commit afe42e935b3ee97bac9a7064157587777259c60e Author: Florian Weimer 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 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 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;