[glibc] elf: Avoid some free (NULL) calls in _dl_update_slotinfo

[Florian Weimer <fw@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=afe42e935b3ee97bac9a7064157587777259c60e

commit afe42e935b3ee97bac9a7064157587777259c60e
Author: Florian Weimer <fweimer@redhat.com>
Date:   Mon Jun 3 10:49:40 2024 +0200

    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>

Diff:
---
 elf/dl-tls.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

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;