commit f8ed116aa574435c6e28260f21963233682d3b57

Author: Florian Weimer <fweimer@redhat.com>

Date:   Fri Dec 13 10:18:46 2019 +0100

    dlopen: Rework handling of pending NODELETE status

    Commit a2e8aa0d9ea648068d8be52dd7b15f1b6a008e23 ("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>

diff --git a/elf/dl-close.c b/elf/dl-close.c

index 243a028c443173c1..fa7f3e8174576e46 100644

--- a/elf/dl-close.c

+++ b/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

@@ -820,7 +819,7 @@ _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));

diff --git a/elf/dl-lookup.c b/elf/dl-lookup.c

index 35a3f96a6296294a..01724a54f8840f9f 100644

--- a/elf/dl-lookup.c

+++ b/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

diff --git a/elf/dl-open.c b/elf/dl-open.c

index c7ed85b7ee99a296..a382bfae8aa3a2f8 100644

--- a/elf/dl-open.c

+++ b/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,

@@ -680,7 +684,7 @@ 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

@@ -879,9 +883,9 @@ 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);

diff --git a/elf/get-dynamic-info.h b/elf/get-dynamic-info.h

index ea286abaea0128d1..78ba7e76db9706cc 100644

--- a/elf/get-dynamic-info.h

+++ b/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

diff --git a/include/link.h b/include/link.h

index a277b77cad6b52b1..e90fa79a0b332087 100644

--- a/include/link.h

+++ b/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>