elf: Avoid some free (NULL) calls in _dl_update_slotinfo

This has been confirmed to work around some interposed mallocs.  Here
is a discussion of the impact test ust/libc-wrapper/test_libc-wrapper
in lttng-tools:

  New TLS usage in libgcc_s.so.1, compatibility impact
  <https://inbox.sourceware.org/libc-alpha/8734v1ieke.fsf@oldenburg.str.redhat.com/>

Reportedly, this patch also papers over a similar issue when tcmalloc
2.9.1 is not compiled with -ftls-model=initial-exec.  Of course the
goal really should be to compile mallocs with the initial-exec TLS
model, but this commit appears to be a useful interim workaround.

Fixes commit d2123d68275acc0f061e73d5f86ca504e0d5a344 ("elf: Fix slow
tls access after dlopen [BZ #19924]").

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/elf/dl-tls.c b/elf/dl-tls.c
index 7b3dd9ab60..670dbc42fc 100644
--- a/elf/dl-tls.c
+++ b/elf/dl-tls.c
@@ -819,7 +819,14 @@ _dl_update_slotinfo (unsigned long int req_modid, size_t new_gen)
 		 dtv entry free it.  Note: this is not AS-safe.  */
 	      /* XXX Ideally we will at some point create a memory
 		 pool.  */
-	      free (dtv[modid].pointer.to_free);
+	      /* Avoid calling free on a null pointer.  Some mallocs
+		 incorrectly use dynamic TLS, and depending on how the
+		 free function was compiled, it could call
+		 __tls_get_addr before the null pointer check in the
+		 free implementation.  Checking here papers over at
+		 least some dynamic TLS usage by interposed mallocs.  */
+	      if (dtv[modid].pointer.to_free != NULL)
+		free (dtv[modid].pointer.to_free);
 	      dtv[modid].pointer.val = TLS_DTV_UNALLOCATED;
 	      dtv[modid].pointer.to_free = NULL;