[PATCH] Fix sparc unwinding

[PATCH] Fix sparc unwinding

[Jakub Jelinek]

Hi!

If sparc* glibc is built with -fasynchronous-unwind-tables
(Fedora does this and perhaps it would be a good idea to do it
in upstream glibc too), gij doesn't work.

The problem can be shown e.g. on:
#include <stdio.h>
#include <unwind.h>

static _Unwind_Reason_Code
backtrace_helper (struct _Unwind_Context *ctx, void *a)
{
  printf ("%p\n", _Unwind_GetIP (ctx));
  return _URC_NO_REASON;
}

int
main (void)
{
  _Unwind_Backtrace (backtrace_helper, NULL);
  return 0;
}
if built with -fasynchronous-unwind-tables, it prints
one address in main, one in __libc_start_main and then
forever keeps repeating the same address inside of _start.
The problem is in start.S unwind directives (and clone.S
as well).  On all other arches these two have no unwind info,
so unwinding stops there.  Alternatively the unwind info
needs to be changed so that it will have context->ra == NULL
in the next iteration and thus stop unwinding.
Something like:

_start:
        cfi_startproc
        cfi_escape (0x16 /* DW_CFA_val_expression */, 15 /* %o7 */, 2 /* len */, 9 /* DW_OP_const1s */, -8)
   
  /* Terminate the stack frame, and reserve space for functions to
     drop their arguments.  */
        mov     %g0, %fp
        cfi_def_cfa_register (%fp)
        sub     %sp, 6*4, %sp
   
  /* Extract the arguments and environment as encoded on the stack.  The
     argument info starts after one register window (16 words) past the SP.  */
        ld      [%sp+22*4], %o1

(and no cfi_adjust_cfa_offset) should be enough, but
1) uses fairly recently implemented DW_CFA_val_expression, in code that is
   linked into the binaries rather than linked dynamically
2) in the above testcase prints an annoying NULL at the end

So IMHO it is better just to remove cfi_* markup from start.S and
the part of clone.S that is executed in the child (well, perhaps even
in the clone fn itself there should be cfi_endproc right before the
syscall, so that if signal comes in for the child right after the syscall
unwinding works too).  Do you agree or would you prefer to go the
cfi_escape route?

2007-09-18  Jakub Jelinek  <jakub@redhat.com>

	* sysdeps/sparc/sparc32/elf/start.S: Remove cfi_* markup.
	* sysdeps/sparc/sparc64/elf/start.S: Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc32/clone.S (__thread_start): Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc64/clone.S (__thread_start): Likewise.

--- libc/sysdeps/sparc/sparc32/elf/start.S.jj	2006-03-06 02:33:04.000000000 +0100
+++ libc/sysdeps/sparc/sparc32/elf/start.S	2007-09-18 11:55:35.000000000 +0200
@@ -1,5 +1,5 @@
 /* Startup code for elf32-sparc
-   Copyright (C) 1997, 1998, 2002, 2004 Free Software Foundation, Inc.
+   Copyright (C) 1997, 1998, 2002, 2004, 2007 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
    Contributed by Richard Henderson <richard@gnu.ai.mit.edu>, 1997.
 
@@ -48,8 +48,6 @@
 	.global _start
 	.type _start,#function
 _start:
-	cfi_startproc
-
 #ifdef SHARED
 	sethi	%hi(_GLOBAL_OFFSET_TABLE_-4), %l7
 	call	.LLGETPC0
@@ -60,7 +58,6 @@ _start:
      drop their arguments.  */
 	mov	%g0, %fp
 	sub	%sp, 6*4, %sp
-	cfi_adjust_cfa_offset(6*4)
 
   /* Extract the arguments and environment as encoded on the stack.  The
      argument info starts after one register window (16 words) past the SP.  */
@@ -93,8 +90,6 @@ _start:
   /* Die very horribly if exit returns.  */
 	unimp
 
-	cfi_endproc
-
 	.size _start, .-_start
 
 /* Define a symbol for the first piece of initialized data.  */
--- libc/sysdeps/sparc/sparc64/elf/start.S.jj	2006-03-06 02:33:03.000000000 +0100
+++ libc/sysdeps/sparc/sparc64/elf/start.S	2007-09-18 11:55:46.000000000 +0200
@@ -1,5 +1,5 @@
 /* Startup code for elf64-sparc
-   Copyright (C) 1997, 1998, 2002, 2004 Free Software Foundation, Inc.
+   Copyright (C) 1997, 1998, 2002, 2004, 2007 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
    Contributed by Richard Henderson <richard@gnu.ai.mit.edu>, 1997.
 
@@ -48,8 +48,6 @@
 	.global _start
 	.type _start,#function
 _start:
-	cfi_startproc
-
 #ifdef SHARED
 	sethi	%hi(_GLOBAL_OFFSET_TABLE_-4), %l7
 	call	.LLGETPC0
@@ -60,7 +58,6 @@ _start:
      drop their arguments.  */
 	mov	%g0, %fp
 	sub	%sp, 6*8, %sp
-	cfi_adjust_cfa_offset(6*8)
 
   /* Extract the arguments and environment as encoded on the stack.  The
      argument info starts after one register window (16 words) past the SP,
@@ -94,8 +91,6 @@ _start:
   /* Die very horribly if exit returns.  */
 	illtrap	0
 
-	cfi_endproc
-
 	.size _start, .-_start
 
 /* Define a symbol for the first piece of initialized data.  */
--- libc/sysdeps/unix/sysv/linux/sparc/sparc32/clone.S.jj	2007-08-16 15:23:31.000000000 +0200
+++ libc/sysdeps/unix/sysv/linux/sparc/sparc32/clone.S	2007-09-18 11:57:34.000000000 +0200
@@ -82,8 +82,6 @@ END(__clone)
 
 	.type	__thread_start,@function
 __thread_start:
-	cfi_startproc
-
 #ifdef RESET_PID
 	sethi	%hi(CLONE_THREAD), %l0
 	andcc	%g4, %l0, %g0
@@ -104,8 +102,6 @@ __thread_start:
 	call	HIDDEN_JUMPTARGET(_exit),0
 	 nop
 
-	cfi_endproc
-
 	.size	__thread_start, .-__thread_start
 
 weak_alias (__clone, clone)
--- libc/sysdeps/unix/sysv/linux/sparc/sparc64/clone.S.jj	2007-08-16 15:25:30.000000000 +0200
+++ libc/sysdeps/unix/sysv/linux/sparc/sparc64/clone.S	2007-09-18 11:58:06.000000000 +0200
@@ -78,7 +78,6 @@ END(__clone)
 
 	.type __thread_start,@function
 __thread_start:
-	cfi_startproc
 #ifdef RESET_PID
 	sethi	%hi(CLONE_THREAD), %l0
 	andcc	%g4, %l0, %g0
@@ -97,7 +96,6 @@ __thread_start:
 	 mov	%g3,%o0
 	call	HIDDEN_JUMPTARGET(_exit),0
 	 nop
-	cfi_endproc
 
 	.size	__thread_start, .-__thread_start
 

	Jakub