/* * safe-syscall.inc.S : host-specific assembly fragment * to handle signals occurring at the same time as system calls. * This is intended to be included by common-user/safe-syscall.S * * Written by Richard Henderson <rth@twiddle.net> * Copyright (C) 2016 Red Hat, Inc. * * This work is licensed under the terms of the GNU GPL, version 2 or later. * See the COPYING file in the top-level directory. */ .global safe_syscall_base .global safe_syscall_start .global safe_syscall_end .type safe_syscall_base, %function .cfi_sections .debug_frame .text .syntax unified .arm .align 2 /* This is the entry point for making a system call. The calling * convention here is that of a C varargs function with the * first argument an 'int *' to the signal_pending flag, the * second one the system call number (as a 'long'), and all further * arguments being syscall arguments (also 'long'). */ safe_syscall_base: .fnstart .cfi_startproc mov r12, sp /* save entry stack */ push { r4, r5, r6, r7, r8, lr } .save { r4, r5, r6, r7, r8, lr } .cfi_adjust_cfa_offset 24 .cfi_rel_offset r4, 0 .cfi_rel_offset r5, 4 .cfi_rel_offset r6, 8 .cfi_rel_offset r7, 12 .cfi_rel_offset r8, 16 .cfi_rel_offset lr, 20 /* The syscall calling convention isn't the same as the C one: * we enter with r0 == &signal_pending * r1 == syscall number * r2, r3, [sp+0] ... [sp+12] == syscall arguments * and return the result in r0 * and the syscall instruction needs * r7 == syscall number * r0 ... r6 == syscall arguments * and returns the result in r0 * Shuffle everything around appropriately. * Note the 16 bytes that we pushed to save registers. */ mov r8, r0 /* copy signal_pending */ mov r7, r1 /* syscall number */ mov r0, r2 /* syscall args */ mov r1, r3 ldm r12, { r2, r3, r4, r5, r6 } /* This next sequence of code works in conjunction with the * rewind_if_safe_syscall_function(). If a signal is taken * and the interrupted PC is anywhere between 'safe_syscall_start' * and 'safe_syscall_end' then we rewind it to 'safe_syscall_start'. * The code sequence must therefore be able to cope with this, and * the syscall instruction must be the final one in the sequence. */ safe_syscall_start: /* if signal_pending is non-zero, don't do the call */ ldr r12, [r8] /* signal_pending */ tst r12, r12 bne 2f swi 0 safe_syscall_end: /* code path for having successfully executed the syscall */ cmp r0, #-4096 neghi r0, r0 bhi 1f pop { r4, r5, r6, r7, r8, pc } /* code path when we didn't execute the syscall */ 2: mov r0, #QEMU_ERESTARTSYS /* code path setting errno */ 1: pop { r4, r5, r6, r7, r8, lr } .cfi_adjust_cfa_offset -24 .cfi_restore r4 .cfi_restore r5 .cfi_restore r6 .cfi_restore r7 .cfi_restore r8 .cfi_restore lr b safe_syscall_set_errno_tail .fnend .cfi_endproc .size safe_syscall_base, .-safe_syscall_base