safe-syscall.inc.S (3907B)
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 .cfi_sections .debug_frame 19 20 .text 21 .syntax unified 22 .arm 23 .align 2 24 25 /* This is the entry point for making a system call. The calling 26 * convention here is that of a C varargs function with the 27 * first argument an 'int *' to the signal_pending flag, the 28 * second one the system call number (as a 'long'), and all further 29 * arguments being syscall arguments (also 'long'). 30 */ 31 safe_syscall_base: 32 .fnstart 33 .cfi_startproc 34 mov r12, sp /* save entry stack */ 35 push { r4, r5, r6, r7, r8, lr } 36 .save { r4, r5, r6, r7, r8, lr } 37 .cfi_adjust_cfa_offset 24 38 .cfi_rel_offset r4, 0 39 .cfi_rel_offset r5, 4 40 .cfi_rel_offset r6, 8 41 .cfi_rel_offset r7, 12 42 .cfi_rel_offset r8, 16 43 .cfi_rel_offset lr, 20 44 45 /* The syscall calling convention isn't the same as the C one: 46 * we enter with r0 == &signal_pending 47 * r1 == syscall number 48 * r2, r3, [sp+0] ... [sp+12] == syscall arguments 49 * and return the result in r0 50 * and the syscall instruction needs 51 * r7 == syscall number 52 * r0 ... r6 == syscall arguments 53 * and returns the result in r0 54 * Shuffle everything around appropriately. 55 * Note the 16 bytes that we pushed to save registers. 56 */ 57 mov r8, r0 /* copy signal_pending */ 58 mov r7, r1 /* syscall number */ 59 mov r0, r2 /* syscall args */ 60 mov r1, r3 61 ldm r12, { r2, r3, r4, r5, r6 } 62 63 /* This next sequence of code works in conjunction with the 64 * rewind_if_safe_syscall_function(). If a signal is taken 65 * and the interrupted PC is anywhere between 'safe_syscall_start' 66 * and 'safe_syscall_end' then we rewind it to 'safe_syscall_start'. 67 * The code sequence must therefore be able to cope with this, and 68 * the syscall instruction must be the final one in the sequence. 69 */ 70 safe_syscall_start: 71 /* if signal_pending is non-zero, don't do the call */ 72 ldr r12, [r8] /* signal_pending */ 73 tst r12, r12 74 bne 2f 75 swi 0 76 safe_syscall_end: 77 78 /* code path for having successfully executed the syscall */ 79 #if defined(__linux__) 80 /* Linux kernel returns (small) negative errno. */ 81 cmp r0, #-4096 82 neghi r0, r0 83 bhi 1f 84 #elif defined(__FreeBSD__) 85 /* FreeBSD kernel returns positive errno and C bit set. */ 86 bcs 1f 87 #else 88 #error "unsupported os" 89 #endif 90 pop { r4, r5, r6, r7, r8, pc } 91 92 /* code path when we didn't execute the syscall */ 93 2: mov r0, #QEMU_ERESTARTSYS 94 95 /* code path setting errno */ 96 1: pop { r4, r5, r6, r7, r8, lr } 97 .cfi_adjust_cfa_offset -24 98 .cfi_restore r4 99 .cfi_restore r5 100 .cfi_restore r6 101 .cfi_restore r7 102 .cfi_restore r8 103 .cfi_restore lr 104 b safe_syscall_set_errno_tail 105 106 .fnend 107 .cfi_endproc 108 .size safe_syscall_base, .-safe_syscall_base