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
18        /* This is the entry point for making a system call. The calling
19         * convention here is that of a C varargs function with the
20         * first argument an 'int *' to the signal_pending flag, the
21         * second one the system call number (as a 'long'), and all further
22         * arguments being syscall arguments (also 'long').
23         */
24safe_syscall_base:
25        .cfi_startproc
26        push    %ebp
27        .cfi_adjust_cfa_offset 4
28        .cfi_rel_offset ebp, 0
29        push    %esi
30        .cfi_adjust_cfa_offset 4
31        .cfi_rel_offset esi, 0
32        push    %edi
33        .cfi_adjust_cfa_offset 4
34        .cfi_rel_offset edi, 0
35        push    %ebx
36        .cfi_adjust_cfa_offset 4
37        .cfi_rel_offset ebx, 0
38
39        /* The syscall calling convention isn't the same as the C one:
40         * we enter with 0(%esp) == return address
41         *               4(%esp) == &signal_pending
42         *               8(%esp) == syscall number
43         *               12(%esp) ... 32(%esp) == syscall arguments
44         *               and return the result in eax
45         * and the syscall instruction needs
46         *               eax == syscall number
47         *               ebx, ecx, edx, esi, edi, ebp == syscall arguments
48         *               and returns the result in eax
49         * Shuffle everything around appropriately.
50         * Note the 16 bytes that we pushed to save registers.
51         */
52        mov     12+16(%esp), %ebx       /* the syscall arguments */
53        mov     16+16(%esp), %ecx
54        mov     20+16(%esp), %edx
55        mov     24+16(%esp), %esi
56        mov     28+16(%esp), %edi
57        mov     32+16(%esp), %ebp
58
59        /* This next sequence of code works in conjunction with the
60         * rewind_if_safe_syscall_function(). If a signal is taken
61         * and the interrupted PC is anywhere between 'safe_syscall_start'
62         * and 'safe_syscall_end' then we rewind it to 'safe_syscall_start'.
63         * The code sequence must therefore be able to cope with this, and
64         * the syscall instruction must be the final one in the sequence.
65         */
66safe_syscall_start:
67        /* if signal_pending is non-zero, don't do the call */
68        mov     4+16(%esp), %eax        /* signal_pending */
69        cmpl    $0, (%eax)
70        jnz     2f
71        mov     8+16(%esp), %eax        /* syscall number */
72        int     $0x80
73safe_syscall_end:
74
75        /* code path for having successfully executed the syscall */
76#if defined(__linux__)
77        /* Linux kernel returns (small) negative errno. */
78        cmp     $-4095, %eax
79        jae     0f
80#elif defined(__FreeBSD__)
81        /* FreeBSD kernel returns positive errno and C bit set. */
82        jc      1f
83#else
84#error "unsupported os"
85#endif
86        pop     %ebx
87        .cfi_remember_state
88        .cfi_adjust_cfa_offset -4
89        .cfi_restore ebx
90        pop     %edi
91        .cfi_adjust_cfa_offset -4
92        .cfi_restore edi
93        pop     %esi
94        .cfi_adjust_cfa_offset -4
95        .cfi_restore esi
96        pop     %ebp
97        .cfi_adjust_cfa_offset -4
98        .cfi_restore ebp
99        ret
100        .cfi_restore_state
101
102#if defined(__linux__)
1030:      neg     %eax
104        jmp     1f
105#endif
106
107        /* code path when we didn't execute the syscall */
1082:      mov     $QEMU_ERESTARTSYS, %eax
109
110        /* code path setting errno */
1111:      pop     %ebx
112        .cfi_adjust_cfa_offset -4
113        .cfi_restore ebx
114        pop     %edi
115        .cfi_adjust_cfa_offset -4
116        .cfi_restore edi
117        pop     %esi
118        .cfi_adjust_cfa_offset -4
119        .cfi_restore esi
120        pop     %ebp
121        .cfi_adjust_cfa_offset -4
122        .cfi_restore ebp
123        jmp     safe_syscall_set_errno_tail
124
125        .cfi_endproc
126        .size   safe_syscall_base, .-safe_syscall_base
127