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 * Written by Richard Henderson <rth@twiddle.net>
7 * Copyright (C) 2016 Red Hat, Inc.
8 *
9 * This work is licensed under the terms of the GNU GPL, version 2 or later.
10 * See the COPYING file in the top-level directory.
11 */
12
13        .global safe_syscall_base
14        .global safe_syscall_start
15        .global safe_syscall_end
16        .type   safe_syscall_base, #function
17        .type   safe_syscall_start, #function
18        .type   safe_syscall_end, #function
19
20        /* This is the entry point for making a system call. The calling
21         * convention here is that of a C varargs function with the
22         * first argument an 'int *' to the signal_pending flag, the
23         * second one the system call number (as a 'long'), and all further
24         * arguments being syscall arguments (also 'long').
25         */
26safe_syscall_base:
27        .cfi_startproc
28        /* The syscall calling convention isn't the same as the
29         * C one:
30         * we enter with x0 == &signal_pending
31         *               x1 == syscall number
32         *               x2 ... x7, (stack) == syscall arguments
33         *               and return the result in x0
34         * and the syscall instruction needs
35         *               x8 == syscall number
36         *               x0 ... x6 == syscall arguments
37         *               and returns the result in x0
38         * Shuffle everything around appropriately.
39         */
40        mov     x9, x0          /* signal_pending pointer */
41        mov     x8, x1          /* syscall number */
42        mov     x0, x2          /* syscall arguments */
43        mov     x1, x3
44        mov     x2, x4
45        mov     x3, x5
46        mov     x4, x6
47        mov     x5, x7
48        ldr     x6, [sp]
49
50        /* This next sequence of code works in conjunction with the
51         * rewind_if_safe_syscall_function(). If a signal is taken
52         * and the interrupted PC is anywhere between 'safe_syscall_start'
53         * and 'safe_syscall_end' then we rewind it to 'safe_syscall_start'.
54         * The code sequence must therefore be able to cope with this, and
55         * the syscall instruction must be the final one in the sequence.
56         */
57safe_syscall_start:
58        /* if signal_pending is non-zero, don't do the call */
59        ldr     w10, [x9]
60        cbnz    w10, 2f
61        svc     0x0
62safe_syscall_end:
63
64        /* code path for having successfully executed the syscall */
65#if defined(__linux__)
66        /* Linux kernel returns (small) negative errno. */
67        cmp     x0, #-4096
68        b.hi    0f
69#elif defined(__FreeBSD__)
70        /* FreeBSD kernel returns positive errno and C bit set. */
71        b.cs    1f
72#else
73#error "unsupported os"
74#endif
75        ret
76
77#if defined(__linux__)
78        /* code path setting errno */
790:      neg     w0, w0
80        b       safe_syscall_set_errno_tail
81#endif
82
83        /* code path when we didn't execute the syscall */
842:      mov     w0, #QEMU_ERESTARTSYS
851:      b       safe_syscall_set_errno_tail
86
87        .cfi_endproc
88        .size   safe_syscall_base, .-safe_syscall_base
89