1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * vdso_test.c: Sample code to test parse_vdso.c on x86
4  * Copyright (c) 2011-2014 Andy Lutomirski
5  *
6  * You can amuse yourself by compiling with:
7  * gcc -std=gnu99 -nostdlib
8  *     -Os -fno-asynchronous-unwind-tables -flto -lgcc_s
9  *      vdso_standalone_test_x86.c parse_vdso.c
10  * to generate a small binary.  On x86_64, you can omit -lgcc_s
11  * if you want the binary to be completely standalone.
12  */
13 
14 #include <sys/syscall.h>
15 #include <sys/time.h>
16 #include <unistd.h>
17 #include <stdint.h>
18 
19 extern void *vdso_sym(const char *version, const char *name);
20 extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
21 extern void vdso_init_from_auxv(void *auxv);
22 
23 /* We need a libc functions... */
24 int strcmp(const char *a, const char *b)
25 {
26 	/* This implementation is buggy: it never returns -1. */
27 	while (*a || *b) {
28 		if (*a != *b)
29 			return 1;
30 		if (*a == 0 || *b == 0)
31 			return 1;
32 		a++;
33 		b++;
34 	}
35 
36 	return 0;
37 }
38 
39 /* ...and two syscalls.  This is x86-specific. */
40 static inline long x86_syscall3(long nr, long a0, long a1, long a2)
41 {
42 	long ret;
43 #ifdef __x86_64__
44 	asm volatile ("syscall" : "=a" (ret) : "a" (nr),
45 		      "D" (a0), "S" (a1), "d" (a2) :
46 		      "cc", "memory", "rcx",
47 		      "r8", "r9", "r10", "r11" );
48 #else
49 	asm volatile ("int $0x80" : "=a" (ret) : "a" (nr),
50 		      "b" (a0), "c" (a1), "d" (a2) :
51 		      "cc", "memory" );
52 #endif
53 	return ret;
54 }
55 
56 static inline long linux_write(int fd, const void *data, size_t len)
57 {
58 	return x86_syscall3(__NR_write, fd, (long)data, (long)len);
59 }
60 
61 static inline void linux_exit(int code)
62 {
63 	x86_syscall3(__NR_exit, code, 0, 0);
64 }
65 
66 void to_base10(char *lastdig, time_t n)
67 {
68 	while (n) {
69 		*lastdig = (n % 10) + '0';
70 		n /= 10;
71 		lastdig--;
72 	}
73 }
74 
75 __attribute__((externally_visible)) void c_main(void **stack)
76 {
77 	/* Parse the stack */
78 	long argc = (long)*stack;
79 	stack += argc + 2;
80 
81 	/* Now we're pointing at the environment.  Skip it. */
82 	while(*stack)
83 		stack++;
84 	stack++;
85 
86 	/* Now we're pointing at auxv.  Initialize the vDSO parser. */
87 	vdso_init_from_auxv((void *)stack);
88 
89 	/* Find gettimeofday. */
90 	typedef long (*gtod_t)(struct timeval *tv, struct timezone *tz);
91 	gtod_t gtod = (gtod_t)vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
92 
93 	if (!gtod)
94 		linux_exit(1);
95 
96 	struct timeval tv;
97 	long ret = gtod(&tv, 0);
98 
99 	if (ret == 0) {
100 		char buf[] = "The time is                     .000000\n";
101 		to_base10(buf + 31, tv.tv_sec);
102 		to_base10(buf + 38, tv.tv_usec);
103 		linux_write(1, buf, sizeof(buf) - 1);
104 	} else {
105 		linux_exit(ret);
106 	}
107 
108 	linux_exit(0);
109 }
110 
111 /*
112  * This is the real entry point.  It passes the initial stack into
113  * the C entry point.
114  */
115 asm (
116 	".text\n"
117 	".global _start\n"
118 	".type _start,@function\n"
119 	"_start:\n\t"
120 #ifdef __x86_64__
121 	"mov %rsp,%rdi\n\t"
122 	"jmp c_main"
123 #else
124 	"push %esp\n\t"
125 	"call c_main\n\t"
126 	"int $3"
127 #endif
128 	);
129