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, 15 insertions, 0 deletions

diff --git a/elf/rtld-debugger-interface.txt b/elf/rtld-debugger-interface.txt
index 61bc99e4b0..f3476d8308 100644
--- a/elf/rtld-debugger-interface.txt
+++ b/elf/rtld-debugger-interface.txt
@@ -9,6 +9,9 @@ structure can be found.
 
 The r_debug structure contains (amongst others) the following fields:
 
+  int r_version:
+    Version number for this protocol.  It should be greater than 0.
+
   struct link_map *r_map:
     A linked list of loaded objects.
 
@@ -32,6 +35,18 @@ but there is no way for the debugger to discover whether any of the
 objects in the link-map have been relocated or not.
 
 
+Extension to the r_debug structure
+==================================
+
+The r_debug_extended structure is an extension of the r_debug interface.
+If r_version is 2, one additional field is available:
+
+  struct r_debug_extended *r_next;
+    Link to the next r_debug_extended structure.  Each r_debug_extended
+    structure represents a different namespace.  A namespace is active
+    if its r_map field isn't NULL.  The first r_debug_extended structure
+    is for the default namespace.
+
 Probe-based debugger interface
 ==============================