Re: [patch] Fix gdb crash on some missing ELF debug info

[Jan Kratochvil <lace@jankratochvil.net>]

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Hi Daniel,

On Fri, 23 Jun 2006 14:39:13 +0200, Daniel Jacobowitz wrote:
...
> The idea is definitely right - thanks!  The patch could be a bit
> better:
> 
> - We really ought to check that it fits within .debug_loc while we're
> here.

OK, fixed (I feel that the DWARF2 processing code is not so foolproof).


> - And I really can't figure out what what you mean by the comment.  Do
> you mean "if we get here, and DW_UNSND (attr) != 0, and we hadn't added
> the next condition, then we'd end up with bogus ->size"?

sorry, it references the code below in dwarf2_symbol_mark_computed().

dwarf2_symbol_mark_computed():
	if (".debug_loc" is missing) {
		assume (dwarf2_per_objfile->loc_size   == 0);
		assume (dwarf2_per_objfile->loc_buffer == NULL);
	}
	assume(DW_UNSND (attr) != 0);
	baton->size = dwarf2_per_objfile->loc_size - DW_UNSND (attr);
	baton->data = dwarf2_per_objfile->loc_buffer + DW_UNSND (attr);
dwarf_expr_frame_base():
	*start = symbaton->data;
	MISSED, *(*start) crashes:
	if (*start == NULL) error ("Could not find the frame base ...");


> You've been submitting a lot of fixes recently; while most of them are
> small, some are big enough to probably require a copyright assignment.
> Have you got one, or shall I send you the forms?

I already signed the copyright assignment to FSF intended for GNU Libtool
but the assignment form looks completely project unspecific to me.
I should be listed there as "Jan Kratochvil", it may be for the e-mail address
<project-libtool@jankratochvil.net>.  Bradley M. Kuhn signed it on 7 Feb 2003.


> - Oh, and no ChangeLog

2006-06-23  Jan Kratochvil  <lace@jankratochvil.net>

	* dwarf2read.c (dwarf2_symbol_mark_computed): Fixed later crash
	on location list reference if the ".debug_loc" section is missing.


Regards,
Jan Kratochvil

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https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=196439

Fedora Core 5:
	gdb /usr/lib/valgrind/x86-linux/memcheck
	break _start_in_C
	run
segfault

eu-readelf -w /usr/lib/valgrind/x86-linux/memcheck
	DWARF section '.debug_info' at offset 0x14759c:
	...
	 [ 176d4]    subprogram
	...
	             name                 "_start_in_C"
	...
	             frame_base           location list [ 116d7]
corrupted as no location lists (.debug_loc) exist there at all.


Index: dwarf2read.c
===================================================================
RCS file: /cvs/src/src/gdb/dwarf2read.c,v
retrieving revision 1.199
diff -u -p -r1.199 dwarf2read.c
--- dwarf2read.c	14 Jun 2006 15:06:35 -0000	1.199
+++ dwarf2read.c	23 Jun 2006 12:46:34 -0000
@@ -9324,7 +9324,9 @@ static void
 dwarf2_symbol_mark_computed (struct attribute *attr, struct symbol *sym,
 			     struct dwarf2_cu *cu)
 {
-  if (attr->form == DW_FORM_data4 || attr->form == DW_FORM_data8)
+  if ((attr->form == DW_FORM_data4 || attr->form == DW_FORM_data8)
+      /* ".debug_loc" may not exist at all.  */
+      && DW_UNSND (attr) < dwarf2_per_objfile->loc_size)
     {
       struct dwarf2_loclist_baton *baton;