Re: Hitting g dwfl->lookup_elts limit in report_r_debug, so not all modules show up and backtracing fails

[Mark Wielaard <mark@klomp.org>]

Hi Florian, Hi Luke,

On Tue, 2023-05-02 at 09:57 +0200, Florian Weimer via Elfutils-devel
wrote:
> * Luke Diamand via Elfutils-devel:
> 
> > I've got a few cores where report_r_debug() in link_map.c fails to
> > find all of the modules - for example I had libc.so missing. This
> > obviously meant that elfutils could not backtrace my core.
> > 
> > It seems to be related to this code:
> > 
> >   /* 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.  */
> >   size_t iterations = 0;
> > 
> >   while (next != 0 && ++iterations < dwfl->lookup_elts)
> > 
> > I've changed this to just keep going until it reaches
> > dwfl->lookup_elts*5, which seems to "fix" it, but I feel there must be
> > a better fix!
> > 
> > The most recent core I saw with this had lookup_elts=36, and hit 109
> > iterations of the loop and then backtraced just fine.
> 
> It's probably another fallout from -z separate-code, which tends to
> create four LOAD segments.  The magic number 5 sounds about right, as
> gold also has -z text-unlikely-segment, which might result in creating
> that number of load segments (but I haven't tried).

Wow, that had never occurred to me. Thanks.

Luke does the binary/libraries from which your core file was generated
contain multiple PT_LOAD segments?

We could add something like:

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,

Does that sound reasonable?

Thanks,

Mark