[Bug dynamic-link/30425] New: Symbol lookup during dlclose may fail unnecessarily

["fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30425

            Bug ID: 30425
           Summary: Symbol lookup during dlclose may fail unnecessarily
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: dynamic-link
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---

Once an object is under removal, all its symbols become unavailable for symbol
resolution, due to this check in do_lookup_x:

      /* Do not look into objects which are going to be removed.  */
      if (map->l_removed)
        continue;

This can cause mysterious crashes during dlclose, particularly if function
symbols are weak due to GCC's C++ vague linkage implementation.

This (admittedly silly) patch fixes the downstream reproducer, but we should
add a C test case for this as well.

diff --git a/elf/dl-lookup.c b/elf/dl-lookup.c
index 05f36a2507..ecc1896c9d 100644
--- a/elf/dl-lookup.c
+++ b/elf/dl-lookup.c
@@ -366,8 +366,10 @@ do_lookup_x (const char *undef_name, unsigned int
new_hash,
       if ((type_class & ELF_RTYPE_CLASS_COPY) && map->l_type == lt_executable)
        continue;

-      /* Do not look into objects which are going to be removed.  */
-      if (map->l_removed)
+      /* Do not look into objects which are going to be removed.
+        References from a map that is under removal are allowed, to
+        enable lazy binding during dlclose.  */
+      if (map->l_removed && !undef_map->l_removed)
        continue;

       /* Print some debugging info if wanted.  */

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