RFC: %ebp-based backtrace patch

[Daniel Jacobowitz <drow@false.org>]

A number of Linux distributions are carrying the attached patch from
Michael Matz.  Frankly, I don't understand i386 frame layout issues
well enough to approve this; I'm posting this in hopes of starting a
discussion between Mark Kettenis and Michael Matz.  The patch
does seem to help in practice.

I took a 32-bit Debian system, with minimal packages installed, and
ran bash inside GDB.  I interrupted it while sitting at a prompt.
Results without the patch:

#0  0xf7fdf430 in __kernel_vsyscall ()
#1  0xf7f0ce93 in __read_nocancel () from /lib/i686/cmov/libc.so.6
#2  0x080cfca6 in rl_getc ()
#3  0x080d0103 in rl_read_key ()
#4  0x080be7d7 in readline_internal_char ()
#5  0x080bebc5 in readline ()
#6  0x080689d1 in ?? ()
#7  0x081b7808 in ?? ()
#8  0x080942c0 in ?? ()
#9  0xffffc5d8 in ?? ()
#10 0x08080e26 in notify_and_cleanup ()
#11 0x0806422b in ?? ()
#12 0x00000000 in ?? ()

With:

#0  0xf7fdf430 in __kernel_vsyscall ()
#1  0xf7f0ce93 in __read_nocancel () from /lib/i686/cmov/libc.so.6
#2  0x080cfca6 in rl_getc ()
#3  0x080d0103 in rl_read_key ()
#4  0x080be7d7 in readline_internal_char ()
#5  0x080bebc5 in readline ()
#6  0x080689d1 in ?? ()
#7  0x0806422b in ?? ()
#8  0x08065492 in ?? ()
#9  0x08068f57 in yyparse ()
#10 0x08061c98 in parse_command ()
#11 0x08061d7f in read_command ()
#12 0x08061fc3 in reader_loop ()
#13 0x08061ae9 in main ()

Michael's explanation of the patch is over here:

  https://bugzilla.novell.com/show_bug.cgi?id=390722#c25

My understanding is that this only affects frames we can't find a
symbol for.  Instead of assuming the frame starts at %esp - 4 (since
sp_offset is initialized to -4 and then not updated if we did not find
a symbol to analyze), assume that it uses and saved %ebp.  We have no
reliable information at this point and the new heuristic seems to be
right more often.

Mark, could you comment on this patch?

-- 
Daniel Jacobowitz
CodeSourcery

2009-07-06  Michael Matz  <matz@suse.de>

	* i386-tdep.c (i386_frame_cache): Assume anonymous functions use
	a frame pointer.

---
 gdb/i386-tdep.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Index: gdb-6.8.50.20090628/gdb/i386-tdep.c
===================================================================
--- gdb-6.8.50.20090628.orig/gdb/i386-tdep.c	2009-07-05 20:38:47.000000000 -0400
+++ gdb-6.8.50.20090628/gdb/i386-tdep.c	2009-07-05 20:39:17.000000000 -0400
@@ -1377,11 +1377,13 @@ i386_frame_cache (struct frame_info *thi
 	  /* This will be added back below.  */
 	  cache->saved_regs[I386_EIP_REGNUM] -= cache->base;
 	}
-      else
+      else if (cache->pc)
 	{
 	  get_frame_register (this_frame, I386_ESP_REGNUM, buf);
 	  cache->base = extract_unsigned_integer (buf, 4) + cache->sp_offset;
 	}
+      else
+	cache->saved_regs[I386_EBP_REGNUM] = 0;
     }
 
   /* Now that we have the base address for the stack frame we can