linux-user/host/i386/safe-syscall.inc.S

/*
 * 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 linux-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

	/* 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').
	 * We return a long which is the syscall's return value, which
	 * may be negative-errno on failure. Conversion to the
	 * -1-and-errno-set convention is done by the calling wrapper.
	 */
safe_syscall_base:
	.cfi_startproc
	push	%ebp
	.cfi_adjust_cfa_offset 4
	.cfi_rel_offset ebp, 0
	push	%esi
	.cfi_adjust_cfa_offset 4
	.cfi_rel_offset esi, 0
	push	%edi
	.cfi_adjust_cfa_offset 4
	.cfi_rel_offset edi, 0
	push	%ebx
	.cfi_adjust_cfa_offset 4
	.cfi_rel_offset ebx, 0

	/* The syscall calling convention isn't the same as the C one:
	 * we enter with 0(%esp) == return address
	 *               4(%esp) == *signal_pending
	 *               8(%esp) == syscall number
	 *               12(%esp) ... 32(%esp) == syscall arguments
	 *               and return the result in eax
	 * and the syscall instruction needs
	 *               eax == syscall number
	 *               ebx, ecx, edx, esi, edi, ebp == syscall arguments
	 *               and returns the result in eax
	 * Shuffle everything around appropriately.
	 * Note the 16 bytes that we pushed to save registers.
	 */
	mov	12+16(%esp), %ebx	/* the syscall arguments */
	mov	16+16(%esp), %ecx
	mov	20+16(%esp), %edx
	mov	24+16(%esp), %esi
	mov	28+16(%esp), %edi
	mov	32+16(%esp), %ebp

	/* 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 */
	mov	4+16(%esp), %eax	/* signal_pending */
	cmpl	$0, (%eax)
	jnz	1f
	mov	8+16(%esp), %eax	/* syscall number */
	int	$0x80
safe_syscall_end:
	/* code path for having successfully executed the syscall */
	pop	%ebx
	.cfi_remember_state
	.cfi_adjust_cfa_offset -4
	.cfi_restore ebx
	pop	%edi
	.cfi_adjust_cfa_offset -4
	.cfi_restore edi
	pop	%esi
	.cfi_adjust_cfa_offset -4
	.cfi_restore esi
	pop	%ebp
	.cfi_adjust_cfa_offset -4
	.cfi_restore ebp
	ret

1:
	/* code path when we didn't execute the syscall */
	.cfi_restore_state
	mov	$-TARGET_ERESTARTSYS, %eax
	jmp	safe_syscall_end
	.cfi_endproc

	.size	safe_syscall_base, .-safe_syscall_base