                   \
		: "memory", "cc", "lr"                                        \
	);                                                                    \
	_arg1;                                                                \
})

#define my_syscall2(num, arg1, arg2)                                          \
({                                                                            \
	register long _num asm("r7") = (num);                                 \
	register long _arg1 asm("r0") = (long)(arg1);                         \
	register long _arg2 asm("r1") = (long)(arg2);                         \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r"(_arg1)                                                 \
		: "r"(_arg1), "r"(_arg2),                                     \
		  "r"(_num)                                                   \
		: "memory", "cc", "lr"                                        \
	);                                                                    \
	_arg1;                                                                \
})

#define my_syscall3(num, arg1, arg2, arg3)                                    \
({                                                                            \
	register long _num asm("r7") = (num);                                 \
	register long _arg1 asm("r0") = (long)(arg1);                         \
	register long _arg2 asm("r1") = (long)(arg2);                         \
	register long _arg3 asm("r2") = (long)(arg3);                         \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r"(_arg1)                                                 \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3),                         \
		  "r"(_num)                                                   \
		: "memory", "cc", "lr"                                        \
	);                                                                    \
	_arg1;                                                                \
})

#define my_syscall4(num, arg1, arg2, arg3, arg4)                              \
({                                                                            \
	register long _num asm("r7") = (num);                                 \
	register long _arg1 asm("r0") = (long)(arg1);                         \
	register long _arg2 asm("r1") = (long)(arg2);                         \
	register long _arg3 asm("r2") = (long)(arg3);                         \
	register long _arg4 asm("r3") = (long)(arg4);                         \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r"(_arg1)                                                 \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4),             \
		  "r"(_num)                                                   \
		: "memory", "cc", "lr"                                        \
	);                                                                    \
	_arg1;                                                                \
})

#define my_syscall5(num, arg1, arg2, arg3, arg4, arg5)                        \
({                                                                            \
	register long _num asm("r7") = (num);                                 \
	register long _arg1 asm("r0") = (long)(arg1);                         \
	register long _arg2 asm("r1") = (long)(arg2);                         \
	register long _arg3 asm("r2") = (long)(arg3);                         \
	register long _arg4 asm("r3") = (long)(arg4);                         \
	register long _arg5 asm("r4") = (long)(arg5);                         \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r" (_arg1)                                                \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4), "r"(_arg5), \
		  "r"(_num)                                                   \
		: "memory", "cc", "lr"                                        \
	);                                                                    \
	_arg1;                                                                \
})

/* startup code */
asm(".section .text\n"
    ".global _start\n"
    "_start:\n"
#if defined(__THUMBEB__) || defined(__THUMBEL__)
    /* We enter here in 32-bit mode but if some previous functions were in
     * 16-bit mode, the assembler cannot know, so we need to tell it we're in
     * 32-bit now, then switch to 16-bit (is there a better way to do it than
     * adding 1 by hand ?) and tell the asm we're now in 16-bit mode so that
     * it generates correct instructions. Note that we do not support thumb1.
     */
    ".code 32\n"
    "add     r0, pc, #1\n"
    "bx      r0\n"
    ".code 16\n"
#endif
    "pop {%r0}\n"                 // argc was in the stack
    "mov %r1, %sp\n"              // argv = sp
    "add %r2, %r1, %r0, lsl #2\n" // envp = argv + 4*argc ...
    "add %r2, %r2, $4\n"          //        ... + 4
    "and %r3, %r1, $-8\n"         // AAPCS : sp must be 8-byte aligned in the
    "mov %sp, %r3\n"              //         callee, an bl doesn't push (lr=pc)
    "bl main\n"                   // main() returns the status code, we'll exit with it.
    "and %r0, %r0, $0xff\n"       // limit exit code to 8 bits
    "movs r7, $1\n"               // NR_exit == 1
    "svc $0x00\n"
    "");

/* fcntl / open */
#define O_RDONLY            0
#define O_WRONLY            1
#define O_RDWR              2
#define O_CREAT          0x40
#define O_EXCL           0x80
#define O_NOCTTY        0x100
#define O_TRUNC         0x200
#define O_APPEND        0x400
#define O_NONBLOCK      0x800
#define O_DIRECTORY    0x4000

/* The struct returned by the stat() syscall, 32-bit only, the syscall returns
 * exactly 56 bytes (stops before the unused array). In big endian, the format
 * differs as devices are returned as short only.
 */
struct sys_stat_struct {
#if defined(__ARMEB__)
	unsigned short st_dev;
	unsigned short __pad1;
#else
	unsigned long  st_dev;
#endif
	unsigned long  st_ino;
	unsigned short st_mode;
	unsigned short st_nlink;
	unsigned short st_uid;
	unsigned short st_gid;
#if defined(__ARMEB__)
	unsigned short st_rdev;
	unsigned short __pad2;
#else
	unsigned long  st_rdev;
#endif
	unsigned long  st_size;
	unsigned long  st_blksize;
	unsigned long  st_blocks;
	unsigned long  st_atime;
	unsigned long  st_atime_nsec;
	unsigned long  st_mtime;
	unsigned long  st_mtime_nsec;
	unsigned long  st_ctime;
	unsigned long  st_ctime_nsec;
	unsigned long  __unused[2];
};

#elif defined(__aarch64__)
/* Syscalls for AARCH64 :
 *   - registers are 64-bit
 *   - stack is 16-byte aligned
 *   - syscall number is passed in x8
 *   - arguments are in x0, x1, x2, x3, x4, x5
 *   - the system call is performed by calling svc 0
 *   - syscall return comes in x0.
 *   - the arguments are cast to long and assigned into the target registers
 *     which are then simply passed as registers to the asm code, so that we
 *     don't have to experience issues with register constraints.
 *
 * On aarch64, select() is not implemented so we have to use pselect6().
 */
#define __ARCH_WANT_SYS_PSELECT6

#define my_syscall0(num)                                                      \
({                                                                            \
	register long _num  asm("x8") = (num);                                \
	register long _arg1 asm("x0");                                        \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r"(_arg1)                                                 \
		: "r"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_arg1;                                                                \
})

#define my_syscall1(num, arg1)                                                \
({                                                                            \
	register long _num  asm("x8") = (num);                                \
	register long _arg1 asm("x0") = (long)(arg1);                         \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r"(_arg1)                                                 \
		: "r"(_arg1),                                                 \
		  "r"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_arg1;                                                                \
})

#define my_syscall2(num, arg1, arg2)                                          \
({                                                                            \
	register long _num  asm("x8") = (num);                                \
	register long _arg1 asm("x0") = (long)(arg1);                         \
	register long _arg2 asm("x1") = (long)(arg2);                         \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r"(_arg1)                                                 \
		: "r"(_arg1), "r"(_arg2),                                     \
		  "r"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_arg1;                                                                \
})

#define my_syscall3(num, arg1, arg2, arg3)                                    \
({                                                                            \
	register long _num  asm("x8") = (num);                                \
	register long _arg1 asm("x0") = (long)(arg1);                         \
	register long _arg2 asm("x1") = (long)(arg2);                         \
	register long _arg3 asm("x2") = (long)(arg3);                         \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r"(_arg1)                                                 \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3),                         \
		  "r"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_arg1;                                                                \
})

#define my_syscall4(num, arg1, arg2, arg3, arg4)                              \
({                                                                            \
	register long _num  asm("x8") = (num);                                \
	register long _arg1 asm("x0") = (long)(arg1);                         \
	register long _arg2 asm("x1") = (long)(arg2);                         \
	register long _arg3 asm("x2") = (long)(arg3);                         \
	register long _arg4 asm("x3") = (long)(arg4);                         \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r"(_arg1)                                                 \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4),             \
		  "r"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_arg1;                                                                \
})

#define my_syscall5(num, arg1, arg2, arg3, arg4, arg5)                        \
({                                                                            \
	register long _num  asm("x8") = (num);                                \
	register long _arg1 asm("x0") = (long)(arg1);                         \
	register long _arg2 asm("x1") = (long)(arg2);                         \
	register long _arg3 asm("x2") = (long)(arg3);                         \
	register long _arg4 asm("x3") = (long)(arg4);                         \
	register long _arg5 asm("x4") = (long)(arg5);                         \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r" (_arg1)                                                \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4), "r"(_arg5), \
		  "r"(_num)                                                   \
		: "memory", "cc"                                              \
	);                                                                    \
	_arg1;                                                                \
})

#define my_syscall6(num, arg1, arg2, arg3, arg4, arg5, arg6)                  \
({                                                                            \
	register long _num  asm("x8") = (num);                                \
	register long _arg1 asm("x0") = (long)(arg1);                         \
	register long _arg2 asm("x1") = (long)(arg2);                         \
	register long _arg3 asm("x2") = (long)(arg3);                         \
	register long _arg4 asm("x3") = (long)(arg4);                         \
	register long _arg5 asm("x4") = (long)(arg5);                         \
	register long _arg6 asm("x5") = (long)(arg6);                         \
									      \
	asm volatile (                                                        \
		"svc #0\n"                                                    \
		: "=r" (_arg1)                                                \
		: "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4), "r"(_arg5), \
		  "r"(_arg6), "r"(_num)                                       \
		: "memory", "cc"                                              \
	);                                                                    \
	_arg1;                                                                \
})

/* startup code */
asm(".section .text\n"
    ".global _start\n"
    "_start:\n"
    "ldr x0, [sp]\n"              // argc (x0) was in the stack
    "add x1, sp, 8\n"             // argv (x1) = sp
    "lsl x2, x0, 3\n"             // envp (x2) = 8*argc ...
    "add x2, x2, 8\n"             //           + 8 (skip null)
    "add x2, x2, x1\n"            //           + argv
    "and sp, x1, -16\n"           // sp must be 16-byte aligned in the callee
    "bl main\n"                   // main() returns the status code, we'll exit with it.
    "and x0, x0, 0xff\n"          // limit exit code to 8 bits
    "mov x8, 93\n"                // NR_exit == 93
    "svc #0\n"
    "");

/* fcntl / open */
#define O_RDONLY            0
#define O_WRONLY            1
#define O_RDWR              2
#define O_CREAT          0x40
#define O_EXCL           0x80
#define O_NOCTTY        0x100
#define O_TRUNC         0x200
#define O_APPEND        0x400
#define O_NONBLOCK      0x800
#define O_DIRECTORY    0x4000

/* The struct returned by the newfstatat() syscall. Differs slightly from the
 * x86_64's stat one by field ordering, so be careful.
 */
struct sys_stat_struct {
	unsigned long   st_dev;
	unsigned long   st_ino;
	unsigned int    st_mode;
	unsigned int    st_nlink;
	unsigned int    st_uid;
	unsigned int    st_gid;

	unsigned long   st_rdev;
	unsigned long   __pad1;
	long            st_size;
	int             st_blksize;
	int             __pad2;

	long            st_blocks;
	long            st_atime;
	unsigned long   st_atime_nsec;
	long            st_mtime;

	unsigned long   st_mtime_nsec;
	long            st_ctime;
	unsigned long   st_ctime_nsec;
	unsigned int    __unused[2];
};

#elif defined(__mips__) && defined(_ABIO32)
/* Syscalls for MIPS ABI O32 :
 *   - WARNING! there's always a delayed slot!
 *   - WARNING again, the syntax is different, registers take a '$' and numbers
 *     do not.
 *   - registers are 32-bit
 *   - stack is 8-byte aligned
 *   - syscall number is passed in v0 (starts at 0xfa0).
 *   - arguments are in a0, a1, a2, a3, then the stack. The caller needs to
 *     leave some room in the stack for the callee to save a0..a3 if needed.
 *   - Many registers are clobbered, in fact only a0..a2 and s0..s8 are
 *     preserved. See: https://www.linux-mips.org/wiki/Syscall as well as
 *     scall32-o32.S in the kernel sources.
 *   - the system call is performed by calling "syscall"
 *   - syscall return comes in v0, and register a3 needs to be checked to know
 *     if an error occured, in which case errno is in v0.
 *   - the arguments are cast to long and assigned into the target registers
 *     which are then simply passed as registers to the asm code, so that we
 *     don't have to experience issues with register constraints.
 */

#define my_syscall0(num)                                                      \
({                                                                            \
	register long _num asm("v0") = (num);                                 \
	register long _arg4 asm("a3");                                        \
									      \
	asm volatile (                                                        \
		"addiu $sp, $sp, -32\n"                                       \
		"syscall\n"                                                   \
		"addiu $sp, $sp, 32\n"                                        \
		: "=r"(_num), "=r"(_arg4)                                     \
		: "r"(_num)                                                   \
		: "memory", "cc", "at", "v1", "hi", "lo",                     \
		  "t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"  \
	);                                                                    \
	_arg4 ? -_num : _num;                                                 \
})

#define my_syscall1(num, arg1)                                                \
({                                                                            \
	register long _num asm("v0") = (num);                                 \
	register long _arg1 asm("a0") = (long)(arg1);                         \
	register long _arg4 asm("a3");                                        \
									      \
	asm volatile (                                                        \
		"addiu $sp, $sp, -32\n"                                       \
		"syscall\n"                                                   \
		"addiu $sp, $sp, 32\n"                                        \
		: "=r"(_num), "=r"(_arg4)                                     \
		: "0"(_num),                                                  \
		  "r"(_arg1)                                                  \
		: "memory", "cc", "at", "v1", "hi", "lo",                     \
		  "t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"  \
	);                                                                    \
	_arg4 ? -_num : _num;                                                 \
})

#define my_syscall2(num, arg1, arg2)                                          \
({                                                                            \
	register long _num asm("v0") = (num);                                 \
	register long _arg1 asm("a0") = (long)(arg1);                         \
	register long _arg2 asm("a1") = (long)(arg2);                         \
	register long _arg4 asm("a3");                                        \
									      \
	asm volatile (                                                        \
		"addiu $sp, $sp, -32\n"                                       \
		"syscall\n"                                                   \
		"addiu $sp, $sp, 32\n"                                        \
		: "=r"(_num), "=r"(_arg4)                                     \
		: "0"(_num),                                                  \
		  "r"(_arg1), "r"(_arg2)                                      \
		: "memory", "cc", "at", "v1", "hi", "lo",                     \
		  "t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"  \
	);                                                                    \
	_arg4 ? -_num : _num;                                                 \
})

#define my_syscall3(num, arg1, arg2, arg3)                                    \
({                                                                            \
	register long _num asm("v0")  = (num);                                \
	register long _arg1 asm("a0") = (long)(arg1);                         \
	register long _arg2 asm("a1") = (long)(arg2);                         \
	register long _arg3 asm("a2") = (long)(arg3);                         \
	register long _arg4 asm("a3");                                        \
									      \
	asm volatile (                                                        \
		"addiu $sp, $sp, -32\n"                                       \
		"syscall\n"                                                   \
		"addiu $sp, $sp, 32\n"                                        \
		: "=r"(_num), "=r"(_arg4)                                     \
		: "0"(_num),                                                  \
		  "r"(_arg1), "r"(_arg2), "r"(_arg3)                          \
		: "memory", "cc", "at", "v1", "hi", "lo",                     \
		  "t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"  \
	);                                                                    \
	_arg4 ? -_num : _num;                                                 \
})

#define my_syscall4(num, arg1, arg2, arg3, arg4)                              \
({                                                                            \
	register long _num asm("v0") = (num);                                 \
	register long _arg1 asm("a0") = (long)(arg1);                         \
	register long _arg2 asm("a1") = (long)(arg2);                         \
	register long _arg3 asm("a2") = (long)(arg3);                         \
	register long _arg4 asm("a3") = (long)(arg4);                         \
									      \
	asm volatile (                                                        \
		"addiu $sp, $sp, -32\n"                                       \
		"syscall\n"                                                   \
		"addiu $sp, $sp, 32\n"                                        \
		: "=r" (_num), "=r"(_arg4)                                    \
		: "0"(_num),                                                  \
		  "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4)              \
		: "memory", "cc", "at", "v1", "hi", "lo",                     \
		  "t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"  \
	);                                                                    \
	_arg4 ? -_num : _num;                                                 \
})

#define my_syscall5(num, arg1, arg2, arg3, arg4, arg5)                        \
({                                                                            \
	register long _num asm("v0") = (num);                                 \
	register long _arg1 asm("a0") = (long)(arg1);                         \
	register long _arg2 asm("a1") = (long)(arg2);                         \
	register long _arg3 asm("a2") = (long)(arg3);                         \
	register long _arg4 asm("a3") = (long)(arg4);                         \
	register long _arg5 = (long)(arg5);				      \
									      \
	asm volatile (                                                        \
		"addiu $sp, $sp, -32\n"                                       \
		"sw %7, 16($sp)\n"                                            \
		"syscall\n  "                                                 \
		"addiu $sp, $sp, 32\n"                                        \
		: "=r" (_num), "=r"(_arg4)                                    \
		: "0"(_num),                                                  \
		  "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4), "r"(_arg5)  \
		: "memory", "cc", "at", "v1", "hi", "lo",                     \
		  "t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"  \
	);                                                                    \
	_arg4 ? -_num : _num;                                                 \
})

/* startup code, note that it's called __start on MIPS */
asm(".section .text\n"
    ".set nomips16\n"
    ".global __start\n"
    ".set    noreorder\n"
    ".option pic0\n"
    ".ent __start\n"
    "__start:\n"
    "lw $a0,($sp)\n"              // argc was in the stack
    "addiu  $a1, $sp, 4\n"        // argv = sp + 4
    "sll $a2, $a0, 2\n"           // a2 = argc * 4
    "add   $a2, $a2, $a1\n"       // envp = argv + 4*argc ...
    "addiu $a2, $a2, 4\n"         //        ... + 4
    "li $t0, -8\n"
    "and $sp, $sp, $t0\n"         // sp must be 8-byte aligned
    "addiu $sp,$sp,-16\n"         // the callee expects to save a0..a3 there!
    "jal main\n"                  // main() returns the status code, we'll exit with it.
    "nop\n"                       // delayed slot
    "and $a0, $v0, 0xff\n"        // limit exit code to 8 bits
    "li $v0, 4001\n"              // NR_exit == 4001
    "syscall\n"
    ".end __start\n"
    "");

/* fcntl / open */
#define O_RDONLY            0
#define O_WRONLY            1
#define O_RDWR              2
#define O_APPEND       0x0008
#define O_NONBLOCK     0x0080
#define O_CREAT        0x0100
#define O_TRUNC        0x0200
#define O_EXCL         0x0400
#define O_NOCTTY       0x0800
#define O_DIRECTORY   0x10000

/* The struct returned by the stat() syscall. 88 bytes are returned by the
 * syscall.
 */
struct sys_stat_struct {
	unsigned int  st_dev;
	long          st_pad1[3];
	unsigned long st_ino;
	unsigned int  st_mode;
	unsigned int  st_nlink;
	unsigned int  st_uid;
	unsigned int  st_gid;
	unsigned int  st_rdev;
	long          st_pad2[2];
	long          st_size;
	long          st_pad3;
	long          st_atime;
	long          st_atime_nsec;
	long          st_mtime;
	long          st_mtime_nsec;
	long          st_ctime;
	long          st_ctime_nsec;
	long          st_blksize;
	long          st_blocks;
	long          st_pad4[14];
};

#endif


/* Below are the C functions used to declare the raw syscalls. They try to be
 * architecture-agnostic, and return either a success or -errno. Declaring them
 * static will lead to them being inlined in most cases, but it's still possible
 * to reference them by a pointer if needed.
 */
static __attribute__((unused))
void *sys_brk(void *addr)
{
	return (void *)my_syscall1(__NR_brk, addr);
}

static __attribute__((noreturn,unused))
void sys_exit(int status)
{
	my_syscall1(__NR_exit, status & 255);
	while(1); // shut the "noreturn" warnings.
}

static __attribute__((unused))
int sys_chdir(const char *path)
{
	return my_syscall1(__NR_chdir, path);
}

static __attribute__((unused))
int sys_chmod(const char *path, mode_t mode)
{
#ifdef __NR_fchmodat
	return my_syscall4(__NR_fchmodat, AT_FDCWD, path, mode, 0);
#else
	return my_syscall2(__NR_chmod, path, mode);
#endif
}

static __attribute__((unused))
int sys_chown(const char *path, uid_t owner, gid_t group)
{
#ifdef __NR_fchownat
	return my_syscall5(__NR_fchownat, AT_FDCWD, path, owner, group, 0);
#else
	return my_syscall3(__NR_chown, path, owner, group);
#endif
}

static __attribute__((unused))
int sys_chroot(const char *path)
{
	return my_syscall1(__NR_chroot, path);
}

static __attribute__((unused))
int sys_close(int fd)
{
	return my_syscall1(__NR_close, fd);
}

static __attribute__((unused))
int sys_dup(int fd)
{
	return my_syscall1(__NR_dup, fd);
}

static __attribute__((unused))
int sys_dup2(int old, int new)
{
	return my_syscall2(__NR_dup2, old, new);
}

static __attribute__((unused))
int sys_execve(const char *filename, char *const argv[], char *const envp[])
{
	return my_syscall3(__NR_execve, filename, argv, envp);
}

static __attribute__((unused))
pid_t sys_fork(void)
{
	return my_syscall0(__NR_fork);
}

static __attribute__((unused))
int sys_fsync(int fd)
{
	return my_syscall1(__NR_fsync, fd);
}

static __attribute__((unused))
int sys_getdents64(int fd, struct linux_dirent64 *dirp, int count)
{
	return my_syscall3(__NR_getdents64, fd, dirp, count);
}

static __attribute__((unused))
pid_t sys_getpgrp(void)
{
	return my_syscall0(__NR_getpgrp);
}

static __attribute__((unused))
pid_t sys_getpid(void)
{
	return my_syscall0(__NR_getpid);
}

static __attribute__((unused))
int sys_gettimeofday(struct timeval *tv, struct timezone *tz)
{
	return my_syscall2(__NR_gettimeofday, tv, tz);
}

static __attribute__((unused))
int sys_ioctl(int fd, unsigned long req, void *value)
{
	return my_syscall3(__NR_ioctl, fd, req, value);
}

static __attribute__((unused))
int sys_kill(pid_t pid, int signal)
{
	return my_syscall2(__NR_kill, pid, signal);
}

static __attribute__((unused))
int sys_link(const char *old, const char *new)
{
#ifdef __NR_linkat
	return my_syscall5(__NR_linkat, AT_FDCWD, old, AT_FDCWD, new, 0);
#else
	return my_syscall2(__NR_link, old, new);
#endif
}

static __attribute__((unused))
off_t sys_lseek(int fd, off_t offset, int whence)
{
	return my_syscall3(__NR_lseek, fd, offset, whence);
}

static __attribute__((unused))
int sys_mkdir(const char *path, mode_t mode)
{
#ifdef __NR_mkdirat
	return my_syscall3(__NR_mkdirat, AT_FDCWD, path, mode);
#else
	return my_syscall2(__NR_mkdir, path, mode);
#endif
}

static __attribute__((unused))
long sys_mknod(const char *path, mode_t mode, dev_t dev)
{
#ifdef __NR_mknodat
	return my_syscall4(__NR_mknodat, AT_FDCWD, path, mode, dev);
#else
	return my_syscall3(__NR_mknod, path, mode, dev);
#endif
}

static __attribute__((unused))
int sys_mount(const char *src, const char *tgt, const char *fst,
	      unsigned long flags, const void *data)
{
	return my_syscall5(__NR_mount, src, tgt, fst, flags, data);
}

static __attribute__((unused))
int sys_open(const char *path, int flags, mode_t mode)
{
#ifdef __NR_openat
	return my_syscall4(__NR_openat, AT_FDCWD, path, flags, mode);
#else
	return my_syscall3(__NR_open, path, flags, mode);
#endif
}

static __attribute__((unused))
int sys_pivot_root(const char *new, const char *old)
{
	return my_syscall2(__NR_pivot_root, new, old);
}

static __attribute__((unused))
int sys_poll(struct pollfd *fds, int nfds, int timeout)
{
	return my_syscall3(__NR_poll, fds, nfds, timeout);
}

static __attribute__((unused))
ssize_t sys_read(int fd, void *buf, size_t count)
{
	return my_syscall3(__NR_read, fd, buf, count);
}

static __attribute__((unused))
ssize_t sys_reboot(int magic1, int magic2, int cmd, void *arg)
{
	return my_syscall4(__NR_reboot, magic1, magic2, cmd, arg);
}

static __attribute__((unused))
int sys_sched_yield(void)
{
	return my_syscall0(__NR_sched_yield);
}

static __attribute__((unused))
int sys_select(int nfds, fd_set *rfds, fd_set *wfds, fd_set *efds, struct timeval *timeout)
{
#if defined(__ARCH_WANT_SYS_OLD_SELECT) && !defined(__NR__newselect)
	struct sel_arg_struct {
		unsigned long n;
		fd_set *r, *w, *e;
		struct timeval *t;
	} arg = { .n = nfds, .r = rfds, .w = wfds, .e = efds, .t = timeout };
	return my_syscall1(__NR_select, &arg);
#elif defined(__ARCH_WANT_SYS_PSELECT6) && defined(__NR_pselect6)
	struct timespec t;

	if (timeout) {
		t.tv_sec  = timeout->tv_sec;
		t.tv_nsec = timeout->tv_usec * 1000;
	}
	return my_syscall6(__NR_pselect6, nfds, rfds, wfds, efds, timeout ? &t : NULL, NULL);
#else
#ifndef __NR__newselect
#define __NR__newselect __NR_select
#endif
	return my_syscall5(__NR__newselect, nfds, rfds, wfds, efds, timeout);
#endif
}

static __attribute__((unused))
int sys_setpgid(pid_t pid, pid_t pgid)
{
	return my_syscall2(__NR_setpgid, pid, pgid);
}

static __attribute__((unused))
pid_t sys_setsid(void)
{
	return my_syscall0(__NR_setsid);
}

static __attribute__((unused))
int sys_stat(const char *path, struct stat *buf)
{
	struct sys_stat_struct stat;
	long ret;

#ifdef __NR_newfstatat
	/* only solution for arm64 */
	ret = my_syscall4(__NR_newfstatat, AT_FDCWD, path, &stat, 0);
#else
	ret = my_syscall2(__NR_stat, path, &stat);
#endif
	buf->st_dev     = stat.st_dev;
	buf->st_ino     = stat.st_ino;
	buf->st_mode    = stat.st_mode;
	buf->st_nlink   = stat.st_nlink;
	buf->st_uid     = stat.st_uid;
	buf->st_gid     = stat.st_gid;
	buf->st_rdev    = stat.st_rdev;
	buf->st_size    = stat.st_size;
	buf->st_blksize = stat.st_blksize;
	buf->st_blocks  = stat.st_blocks;
	buf->st_atime   = stat.st_atime;
	buf->st_mtime   = stat.st_mtime;
	buf->st_ctime   = stat.st_ctime;
	return ret;
}


static __attribute__((unused))
int sys_symlink(const char *old, const char *new)
{
#ifdef __NR_symlinkat
	return my_syscall3(__NR_symlinkat, old, AT_FDCWD, new);
#else
	return my_syscall2(__NR_symlink, old, new);
#endif
}

static __attribute__((unused))
mode_t sys_umask(mode_t mode)
{
	return my_syscall1(__NR_umask, mode);
}

static __attribute__((unused))
int sys_umount2(const char *path, int flags)
{
	return my_syscall2(__NR_umount2, path, flags);
}

static __attribute__((unused))
int sys_unlink(const char *path)
{
#ifdef __NR_unlinkat
	return my_syscall3(__NR_unlinkat, AT_FDCWD, path, 0);
#else
	return my_syscall1(__NR_unlink, path);
#endif
}

static __attribute__((unused))
pid_t sys_wait4(pid_t pid, int *status, int options, struct rusage *rusage)
{
	return my_syscall4(__NR_wait4, pid, status, options, rusage);
}

static __attribute__((unused))
pid_t sys_waitpid(pid_t pid, int *status, int options)
{
	return sys_wait4(pid, status, options, 0);
}

static __attribute__((unused))
pid_t sys_wait(int *status)
{
	return sys_waitpid(-1, status, 0);
}

static __attribute__((unused))
ssize_t sys_write(int fd, const void *buf, size_t count)
{
	return my_syscall3(__NR_write, fd, buf, count);
}


/* Below are the libc-compatible syscalls which return x or -1 and set errno.
 * They rely on the functions above. Similarly they're marked static so that it
 * is possible to assign pointers to them if needed.
 */

static __attribute__((unused))
int brk(void *addr)
{
	void *ret = sys_brk(addr);

	if (!ret) {
		SET_ERRNO(ENOMEM);
		return -1;
	}
	return 0;
}

static __attribute__((noreturn,unused))
void exit(int status)
{
	sys_exit(status);
}

static __attribute__((unused))
int chdir(const char *path)
{
	int ret = sys_chdir(path);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int chmod(const char *path, mode_t mode)
{
	int ret = sys_chmod(path, mode);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int chown(const char *path, uid_t owner, gid_t group)
{
	int ret = sys_chown(path, owner, group);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int chroot(const char *path)
{
	int ret = sys_chroot(path);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int close(int fd)
{
	int ret = sys_close(fd);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int dup2(int old, int new)
{
	int ret = sys_dup2(old, new);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int execve(const char *filename, char *const argv[], char *const envp[])
{
	int ret = sys_execve(filename, argv, envp);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
pid_t fork(void)
{
	pid_t ret = sys_fork();

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int fsync(int fd)
{
	int ret = sys_fsync(fd);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int getdents64(int fd, struct linux_dirent64 *dirp, int count)
{
	int ret = sys_getdents64(fd, dirp, count);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
pid_t getpgrp(void)
{
	pid_t ret = sys_getpgrp();

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
pid_t getpid(void)
{
	pid_t ret = sys_getpid();

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int gettimeofday(struct timeval *tv, struct timezone *tz)
{
	int ret = sys_gettimeofday(tv, tz);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int ioctl(int fd, unsigned long req, void *value)
{
	int ret = sys_ioctl(fd, req, value);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int kill(pid_t pid, int signal)
{
	int ret = sys_kill(pid, signal);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int link(const char *old, const char *new)
{
	int ret = sys_link(old, new);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
off_t lseek(int fd, off_t offset, int whence)
{
	off_t ret = sys_lseek(fd, offset, whence);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int mkdir(const char *path, mode_t mode)
{
	int ret = sys_mkdir(path, mode);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int mknod(const char *path, mode_t mode, dev_t dev)
{
	int ret = sys_mknod(path, mode, dev);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int mount(const char *src, const char *tgt,
	  const char *fst, unsigned long flags,
	  const void *data)
{
	int ret = sys_mount(src, tgt, fst, flags, data);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int open(const char *path, int flags, mode_t mode)
{
	int ret = sys_open(path, flags, mode);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int pivot_root(const char *new, const char *old)
{
	int ret = sys_pivot_root(new, old);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int poll(struct pollfd *fds, int nfds, int timeout)
{
	int ret = sys_poll(fds, nfds, timeout);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
ssize_t read(int fd, void *buf, size_t count)
{
	ssize_t ret = sys_read(fd, buf, count);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int reboot(int cmd)
{
	int ret = sys_reboot(LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, cmd, 0);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
void *sbrk(intptr_t inc)
{
	void *ret;

	/* first call to find current end */
	if ((ret = sys_brk(0)) && (sys_brk(ret + inc) == ret + inc))
		return ret + inc;

	SET_ERRNO(ENOMEM);
	return (void *)-1;
}

static __attribute__((unused))
int sched_yield(void)
{
	int ret = sys_sched_yield();

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int select(int nfds, fd_set *rfds, fd_set *wfds, fd_set *efds, struct timeval *timeout)
{
	int ret = sys_select(nfds, rfds, wfds, efds, timeout);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int setpgid(pid_t pid, pid_t pgid)
{
	int ret = sys_setpgid(pid, pgid);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
pid_t setsid(void)
{
	pid_t ret = sys_setsid();

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
unsigned int sleep(unsigned int seconds)
{
	struct timeval my_timeval = { seconds, 0 };

	if (sys_select(0, 0, 0, 0, &my_timeval) < 0)
		return my_timeval.tv_sec + !!my_timeval.tv_usec;
	else
		return 0;
}

static __attribute__((unused))
int stat(const char *path, struct stat *buf)
{
	int ret = sys_stat(path, buf);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int symlink(const char *old, const char *new)
{
	int ret = sys_symlink(old, new);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int tcsetpgrp(int fd, pid_t pid)
{
	return ioctl(fd, TIOCSPGRP, &pid);
}

static __attribute__((unused))
mode_t umask(mode_t mode)
{
	return sys_umask(mode);
}

static __attribute__((unused))
int umount2(const char *path, int flags)
{
	int ret = sys_umount2(path, flags);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
int unlink(const char *path)
{
	int ret = sys_unlink(path);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
pid_t wait4(pid_t pid, int *status, int options, struct rusage *rusage)
{
	pid_t ret = sys_wait4(pid, status, options, rusage);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
pid_t waitpid(pid_t pid, int *status, int options)
{
	pid_t ret = sys_waitpid(pid, status, options);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
pid_t wait(int *status)
{
	pid_t ret = sys_wait(status);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

static __attribute__((unused))
ssize_t write(int fd, const void *buf, size_t count)
{
	ssize_t ret = sys_write(fd, buf, count);

	if (ret < 0) {
		SET_ERRNO(-ret);
		ret = -1;
	}
	return ret;
}

/* some size-optimized reimplementations of a few common str* and mem*
 * functions. They're marked static, except memcpy() and raise() which are used
 * by libgcc on ARM, so they are marked weak instead in order not to cause an
 * error when building a program made of multiple files (not recommended).
 */

static __attribute__((unused))
void *memmove(void *dst, const void *src, size_t len)
{
	ssize_t pos = (dst <= src) ? -1 : (long)len;
	void *ret = dst;

	while (len--) {
		pos += (dst <= src) ? 1 : -1;
		((char *)dst)[pos] = ((char *)src)[pos];
	}
	return ret;
}

static __attribute__((unused))
void *memset(void *dst, int b, size_t len)
{
	char *p = dst;

	while (len--)
		*(p++) = b;
	return dst;
}

static __attribute__((unused))
int memcmp(const void *s1, const void *s2, size_t n)
{
	size_t ofs = 0;
	char c1 = 0;

	while (ofs < n && !(c1 = ((char *)s1)[ofs] - ((char *)s2)[ofs])) {
		ofs++;
	}
	return c1;
}

static __attribute__((unused))
char *strcpy(char *dst, const char *src)
{
	char *ret = dst;

	while ((*dst++ = *src++));
	return ret;
}

static __attribute__((unused))
char *strchr(const char *s, int c)
{
	while (*s) {
		if (*s == (char)c)
			return (char *)s;
		s++;
	}
	return NULL;
}

static __attribute__((unused))
char *strrchr(const char *s, int c)
{
	const char *ret = NULL;

	while (*s) {
		if (*s == (char)c)
			ret = s;
		s++;
	}
	return (char *)ret;
}

static __attribute__((unused))
size_t nolibc_strlen(const char *str)
{
	size_t len;

	for (len = 0; str[len]; len++);
	return len;
}

#define strlen(str) ({                          \
	__builtin_constant_p((str)) ?           \
		__builtin_strlen((str)) :       \
		nolibc_strlen((str));           \
})

static __attribute__((unused))
int isdigit(int c)
{
	return (unsigned int)(c - '0') <= 9;
}

static __attribute__((unused))
long atol(const char *s)
{
	unsigned long ret = 0;
	unsigned long d;
	int neg = 0;

	if (*s == '-') {
		neg = 1;
		s++;
	}

	while (1) {
		d = (*s++) - '0';
		if (d > 9)
			break;
		ret *= 10;
		ret += d;
	}

	return neg ? -ret : ret;
}

static __attribute__((unused))
int atoi(const char *s)
{
	return atol(s);
}

static __attribute__((unused))
const char *ltoa(long in)
{
	/* large enough for -9223372036854775808 */
	static char buffer[21];
	char       *pos = buffer + sizeof(buffer) - 1;
	int         neg = in < 0;
	unsigned long n = neg ? -in : in;

	*pos-- = '\0';
	do {
		*pos-- = '0' + n % 10;
		n /= 10;
		if (pos < buffer)
			return pos + 1;
	} while (n);

	if (neg)
		*pos-- = '-';
	return pos + 1;
}

__attribute__((weak,unused))
void *memcpy(void *dst, const void *src, size_t len)
{
	return memmove(dst, src, len);
}

/* needed by libgcc for divide by zero */
__attribute__((weak,unused))
int raise(int signal)
{
	return kill(getpid(), signal);
}

/* Here come a few helper functions */

static __attribute__((unused))
void FD_ZERO(fd_set *set)
{
	memset(set, 0, sizeof(*set));
}

static __attribute__((unused))
void FD_SET(int fd, fd_set *set)
{
	if (fd < 0 || fd >= FD_SETSIZE)
		return;
	set->fd32[fd / 32] |= 1 << (fd & 31);
}

/* WARNING, it only deals with the 4096 first majors and 256 first minors */
static __attribute__((unused))
dev_t makedev(unsigned int major, unsigned int minor)
{
	return ((major & 0xfff) << 8) | (minor & 0xff);
}