[Qemu-devel] [PATCH v2 1/5] linux-user: Provide safe_syscall for i386

[Richard Henderson <rth@twiddle.net>]

Signed-off-by: Richard Henderson <rth@twiddle.net>
---
 linux-user/host/i386/hostdep.h          |  23 +++++++
 linux-user/host/i386/safe-syscall.inc.S | 112 ++++++++++++++++++++++++++++++++
 2 files changed, 135 insertions(+)
 create mode 100644 linux-user/host/i386/safe-syscall.inc.S

diff --git a/linux-user/host/i386/hostdep.h b/linux-user/host/i386/hostdep.h
index 7609bf5..5a12f4a 100644
--- a/linux-user/host/i386/hostdep.h
+++ b/linux-user/host/i386/hostdep.h
@@ -12,4 +12,27 @@
 #ifndef QEMU_HOSTDEP_H
 #define QEMU_HOSTDEP_H
 
+/* We have a safe-syscall.inc.S */
+#define HAVE_SAFE_SYSCALL
+
+#ifndef __ASSEMBLER__
+
+/* These are defined by the safe-syscall.inc.S file */
+extern char safe_syscall_start[];
+extern char safe_syscall_end[];
+
+/* Adjust the signal context to rewind out of safe-syscall if we're in it */
+static inline void rewind_if_in_safe_syscall(void *puc)
+{
+    struct ucontext *uc = puc;
+    greg_t *pcreg = &uc->uc_mcontext.gregs[REG_EIP];
+
+    if (*pcreg > (uintptr_t)safe_syscall_start
+        && *pcreg < (uintptr_t)safe_syscall_end) {
+        *pcreg = (uintptr_t)safe_syscall_start;
+    }
+}
+
+#endif /* __ASSEMBLER__ */
+
 #endif
diff --git a/linux-user/host/i386/safe-syscall.inc.S b/linux-user/host/i386/safe-syscall.inc.S
new file mode 100644
index 0000000..766d0de
--- /dev/null
+++ b/linux-user/host/i386/safe-syscall.inc.S
@@ -0,0 +1,112 @@
+/*
+ * 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 */
+	cmp	$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_def_cfa_offset -4
+	.cfi_restore ebx
+	pop	%edi
+	.cfi_def_cfa_offset -4
+	.cfi_restore edi
+	pop	%esi
+	.cfi_def_cfa_offset -4
+	.cfi_restore esi
+	pop	%ebp
+	.cfi_def_cfa_offset -4
+	.cfi_restore ebp
+	ret
+
+1:
+	/* code path when we didn't execute the syscall */
+	.cfi_restore_state
+	mov	$-TARGET_ERESTARTSYS, %eax
+	pop	%ebx
+	.cfi_def_cfa_offset -4
+	.cfi_restore ebx
+	pop	%edi
+	.cfi_def_cfa_offset -4
+	.cfi_restore edi
+	pop	%esi
+	.cfi_def_cfa_offset -4
+	.cfi_restore esi
+	pop	%ebp
+	.cfi_def_cfa_offset -4
+	.cfi_restore ebp
+	ret
+	.cfi_endproc
+
+	.size	safe_syscall_base, .-safe_syscall_base
-- 
2.5.5