elf: Relocate libc.so early during startup and dlmopen (bug 31083)

This makes it more likely that objects without dependencies can
use IFUNC resolvers in libc.so.

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

elf/Makefile

@@ -433,6 +433,8 @@ tests += \
   tst-nodelete-dlclose \
   tst-nodelete-opened \
   tst-nodelete2 \
+  tst-nodeps1 \
+  tst-nodeps2 \
   tst-noload \
   tst-non-directory-path \
   tst-null-argv \
@@ -863,6 +865,8 @@ modules-names += \
   tst-nodelete-dlclose-plugin \
   tst-nodelete-opened-lib \
   tst-nodelete2mod \
+  tst-nodeps1-mod \
+  tst-nodeps2-mod \
   tst-non-directory-mod \
   tst-null-argv-lib \
   tst-p_alignmod-base \
@@ -1030,6 +1034,8 @@ modules-names-nobuild += \
   tst-audit24bmod1 \
   tst-audit24bmod2 \
   tst-big-note-lib \
+  tst-nodeps1-mod \
+  tst-nodeps2-mod \
   tst-ro-dynamic-mod \
   # modules-names-nobuild
 
@@ -3009,3 +3015,18 @@ tst-env-setuid-ARGS = -- $(host-test-program-cmd)
 
 # Reuse a module with a SONAME, to specific as the LD_PROFILE.
 $(objpfx)tst-env-setuid: $(objpfx)tst-sonamemove-runmod2.so
+
+# The object tst-nodeps1-mod.so has no explicit dependencies on libc.so.
+$(objpfx)tst-nodeps1-mod.so: $(objpfx)tst-nodeps1-mod.os
+	$(LINK.o) -nostartfiles -nostdlib -shared -o $@ $^
+tst-nodeps1.so-no-z-defs = yes
+# Link libc.so before the test module with the IFUNC resolver reference.
+LDFLAGS-tst-nodeps1 = $(common-objpfx)libc.so $(objpfx)tst-nodeps1-mod.so
+$(objpfx)tst-nodeps1: $(objpfx)tst-nodeps1-mod.so
+# Reuse the tst-nodeps1 module.  Link libc.so before the test module
+# with the IFUNC resolver reference.
+$(objpfx)tst-nodeps2-mod.so: $(common-objpfx)libc.so \
+  $(objpfx)tst-nodeps1-mod.so $(objpfx)tst-nodeps2-mod.os
+	$(LINK.o) -Wl,--no-as-needed -nostartfiles -nostdlib -shared -o $@ $^
+$(objpfx)tst-nodeps2.out: \
+  $(objpfx)tst-nodeps1-mod.so $(objpfx)tst-nodeps2-mod.so

elf/dl-open.c

@@ -708,6 +708,17 @@ dl_open_worker_begin (void *a)
      them.  However, such relocation dependencies in IFUNC resolvers
      are undefined anyway, so this is not a problem.  */
 
+  /* Ensure that libc is relocated first.  This helps with the
+     execution of IFUNC resolvers in libc, and matters only to newly
+     created dlmopen namespaces.  Do not do this for static dlopen
+     because libc has relocations against ld.so, which may not have
+     been relocated at this point.  */
+#ifdef SHARED
+  if (GL(dl_ns)[args->nsid].libc_map != NULL)
+    _dl_open_relocate_one_object (args, r, GL(dl_ns)[args->nsid].libc_map,
+				  reloc_mode, &relocation_in_progress);
+#endif
+
   for (unsigned int i = last; i-- > first; )
     _dl_open_relocate_one_object (args, r, new->l_initfini[i], reloc_mode,
 				  &relocation_in_progress);

elf/rtld.c

@@ -2272,11 +2272,17 @@ dl_main (const ElfW(Phdr) *phdr,
      objects.  We do not re-relocate the dynamic linker itself in this
      loop because that could result in the GOT entries for functions we
      call being changed, and that would break us.  It is safe to relocate
-     the dynamic linker out of order because it has no copy relocs (we
+     the dynamic linker out of order because it has no copy relocations.
-     know that because it is self-contained).  */
+     Likewise for libc, which is relocated early to ensure that IFUNC
+     resolvers in libc work.  */
 
   int consider_profiling = GLRO(dl_profile) != NULL;
 
+  if (GL(dl_ns)[LM_ID_BASE].libc_map != NULL)
+    _dl_relocate_object (GL(dl_ns)[LM_ID_BASE].libc_map,
+			 GL(dl_ns)[LM_ID_BASE].libc_map->l_scope,
+			 GLRO(dl_lazy) ? RTLD_LAZY : 0, consider_profiling);
+
   /* If we are profiling we also must do lazy reloaction.  */
   GLRO(dl_lazy) |= consider_profiling;
 

elf/tst-nodeps1-mod.c

@@ -0,0 +1,25 @@
+/* Test module with no libc.so dependency and string function references.
+   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 <string.h>
+
+/* Some references to libc symbols which are likely to have IFUNC
+   resolvers.  If they do not, this module does not exercise bug 31083.  */
+void *memcpy_pointer = memcpy;
+void *memmove_pointer = memmove;
+void *memset_pointer = memset;

elf/tst-nodeps1.c

@@ -0,0 +1,23 @@
+/* Test initially loaded module with implicit libc.so dependency (bug 31083).
+   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/>.  */
+
+/* Testing happens before main.  */
+int
+main (void)
+{
+}

elf/tst-nodeps2-mod.c

@@ -0,0 +1 @@
+/* Empty test module which depends on tst-nodeps1-mod.so.  */

elf/tst-nodeps2.c

@@ -0,0 +1,29 @@
+/* Test dlmopen with implicit libc.so dependency (bug 31083).
+   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 <support/xdlfcn.h>
+
+static int
+do_test (void)
+{
+  void *handle = xdlmopen (LM_ID_NEWLM, "tst-nodeps2-mod.so", RTLD_NOW);
+  xdlclose (handle);
+  return 0;
+}
+
+#include <support/test-driver.c>