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[Qemu-devel] [PULL v2 20/24] linux-user: Provide safe_syscall for i386


From: riku . voipio
Subject: [Qemu-devel] [PULL v2 20/24] linux-user: Provide safe_syscall for i386
Date: Tue, 28 Jun 2016 22:12:54 +0300

From: Richard Henderson <address@hidden>

Signed-off-by: Richard Henderson <address@hidden>
Reviewed-by: Peter Maydell <address@hidden>
Signed-off-by: Riku Voipio <address@hidden>
---
 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 <address@hidden>
+ * 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
-- 
2.1.4




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