1/*
2 * safe-syscall.inc.S : host-specific assembly fragment
3 * to handle signals occurring at the same time as system calls.
4 * This is intended to be included by common-user/safe-syscall.S
5 *
6 * Copyright (C) 2015 Timothy Edward Baldwin <T.E.Baldwin99@members.leeds.ac.uk>
7 *
8 * This work is licensed under the terms of the GNU GPL, version 2 or later.
9 * See the COPYING file in the top-level directory.
10 */
11
12        .global safe_syscall_base
13        .global safe_syscall_start
14        .global safe_syscall_end
15        .type   safe_syscall_base, @function
16
17        /* This is the entry point for making a system call. The calling
18         * convention here is that of a C varargs function with the
19         * first argument an 'int *' to the signal_pending flag, the
20         * second one the system call number (as a 'long'), and all further
21         * arguments being syscall arguments (also 'long').
22         */
23safe_syscall_base:
24        .cfi_startproc
25        /* This saves a frame pointer and aligns the stack for the syscall.
26         * (It's unclear if the syscall ABI has the same stack alignment
27         * requirements as the userspace function call ABI, but better safe than
28         * sorry. Appendix A2 of http://www.x86-64.org/documentation/abi.pdf
29         * does not list any ABI differences regarding stack alignment.)
30         */
31        push    %rbp
32        .cfi_adjust_cfa_offset 8
33        .cfi_rel_offset rbp, 0
34
35        /*
36         * The syscall calling convention isn't the same as the C one:
37         * we enter with rdi == &signal_pending
38         *               rsi == syscall number
39         *               rdx, rcx, r8, r9, (stack), (stack) == syscall arguments
40         *               and return the result in rax
41         * and the syscall instruction needs
42         *               rax == syscall number
43         *               rdi, rsi, rdx, r10, r8, r9 == syscall arguments
44         *               and returns the result in rax
45         * Shuffle everything around appropriately.
46         * Note that syscall will trash rcx and r11.
47         */
48        mov     %rsi, %rax /* syscall number */
49        mov     %rdi, %rbp /* signal_pending pointer */
50        /* and the syscall arguments */
51        mov     %rdx, %rdi
52        mov     %rcx, %rsi
53        mov     %r8,  %rdx
54        mov     %r9,  %r10
55        mov     16(%rsp), %r8
56        mov     24(%rsp), %r9
57
58        /* This next sequence of code works in conjunction with the
59         * rewind_if_safe_syscall_function(). If a signal is taken
60         * and the interrupted PC is anywhere between 'safe_syscall_start'
61         * and 'safe_syscall_end' then we rewind it to 'safe_syscall_start'.
62         * The code sequence must therefore be able to cope with this, and
63         * the syscall instruction must be the final one in the sequence.
64         */
65safe_syscall_start:
66        /* if signal_pending is non-zero, don't do the call */
67        cmpl    $0, (%rbp)
68        jnz     2f
69        syscall
70safe_syscall_end:
71
72        /* code path for having successfully executed the syscall */
73#if defined(__linux__)
74        /* Linux kernel returns (small) negative errno. */
75        cmp     $-4095, %rax
76        jae     0f
77#elif defined(__FreeBSD__)
78        /* FreeBSD kernel returns positive errno and C bit set. */
79        jc      1f
80#else
81#error "unsupported os"
82#endif
83        pop     %rbp
84        .cfi_remember_state
85        .cfi_def_cfa_offset 8
86        .cfi_restore rbp
87        ret
88        .cfi_restore_state
89
90#if defined(__linux__)
910:      neg     %eax
92        jmp     1f
93#endif
94
95        /* code path when we didn't execute the syscall */
962:      mov     $QEMU_ERESTARTSYS, %eax
97
98        /* code path setting errno */
991:      pop     %rbp
100        .cfi_def_cfa_offset 8
101        .cfi_restore rbp
102        jmp     safe_syscall_set_errno_tail
103        .cfi_endproc
104
105        .size   safe_syscall_base, .-safe_syscall_base
106