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elf: Avoid some free (NULL) calls in _dl_update_slotinfo

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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 d2123d6827 ("elf: Fix slow
tls access after dlopen [BZ #19924]").

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
This commit is contained in:
Florian Weimer 2024-06-03 10:49:40 +02:00
parent 46b5e98ef6
commit afe42e935b
1 changed files with 8 additions and 1 deletions
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9
elf/dl-tls.c
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@ -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;