elf: Make more functions available for binding during dlclose (bug 30425)

Previously, after destructors for a DSO have been invoked, ld.so refused
to bind against that DSO in all cases.  Relax this restriction somewhat
if the referencing object is itself a DSO that is being unloaded.  This
assumes that the symbol reference is not going to be stored anywhere.

The situation in the test case can arise fairly easily with C++ and
objects that are built with different optimization levels and therefore
define different functions with vague linkage.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

elf/Makefile

@@ -396,6 +396,7 @@ tests += \
   tst-debug1 \
   tst-deep1 \
   tst-dl-is_dso \
+  tst-dlclose-lazy \
   tst-dlmodcount \
   tst-dlmopen-dlerror \
   tst-dlmopen-gethostbyname \
@@ -816,6 +817,8 @@ modules-names += \
   tst-dl_find_object-mod7 \
   tst-dl_find_object-mod8 \
   tst-dl_find_object-mod9 \
+  tst-dlclose-lazy-mod1 \
+  tst-dlclose-lazy-mod2 \
   tst-dlmopen-dlerror-mod \
   tst-dlmopen-gethostbyname-mod \
   tst-dlmopen-twice-mod1 \
@@ -3001,3 +3004,8 @@ $(objpfx)tst-sprof-basic.out: tst-sprof-basic.sh $(objpfx)tst-sprof-basic
 		 '$(run-program-env)' > $@; \
 	$(evaluate-test)
 generated += tst-sprof-mod.so.profile
+
+LDFLAGS-tst-dlclose-lazy-mod1.so = -Wl,-z,lazy,--no-as-needed
+$(objpfx)tst-dlclose-lazy-mod1.so: $(objpfx)tst-dlclose-lazy-mod2.so
+$(objpfx)tst-dlclose-lazy.out: \
+  $(objpfx)tst-dlclose-lazy-mod1.so $(objpfx)tst-dlclose-lazy-mod2.so

elf/dl-lookup.c

@@ -366,8 +366,25 @@ 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.  */
+      /* Do not look into objects which are going to be removed,
-      if (map->l_removed)
+	 except when the referencing object itself is being removed.
+
+	 The second part covers the situation when an object lazily
+	 binds to another object while running its destructor, but the
+	 destructor of the other object has already run, so that
+	 dlclose has set l_removed.  It may not always be obvious how
+	 to avoid such a scenario to programmers creating DSOs,
+	 particularly if C++ vague linkage is involved and triggers
+	 symbol interposition.
+
+	 Accepting these to-be-removed objects makes the lazy and
+	 BIND_NOW cases more similar.  (With BIND_NOW, the symbol is
+	 resolved early, before the destructor call, so the issue does
+	 not arise.).  Behavior matches the constructor scenario: the
+	 implementation allows binding to symbols of objects whose
+	 constructors have not run.  In fact, not doing this would be
+	 mostly incompatible with symbol interposition.  */
+      if (map->l_removed && !(undef_map != NULL && undef_map->l_removed))
 	continue;
 
       /* Print some debugging info if wanted.  */

elf/tst-dlclose-lazy-mod1.c

@@ -0,0 +1,36 @@
+/* Lazy binding during dlclose.  Directly loaded module.
+   Copyright (C) 2023 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* This function is called from exported_function below.  It is only
+   defined in this module.  The weak attribute mimics how G++
+   implements vague linkage for C++.  */
+void __attribute__ ((weak))
+lazily_bound_exported_function (void)
+{
+}
+
+/* Called from tst-dlclose-lazy-mod2.so.  */
+void
+exported_function (int call_it)
+{
+  if (call_it)
+    /* Previous to the fix this would crash when called during dlclose
+       since symbols from the DSO were no longer available for binding
+       (bug 30425) after the DSO started being closed by dlclose.  */
+    lazily_bound_exported_function ();
+}

elf/tst-dlclose-lazy-mod2.c

@@ -0,0 +1,49 @@
+/* Lazy binding during dlclose.  Indirectly loaded module.
+   Copyright (C) 2023 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+void
+exported_function (int ignored)
+{
+  /* This function is interposed from tst-dlclose-lazy-mod1.so and
+     thus never called.  */
+  abort ();
+}
+
+static void __attribute__ ((constructor))
+init (void)
+{
+  puts ("info: tst-dlclose-lazy-mod2.so constructor called");
+
+  /* Trigger lazy binding to the definition in
+     tst-dlclose-lazy-mod1.so, but not for
+     lazily_bound_exported_function in that module.  */
+  exported_function (0);
+}
+
+static void __attribute__ ((destructor))
+fini (void)
+{
+  puts ("info: tst-dlclose-lazy-mod2.so destructor called");
+
+  /* Trigger the lazily_bound_exported_function call in
+     exported_function in tst-dlclose-lazy-mod1.so.  */
+  exported_function (1);
+}

elf/tst-dlclose-lazy.c

@@ -0,0 +1,47 @@
+/* Test lazy binding during dlclose (bug 30425).
+   Copyright (C) 2023 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* This test re-creates a situation that can arise naturally for C++
+   applications due to the use of vague linkage and differences in the
+   set of compiler-emitted functions.  A function in
+   tst-dlclose-lazy-mod1.so (exported_function) interposes a function
+   in tst-dlclose-lazy-mod2.so.  This function is called from the
+   destructor in tst-dlclose-lazy-mod2.so, after the destructor for
+   tst-dlclose-lazy-mod1.so has already completed.  Prior to the fix
+   for bug 30425, this would lead to a lazy binding failure in
+   tst-dlclose-lazy-mod1.so because dlclose had already marked the DSO
+   as unavailable for binding (by setting l_removed).  */
+
+#include <dlfcn.h>
+#include <support/xdlfcn.h>
+#include <support/check.h>
+
+int
+main (void)
+{
+  /* Load tst-dlclose-lazy-mod1.so, indirectly loading
+     tst-dlclose-lazy-mod2.so.  */
+  void *handle = xdlopen ("tst-dlclose-lazy-mod1.so", RTLD_GLOBAL | RTLD_LAZY);
+
+  /* Invoke the destructor of tst-dlclose-lazy-mod2.so, which calls
+     into tst-dlclose-lazy-mod1.so after its destructor has been
+     called.  */
+  xdlclose (handle);
+
+  return 0;
+}