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[Qemu-devel] [PATCH] linux-user: provide frame information in x86-64 saf

From: Peter Maydell
Subject: [Qemu-devel] [PATCH] linux-user: provide frame information in x86-64 safe_syscall
Date: Fri, 27 May 2016 16:06:38 +0100

Use cfi directives in the x86-64 safe_syscall to allow gdb to get
backtraces right from within it. (In particular this will be
quite a common situation if the user interrupts QEMU while it's
in a blocked safe-syscall: at the point of the syscall insn RBP
is in use for something else, and so gdb can't find the frame then
without assistance.)

Signed-off-by: Peter Maydell <address@hidden>
The requirements for frame information annotations seem to be a bit
of an undocumented black art, but I think I have these right. At
any rate, gdb now gives correct backtraces from all points in
the routine as far as I can see. Review appreciated...

 linux-user/host/x86_64/safe-syscall.inc.S | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/linux-user/host/x86_64/safe-syscall.inc.S 
index dde434c..bbb1eca 100644
--- a/linux-user/host/x86_64/safe-syscall.inc.S
+++ b/linux-user/host/x86_64/safe-syscall.inc.S
@@ -24,6 +24,7 @@
          * -1-and-errno-set convention is done by the calling wrapper.
+        .cfi_startproc
         /* This saves a frame pointer and aligns the stack for the syscall.
          * (It's unclear if the syscall ABI has the same stack alignment
          * requirements as the userspace function call ABI, but better safe 
@@ -31,6 +32,8 @@ safe_syscall_base:
          * does not list any ABI differences regarding stack alignment.)
         push    %rbp
+        .cfi_def_cfa_offset 16
+        .cfi_offset rbp,-16
         /* The syscall calling convention isn't the same as the
          * C one:
@@ -70,12 +73,19 @@ safe_syscall_start:
         /* code path for having successfully executed the syscall */
         pop     %rbp
+        .cfi_remember_state
+        .cfi_def_cfa_offset 8
+        .cfi_restore ebp
         /* code path when we didn't execute the syscall */
+        .cfi_restore_state
         mov     $-TARGET_ERESTARTSYS, %rax
         pop     %rbp
+        .cfi_def_cfa_offset 8
+        .cfi_restore ebp
+        .cfi_endproc
         .size   safe_syscall_base, .-safe_syscall_base

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