[PATCHv2 0/1] In report_r_debug, iterate more segments

[PATCHv2 0/1] In report_r_debug, iterate more segments

[Luke Diamand]

Further to:

    https://sourceware.org/pipermail/elfutils-devel/2023q2/006149.html

This is a patch to increase the number of iterations looking for
segments to load.

This lets eu-stack backtrace cores that would otherwise fail.

Florian Weimer and Mark Wielaard suggested both the explanation and the fix.

Luke Diamand (1):
  report_r_debug: handle `-z separate-code' and find more modules

 libdwfl/link_map.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

-- 
2.40.1

[PATCHv2 1/1] report_r_debug: handle `-z separate-code' and find more modules

[Luke Diamand]

Looking at some cores in eu-stack, I found that they were not being
backtraced.

This was because elfutils had not found some modules (e.g. libc-2.22.so)
in report_r_debug.

That is because it has a limit on the number of link map entries it will
look at, to avoid loops in corrupted core files.

The example I found had:
- 36 elements
- 109 iterations

See also discussion here:

    https://sourceware.org/pipermail/elfutils-devel/2023q2/006149.html

Signed-off-by: Luke Diamand <ldiamand@roku.com>
---
 libdwfl/link_map.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/libdwfl/link_map.c b/libdwfl/link_map.c
index 06d85eb6..76f23354 100644
--- a/libdwfl/link_map.c
+++ b/libdwfl/link_map.c
@@ -331,11 +331,17 @@ report_r_debug (uint_fast8_t elfclass, uint_fast8_t elfdata,
   int result = 0;
 
   /* There can't be more elements in the link_map list than there are
-     segments.  DWFL->lookup_elts is probably twice that number, so it
-     is certainly above the upper bound.  If we iterate too many times,
-     there must be a loop in the pointers due to link_map clobberation.  */
+     segments.  A segment is created for each PT_LOAD and there can be
+     up to 5 per module (-z separate-code, tends to create four LOAD
+     segments, gold has -z text-unlikely-segment, which might result
+     in creating that number of load segments) DWFL->lookup_elts is
+     probably twice the number of modules, so that multiplied by max
+     PT_LOADs is certainly above the upper bound.  If we iterate too
+     many times, there must be a loop in the pointers due to link_map
+     clobberation.  */
+#define MAX_PT_LOAD 5
   size_t iterations = 0;
-  while (next != 0 && ++iterations < dwfl->lookup_elts)
+  while (next != 0 && ++iterations < dwfl->lookup_elts * MAX_PT_LOAD)
     {
       if (read_addrs (&memory_closure, elfclass, elfdata,
 		      &buffer, &buffer_available, next, &read_vaddr,
-- 
2.40.1

Re: [PATCHv2 1/1] report_r_debug: handle `-z separate-code' and find more modules

[Mark Wielaard]

Hi Luke,

On Fri, May 12, 2023 at 08:11:45PM +0100, Luke Diamand wrote:
> Looking at some cores in eu-stack, I found that they were not being
> backtraced.
> 
> This was because elfutils had not found some modules (e.g. libc-2.22.so)
> in report_r_debug.
> 
> That is because it has a limit on the number of link map entries it will
> look at, to avoid loops in corrupted core files.

Looks good. Pushed.

Thanks,

Mark