[RFC PATCH 0/1] In report_r_debug, iterate more segments

[RFC PATCH 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 draft patch to increase the number of iterations looking for
segments to load.

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

I have not included any unit tests as so far I have been unable to
reproduce the problem in a suitably small example.

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

 libdwfl/link_map.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

-- 
2.39.1

[RFC PATCH 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

I have increased the limit, and this seems to solve the problem.

Florian Weimer suggested that the problem is caused by use of
`-z separate-code' although I have not yet managed to confirm this.

See also:

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

Signed-off-by: Luke Diamand <ldiamand@roku.com>
---
 libdwfl/link_map.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/libdwfl/link_map.c b/libdwfl/link_map.c
index 06d85eb6..975910a9 100644
--- a/libdwfl/link_map.c
+++ b/libdwfl/link_map.c
@@ -330,12 +330,13 @@ report_r_debug (uint_fast8_t elfclass, uint_fast8_t elfdata,
   Dwfl_Module **lastmodp = &dwfl->modulelist;
   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.  */
+  /* Keep an upper bound on the number of iterations - if we iterate
+   * too many times, there must be a loop in the pointers due to link_map
+   * clobberation.
+   */
   size_t iterations = 0;
-  while (next != 0 && ++iterations < dwfl->lookup_elts)
+
+  while (next != 0 && ++iterations < dwfl->lookup_elts * 5)
     {
       if (read_addrs (&memory_closure, elfclass, elfdata,
 		      &buffer, &buffer_available, next, &read_vaddr,
-- 
2.39.1