[PATCH] Make elf section sorting more deterministic

Mark Wielaard mark@klomp.org
Fri Apr 28 00:35:00 GMT 2017

On Thu, Apr 20, 2017 at 04:54:26PM +0200, Ulf Hermann wrote:
> At least one test (dwfl-addr-sect) depends on the order of elf sections
> with equal addresses. This is not guaranteed by the code. Compare also
> by end address and name to tell entries apart.

O, interesting find. If the start addresses match the order depends on
the specific qsort algorithm. So you need a real tie breaker.

I think it is simpler and more predictable if we just take the section
number into account. It seem to have the added benefit that it provide
the same ordering as before with the glibc qsort, so no testcases need
to be adjusted. Does the following work for you?

diff --git a/libdwfl/derelocate.c b/libdwfl/derelocate.c
index 439a24e..0d10672 100644
--- a/libdwfl/derelocate.c
+++ b/libdwfl/derelocate.c
@@ -63,7 +63,10 @@ compare_secrefs (const void *a, const void *b)
   if ((*p1)->start > (*p2)->start)
     return 1;
-  return 0;
+  /* Same start address, then just compare which section came first.  */
+  size_t n1 = elf_ndxscn ((*p1)->scn);
+  size_t n2 = elf_ndxscn ((*p2)->scn);
+  return n1 - n2;
 static int

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