_dl_build_local_scope - BZ #20488

_dl_build_local_scope - BZ #20488

[Mark Hatle]

This is somewhat a followup to Alexander Miller's email for a few months back:

https://sourceware.org/ml/libc-alpha/2016-05/msg00034.html

Independently we identify a similar problem with the emulated rtld in the cross
prelinker.  The ELF spec indicates that the symbol resolution is breadth first,
not depth first.  Without doing this a failure can occur.

The current prelinker (git://git.yoctoproject.org/prelink-cross) has a specific
test case for the ordering issue.  See 'order' test case.

The patch below fixes the issue in glibc and was developed independently.  My
employer (Wind River) has a copyright assigned with the FSF if the change is
substantial enough to require one.

The patch is attached to BZ 20488, as well as included below.


From 6e4ec5a3c5fe63b6458036f18d43124de4a7e724 Mon Sep 17 00:00:00 2001
From: Mark Hatle <mark.hatle@windriver.com>
Date: Thu, 18 Aug 2016 14:07:58 -0500
Subject: [PATCH] elf/dl-deps.c: Make _dl_build_local_scope breadth first

According to the ELF specification:

When resolving symbolic references, the dynamic linker examines the symbol
tables with a breadth-first search.

This function was using a depth first search.  By doing so the conflict
resolution reported to the prelinker (when LD_TRACE_PRELINKING=1 is set)
was incorrect.  This caused problems when their were various circular
dependencies between libraries.  The problem usually manifested itself by
the wrong IFUNC being executed.

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
---
 elf/dl-deps.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/elf/dl-deps.c b/elf/dl-deps.c
index 6a82987..fc37c87 100644
--- a/elf/dl-deps.c
+++ b/elf/dl-deps.c
@@ -73,13 +73,19 @@ _dl_build_local_scope (struct link_map **list, struct
link_map *map)
 {
   struct link_map **p = list;
   struct link_map **q;
+  struct link_map **r;

   *p++ = map;
   map->l_reserved = 1;
-  if (map->l_initfini)
-    for (q = map->l_initfini + 1; *q; ++q)
-      if (! (*q)->l_reserved)
-	p += _dl_build_local_scope (p, *q);
+
+  for (r = list; r < p; ++r)
+    if ((*r)->l_initfini)
+      for (q = (*r)->l_initfini + 1; *q; ++q)
+	if (! (*q)->l_reserved)
+	  {
+	    *p++ = *q;
+	    (*q)->l_reserved = 1;
+	  }
   return p - list;
 }

-- 
2.5.5