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dlopen: Rework handling of pending NODELETE status

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Commit a2e8aa0d9e ("Block signals during
the initial part of dlopen") was deemed necessary because of
read-modify-write operations like the one in  add_dependency in
elf/dl-lookup.c.  In the old code, we check for any kind of NODELETE
status and bail out:

      /* Redo the NODELETE check, as when dl_load_lock wasn't held
	 yet this could have changed.  */
      if (map->l_nodelete != link_map_nodelete_inactive)
	goto out;

And then set pending status (during relocation):

	  if (flags & DL_LOOKUP_FOR_RELOCATE)
	    map->l_nodelete = link_map_nodelete_pending;
	  else
	    map->l_nodelete = link_map_nodelete_active;

If a signal arrives during relocation and the signal handler, through
lazy binding, adds a global scope dependency on the same map, it will
set map->l_nodelete to link_map_nodelete_active.  This will be
overwritten with link_map_nodelete_pending by the dlopen relocation
code.

To avoid such problems in relation to the l_nodelete member, this
commit introduces two flags for active NODELETE status (irrevocable)
and pending NODELETE status (revocable until activate_nodelete is
invoked).  As a result, NODELETE processing in dlopen does not
introduce further reasons why lazy binding from signal handlers
is unsafe during dlopen, and a subsequent commit can remove signal
blocking from dlopen.

This does not address pre-existing issues (unrelated to the NODELETE
changes) which make lazy binding in a signal handler during dlopen
unsafe, such as the use of malloc in both cases.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
This commit is contained in:
Florian Weimer
2019-12-13 10:18:46 +01:00
parent 365624e2d2
commit f8ed116aa5
5 changed files with 62 additions and 58 deletions
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22
elf/dl-open.c
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@ -440,13 +440,17 @@ activate_nodelete (struct link_map *new)
NODELETE status for objects outside the local scope. */
for (struct link_map *l = GL (dl_ns)[new->l_ns]._ns_loaded; l != NULL;
l = l->l_next)
if (l->l_nodelete == link_map_nodelete_pending)
if (l->l_nodelete_pending)
{
if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_FILES))
_dl_debug_printf ("activating NODELETE for %s [%lu]\n",
l->l_name, l->l_ns);
l->l_nodelete = link_map_nodelete_active;
l->l_nodelete_active = true;
/* This is just a debugging aid, to indicate that
activate_nodelete has run for this map. */
l->l_nodelete_pending = false;
}
}
@ -549,10 +553,10 @@ dl_open_worker (void *a)
if (__glibc_unlikely (mode & RTLD_NODELETE))
{
if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_FILES)
&& new->l_nodelete == link_map_nodelete_inactive)
&& !new->l_nodelete_active)
_dl_debug_printf ("marking %s [%lu] as NODELETE\n",
new->l_name, new->l_ns);
new->l_nodelete = link_map_nodelete_active;
new->l_nodelete_active = true;
}
/* Finalize the addition to the global scope. */
@ -568,7 +572,7 @@ dl_open_worker (void *a)
/* Schedule NODELETE marking for the directly loaded object if
requested. */
if (__glibc_unlikely (mode & RTLD_NODELETE))
new->l_nodelete = link_map_nodelete_pending;
new->l_nodelete_pending = true;
/* Load that object's dependencies. */
_dl_map_object_deps (new, NULL, 0, 0,
@ -683,7 +687,7 @@ dl_open_worker (void *a)
_dl_start_profile ();
/* Prevent unloading the object. */
GL(dl_profile_map)->l_nodelete = link_map_nodelete_active;
GL(dl_profile_map)->l_nodelete_active = true;
}
}
else
@ -882,9 +886,9 @@ no more namespaces available for dlmopen()"));
happens inside dl_open_worker. */
__libc_signal_restore_set (&args.original_signal_mask);
/* All link_map_nodelete_pending objects should have been
deleted at this point, which is why it is not necessary
to reset the flag here. */
/* All l_nodelete_pending objects should have been deleted
at this point, which is why it is not necessary to reset
the flag here. */
}
else
__libc_signal_restore_set (&args.original_signal_mask);