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
