elf: Fix TLS modid reuse generation assignment (BZ 29039)

_dl_assign_tls_modid() assigns a slotinfo entry for a new module, but
does *not* do anything to the generation counter. The first time this
happens, the generation is zero and map_generation() returns the current
generation to be used during relocation processing. However, if
a slotinfo entry is later reused, it will already have a generation
assigned. If this generation has fallen behind the current global max
generation, then this causes an obsolete generation to be assigned
during relocation processing, as map_generation() returns this
generation if nonzero. _dl_add_to_slotinfo() eventually resets the
generation, but by then it is too late. This causes DTV updates to be
skipped, leading to NULL or broken TLS slot pointers and segfaults.

Fix this by resetting the generation to zero in _dl_assign_tls_modid(),
so it behaves the same as the first time a slot is assigned.
_dl_add_to_slotinfo() will still assign the correct static generation
later during module load, but relocation processing will no longer use
an obsolete generation.

Note that slotinfo entry (aka modid) reuse typically happens after a
dlclose and only TLS access via dynamic tlsdesc is affected. Because
tlsdesc is optimized to use the optional part of static TLS, dynamic
tlsdesc can be avoided by increasing the glibc.rtld.optional_static_tls
tunable to a large enough value, or by LD_PRELOAD-ing the affected
modules.

Fixes bug 29039.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

elf/dl-tls.c

@@ -154,6 +154,7 @@ _dl_assign_tls_modid (struct link_map *l)
 	      {
 		/* Mark the entry as used, so any dependency see it.  */
 		atomic_store_relaxed (&runp->slotinfo[result - disp].map, l);
+		atomic_store_relaxed (&runp->slotinfo[result - disp].gen, 0);
 		break;
 	      }