dlopen: Rework handling of pending NODELETE status

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>

elf/dl-close.c

@@ -197,7 +197,7 @@ _dl_close_worker (struct link_map *map, bool force)
       /* Check whether this object is still used.  */
       if (l->l_type == lt_loaded
 	  && l->l_direct_opencount == 0
-	  && l->l_nodelete != link_map_nodelete_active
+	  && !l->l_nodelete_active
 	  /* See CONCURRENCY NOTES in cxa_thread_atexit_impl.c to know why
 	     acquire is sufficient and correct.  */
 	  && atomic_load_acquire (&l->l_tls_dtor_count) == 0
@@ -279,8 +279,7 @@ _dl_close_worker (struct link_map *map, bool force)
 
       if (!used[i])
 	{
-	  assert (imap->l_type == lt_loaded
-		  && imap->l_nodelete != link_map_nodelete_active);
+	  assert (imap->l_type == lt_loaded && !imap->l_nodelete_active);
 
 	  /* Call its termination function.  Do not do it for
 	     half-cooked objects.  Temporarily disable exception
@@ -830,7 +829,7 @@ _dl_close (void *_map)
      before we took the lock. There is no way to detect this (see below)
      so we proceed assuming this isn't the case.  First see whether we
      can remove the object at all.  */
-  if (__glibc_unlikely (map->l_nodelete == link_map_nodelete_active))
+  if (__glibc_unlikely (map->l_nodelete_active))
     {
       /* Nope.  Do nothing.  */
       __rtld_lock_unlock_recursive (GL(dl_load_lock));

elf/dl-lookup.c

@@ -187,6 +187,28 @@ enter_unique_sym (struct unique_sym *table, size_t size,
   table[idx].map = map;
 }
 
+/* Mark MAP as NODELETE according to the lookup mode in FLAGS.  During
+   initial relocation, NODELETE state is pending only.  */
+static void
+mark_nodelete (struct link_map *map, int flags)
+{
+  if (flags & DL_LOOKUP_FOR_RELOCATE)
+    map->l_nodelete_pending = true;
+  else
+    map->l_nodelete_active = true;
+}
+
+/* Return true if MAP is marked as NODELETE according to the lookup
+   mode in FLAGS> */
+static bool
+is_nodelete (struct link_map *map, int flags)
+{
+  /* Non-pending NODELETE always counts.  Pending NODELETE only counts
+     during initial relocation processing.  */
+  return map->l_nodelete_active
+    || ((flags & DL_LOOKUP_FOR_RELOCATE) && map->l_nodelete_pending);
+}
+
 /* Utility function for do_lookup_x. Lookup an STB_GNU_UNIQUE symbol
    in the unique symbol table, creating a new entry if necessary.
    Return the matching symbol in RESULT.  */
@@ -311,8 +333,7 @@ do_lookup_unique (const char *undef_name, uint_fast32_t new_hash,
       enter_unique_sym (entries, size,
                         new_hash, strtab + sym->st_name, sym, map);
 
-      if (map->l_type == lt_loaded
-	  && map->l_nodelete == link_map_nodelete_inactive)
+      if (map->l_type == lt_loaded && !is_nodelete (map, flags))
 	{
 	  /* Make sure we don't unload this object by
 	     setting the appropriate flag.  */
@@ -320,10 +341,7 @@ do_lookup_unique (const char *undef_name, uint_fast32_t new_hash,
 	    _dl_debug_printf ("\
 marking %s [%lu] as NODELETE due to unique symbol\n",
 			      map->l_name, map->l_ns);
-	  if (flags & DL_LOOKUP_FOR_RELOCATE)
-	    map->l_nodelete = link_map_nodelete_pending;
-	  else
-	    map->l_nodelete = link_map_nodelete_active;
+	  mark_nodelete (map, flags);
 	}
     }
   ++tab->n_elements;
@@ -586,7 +604,7 @@ add_dependency (struct link_map *undef_map, struct link_map *map, int flags)
      dependencies may pick an dependency which can be dlclose'd, but
      such IFUNC resolvers are undefined anyway.  */
   assert (map->l_type == lt_loaded);
-  if (map->l_nodelete != link_map_nodelete_inactive)
+  if (is_nodelete (map, flags))
     return 0;
 
   struct link_map_reldeps *l_reldeps
@@ -694,17 +712,16 @@ add_dependency (struct link_map *undef_map, struct link_map *map, int flags)
 
       /* 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)
+      if (is_nodelete (map, flags))
 	goto out;
 
       /* If the object with the undefined reference cannot be removed ever
 	 just make sure the same is true for the object which contains the
 	 definition.  */
-      if (undef_map->l_type != lt_loaded
-	  || (undef_map->l_nodelete != link_map_nodelete_inactive))
+      if (undef_map->l_type != lt_loaded || is_nodelete (map, flags))
 	{
 	  if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_BINDINGS)
-	      && map->l_nodelete == link_map_nodelete_inactive)
+	      && !is_nodelete (map, flags))
 	    {
 	      if (undef_map->l_name[0] == '\0')
 		_dl_debug_printf ("\
@@ -716,11 +733,7 @@ marking %s [%lu] as NODELETE due to reference to %s [%lu]\n",
 				  map->l_name, map->l_ns,
 				  undef_map->l_name, undef_map->l_ns);
 	    }
-
-	  if (flags & DL_LOOKUP_FOR_RELOCATE)
-	    map->l_nodelete = link_map_nodelete_pending;
-	  else
-	    map->l_nodelete = link_map_nodelete_active;
+	  mark_nodelete (map, flags);
 	  goto out;
 	}
 
@@ -746,17 +759,14 @@ marking %s [%lu] as NODELETE due to reference to %s [%lu]\n",
 		 cannot be unloaded.  This is semantically the correct
 		 behavior.  */
 	      if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_BINDINGS)
-		  && map->l_nodelete == link_map_nodelete_inactive)
+		  && !is_nodelete (map, flags))
 		_dl_debug_printf ("\
 marking %s [%lu] as NODELETE due to memory allocation failure\n",
 				  map->l_name, map->l_ns);
-	      if (flags & DL_LOOKUP_FOR_RELOCATE)
-		/* In case of non-lazy binding, we could actually
-		   report the memory allocation error, but for now, we
-		   use the conservative approximation as well. */
-		map->l_nodelete = link_map_nodelete_pending;
-	      else
-		map->l_nodelete = link_map_nodelete_active;
+	      /* In case of non-lazy binding, we could actually report
+		 the memory allocation error, but for now, we use the
+		 conservative approximation as well.  */
+	      mark_nodelete (map, flags);
 	      goto out;
 	    }
 	  else

elf/dl-open.c

@@ -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);

elf/get-dynamic-info.h

@@ -164,7 +164,7 @@ elf_get_dynamic_info (struct link_map *l, ElfW(Dyn) *temp)
     {
       l->l_flags_1 = info[VERSYMIDX (DT_FLAGS_1)]->d_un.d_val;
       if (l->l_flags_1 & DF_1_NODELETE)
-	l->l_nodelete = link_map_nodelete_pending;
+	l->l_nodelete_pending = true;
 
       /* Only DT_1_SUPPORTED_MASK bits are supported, and we would like
 	 to assert this, but we can't. Users have been setting

include/link.h

@@ -79,22 +79,6 @@ struct r_search_path_struct
     int malloced;
   };
 
-/* Type used by the l_nodelete member.  */
-enum link_map_nodelete
-{
- /* This link map can be deallocated.  */
- link_map_nodelete_inactive = 0, /* Zero-initialized in _dl_new_object.  */
-
- /* This link map cannot be deallocated.  */
- link_map_nodelete_active,
-
- /* This link map cannot be deallocated after dlopen has succeded.
-    dlopen turns this into link_map_nodelete_active.  dlclose treats
-    this intermediate state as link_map_nodelete_active.  */
- link_map_nodelete_pending,
-};
-
-
 /* Structure describing a loaded shared object.  The `l_next' and `l_prev'
    members form a chain of all the shared objects loaded at startup.
 
@@ -218,10 +202,17 @@ struct link_map
 				       freed, ie. not allocated with
 				       the dummy malloc in ld.so.  */
 
-    /* Actually of type enum link_map_nodelete.  Separate byte due to
-       a read in add_dependency in elf/dl-lookup.c outside the loader
-       lock.  Only valid for l_type == lt_loaded.  */
-    unsigned char l_nodelete;
+    /* NODELETE status of the map.  Only valid for maps of type
+       lt_loaded.  Lazy binding sets l_nodelete_active directly,
+       potentially from signal handlers.  Initial loading of an
+       DF_1_NODELETE object set l_nodelete_pending.  Relocation may
+       set l_nodelete_pending as well.  l_nodelete_pending maps are
+       promoted to l_nodelete_active status in the final stages of
+       dlopen, prior to calling ELF constructors.  dlclose only
+       refuses to unload l_nodelete_active maps, the pending status is
+       ignored.  */
+    bool l_nodelete_active;
+    bool l_nodelete_pending;
 
 #include <link_map.h>