Extend struct r_debug to support multiple namespaces [BZ #15971]

Glibc does not provide an interface for debugger to access libraries
loaded in multiple namespaces via dlmopen.

The current rtld-debugger interface is described in the file:

elf/rtld-debugger-interface.txt

under the "Standard debugger interface" heading.  This interface only
provides access to the first link-map (LM_ID_BASE).

1. Bump r_version to 2 when multiple namespaces are used.  This triggers
the GDB bug:

https://sourceware.org/bugzilla/show_bug.cgi?id=28236

2. Add struct r_debug_extended to extend struct r_debug into a linked-list,
where each element correlates to an unique namespace.
3. Initialize the r_debug_extended structure.  Bump r_version to 2 for
the new namespace and add the new namespace to the namespace linked list.
4. Add _dl_debug_update to return the address of struct r_debug' of a
namespace.
5. Add a hidden symbol, _r_debug_extended, for struct r_debug_extended.
6. Provide the symbol, _r_debug, with size of struct r_debug, as an alias
of _r_debug_extended, for programs which reference _r_debug.

This fixes BZ #15971.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

1 files changed, 72 insertions, 0 deletions

diff --git a/elf/tst-dlmopen4.c b/elf/tst-dlmopen4.c
new file mode 100644
index 0000000000..3fe150e50b
--- /dev/null
+++ b/elf/tst-dlmopen4.c
@@ -0,0 +1,72 @@
+/* Test struct r_debug_extended via DT_DEBUG.
+   Copyright (C) 2021 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 <link.h>
+#include <stdlib.h>
+#include <string.h>
+#include <gnu/lib-names.h>
+#include <support/xdlfcn.h>
+#include <support/check.h>
+#include <support/test-driver.h>
+
+#ifndef ELF_MACHINE_GET_R_DEBUG
+# define ELF_MACHINE_GET_R_DEBUG(d) \
+    (__extension__ ({ 						\
+      struct r_debug_extended *debug;				\
+      if ((d)->d_tag == DT_DEBUG)				\
+	debug = (struct r_debug_extended *) (d)->d_un.d_ptr;	\
+      else							\
+	debug = NULL;						\
+      debug; }))
+#endif
+
+static int
+do_test (void)
+{
+  ElfW(Dyn) *d;
+  struct r_debug_extended *debug = NULL;
+
+  for (d = _DYNAMIC; d->d_tag != DT_NULL; ++d)
+    {
+      debug = ELF_MACHINE_GET_R_DEBUG (d);
+      if (debug != NULL)
+	break;
+    }
+
+  TEST_VERIFY_EXIT (debug != NULL);
+  TEST_COMPARE (debug->base.r_version, 1);
+  TEST_VERIFY_EXIT (debug->r_next == NULL);
+
+  void *h = xdlmopen (LM_ID_NEWLM, "$ORIGIN/tst-dlmopen1mod.so",
+		      RTLD_LAZY);
+
+  TEST_COMPARE (debug->base.r_version, 2);
+  TEST_VERIFY_EXIT (debug->r_next != NULL);
+  TEST_VERIFY_EXIT (debug->r_next->r_next == NULL);
+  TEST_VERIFY_EXIT (debug->r_next->base.r_map != NULL);
+  TEST_VERIFY_EXIT (debug->r_next->base.r_map->l_name != NULL);
+  const char *name = basename (debug->r_next->base.r_map->l_name);
+  TEST_COMPARE_STRING (name, "tst-dlmopen1mod.so");
+
+  xdlclose (h);
+
+  return 0;
+}
+
+#include <support/test-driver.c>