elf: Second ld.so relocation only if libc.so has been loaded

Commit 8f8dd90 (“elf:
rtld_multiple_ref is always true”) removed some code that happened
to enable compatibility with programs that do not link against
libc.so.  Such programs cannot call dlopen or any dynamic linker
functions (except __tls_get_addr), so this is not really useful.
Still ld.so should not crash with a null-pointer dereference
or undefined symbol reference in these cases.

In the main relocation loop, call _dl_relocate_object unconditionally
because it already checks if the object has been relocated.

If libc.so was loaded, self-relocate ld.so against it and call
__rtld_mutex_init and __rtld_malloc_init_real to activate the full
implementations.  Those are available only if libc.so is there,
so skip these initialization steps if libc.so is absent.  Without
libc.so, the global scope can be completely empty.  This can cause
ld.so self-relocation to fail because if it uses symbol-based
relocations, which is why the second ld.so self-relocation is not
performed if libc.so is missing.

The previous concern regarding GOT updates through self-relocation
no longer applies because function pointers are updated
explicitly through __rtld_mutex_init and __rtld_malloc_init_real,
and not through relocation.  However, the second ld.so self-relocation
is still delayed, in case there are other symbols being used.

Fixes commit 8f8dd90 (“elf:
rtld_multiple_ref is always true”).

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

elf/Makefile

@@ -3372,3 +3372,20 @@ endef
 $(foreach m,$(modules-semantic-interposition),\
   $(eval $(call enable-semantic-interposition,$(m))))
 endif
+
+# These rules link and run the special elf/tst-nolink-libc-* tests if
+# a port adds them to the tests variables.  Neither test variant is
+# linked against libc.so, but tst-nolink-libc-1 is linked against
+# ld.so.  The test is always run directly, not under the dynamic
+# linker.
+CFLAGS-tst-nolink-libc.c += $(no-stack-protector)
+$(objpfx)tst-nolink-libc-1: $(objpfx)tst-nolink-libc.o $(objpfx)ld.so
+	$(LINK.o) -nostdlib -nostartfiles -o $@ $< \
+	  -Wl,--dynamic-linker=$(objpfx)ld.so,--no-as-needed $(objpfx)ld.so
+$(objpfx)tst-nolink-libc-1.out: $(objpfx)tst-nolink-libc-1 $(objpfx)ld.so
+	$< > $@ 2>&1; $(evaluate-test)
+$(objpfx)tst-nolink-libc-2: $(objpfx)tst-nolink-libc.o
+	$(LINK.o) -nostdlib -nostartfiles -o $@ $< \
+	  -Wl,--dynamic-linker=$(objpfx)ld.so
+$(objpfx)tst-nolink-libc-2.out: $(objpfx)tst-nolink-libc-2 $(objpfx)ld.so
+	$< > $@ 2>&1; $(evaluate-test)

elf/rtld.c

@@ -2242,25 +2242,25 @@ dl_main (const ElfW(Phdr) *phdr,
 
   _rtld_main_check (main_map, _dl_argv[0]);
 
-  /* Now we have all the objects loaded.  Relocate them all except for
-     the dynamic linker itself.  We do this in reverse order so that copy
-     relocs of earlier objects overwrite the data written by later
-     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 relocations.
-     Likewise for libc, which is relocated early to ensure that IFUNC
-     resolvers in libc work.  */
+  /* Now we have all the objects loaded.  */
 
   int consider_profiling = GLRO(dl_profile) != NULL;
 
   /* If we are profiling we also must do lazy reloaction.  */
   GLRO(dl_lazy) |= consider_profiling;
 
+  /* If libc.so has been loaded, relocate it early, after the dynamic
+     loader itself.  The initial self-relocation of ld.so should be
+     sufficient for IFUNC resolvers in libc.so.  */
   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);
+    {
+      RTLD_TIMING_VAR (start);
+      rtld_timer_start (&start);
+      _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);
+      rtld_timer_accum (&relocate_time, start);
+  }
 
   RTLD_TIMING_VAR (start);
   rtld_timer_start (&start);
@@ -2283,9 +2283,8 @@ dl_main (const ElfW(Phdr) *phdr,
 	/* Also allocated with the fake malloc().  */
 	l->l_free_initfini = 0;
 
-	if (l != &_dl_rtld_map)
-	  _dl_relocate_object (l, l->l_scope, GLRO(dl_lazy) ? RTLD_LAZY : 0,
-			       consider_profiling);
+	_dl_relocate_object (l, l->l_scope, GLRO(dl_lazy) ? RTLD_LAZY : 0,
+			     consider_profiling);
 
 	/* Add object to slot information data if necessasy.  */
 	if (l->l_tls_blocksize != 0 && __rtld_tls_init_tp_called)
@@ -2323,27 +2322,22 @@ dl_main (const ElfW(Phdr) *phdr,
   /* Set up the object lookup structures.  */
   _dl_find_object_init ();
 
-  /* Likewise for the locking implementation.  */
-  __rtld_mutex_init ();
-
-  /* Re-relocate ourselves with user-controlled symbol definitions.  */
-
-  {
-    RTLD_TIMING_VAR (start);
-    rtld_timer_start (&start);
-
-    _dl_relocate_object_no_relro (&_dl_rtld_map, main_map->l_scope, 0, 0);
-
-    /* The malloc implementation has been relocated, so resolving
-       its symbols (and potentially calling IFUNC resolvers) is safe
-       at this point.  */
-    __rtld_malloc_init_real (main_map);
+  /* If libc.so was loaded, relocate ld.so against it.  Complete ld.so
+     initialization with mutex symbols from libc.so and malloc symbols
+     from the global scope.  */
+  if (GL(dl_ns)[LM_ID_BASE].libc_map != NULL)
+    {
+      RTLD_TIMING_VAR (start);
+      rtld_timer_start (&start);
+      _dl_relocate_object_no_relro (&_dl_rtld_map, main_map->l_scope, 0, 0);
+      rtld_timer_accum (&relocate_time, start);
 
-    if (_dl_rtld_map.l_relro_size != 0)
-      _dl_protect_relro (&_dl_rtld_map);
+      __rtld_mutex_init ();
+      __rtld_malloc_init_real (main_map);
+    }
 
-    rtld_timer_accum (&relocate_time, start);
-  }
+  /* All ld.so initialization is complete.  Apply RELRO.  */
+  _dl_protect_relro (&_dl_rtld_map);
 
   /* Relocation is complete.  Perform early libc initialization.  This
      is the initial libc, even if audit modules have been loaded with

sysdeps/unix/sysv/linux/Makefile

@@ -652,7 +652,15 @@ install-bin += \
   # install-bin
 
 $(objpfx)pldd: $(objpfx)xmalloc.o
+
+test-internal-extras += tst-nolink-libc
+ifeq ($(run-built-tests),yes)
+tests-special += \
+  $(objpfx)tst-nolink-libc-1.out \
+  $(objpfx)tst-nolink-libc-2.out \
+  # tests-special
 endif
+endif # $(subdir) == elf
 
 ifeq ($(subdir),rt)
 CFLAGS-mq_send.c += -fexceptions

sysdeps/unix/sysv/linux/arm/Makefile

@@ -1,5 +1,8 @@
 ifeq ($(subdir),elf)
 sysdep-rtld-routines += aeabi_read_tp libc-do-syscall
+# The test uses INTERNAL_SYSCALL_CALL.  In thumb mode, this uses
+# an undefined reference to __libc_do_syscall.
+CFLAGS-tst-nolink-libc.c += -marm
 endif
 
 ifeq ($(subdir),misc)

sysdeps/unix/sysv/linux/tst-nolink-libc.c

@@ -0,0 +1,25 @@
+/* Test program not linked against libc.so and not using any glibc functions.
+   Copyright (C) 2024 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 <sysdep.h>
+
+void
+_start (void)
+{
+  INTERNAL_SYSCALL_CALL (exit_group, 0);
+}