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, 36 insertions, 0 deletions

diff --git a/elf/dl-debug-symbols.S b/elf/dl-debug-symbols.S
new file mode 100644
index 0000000000..b7e9f5d947
--- /dev/null
+++ b/elf/dl-debug-symbols.S
@@ -0,0 +1,36 @@
+/* Define symbols used to communicate dynamic linker state to the
+   debugger at runtime.
+   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 <rtld-sizes.h>
+
+/* Define 2 symbols, _r_debug_extended and _r_debug, which is an alias
+   of _r_debug_extended, but with the size of struct r_debug.  */
+
+	.globl	_r_debug
+	.type	_r_debug, %object
+	.size	_r_debug, R_DEBUG_SIZE
+	.hidden	_r_debug_extended
+	.globl	_r_debug_extended
+	.type	_r_debug_extended, %object
+	.size	_r_debug_extended, R_DEBUG_EXTENDED_SIZE
+	.section .bss
+	.balign	R_DEBUG_EXTENDED_ALIGN
+_r_debug:
+_r_debug_extended:
+	.zero	R_DEBUG_EXTENDED_SIZE