[RFC PATCH 3/3] add r_debug multiple namespaces support

[Daniel Walker via Libc-alpha <libc-alpha@sourceware.org>]

From: Conan C Huang <conhuang@cisco.com>

GLIBC Bugzilla: 15971
GDB Bugzilla: 11839

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).

This solution converts r_debug into a linked-list, where each element correlates
to an unique namespace. Debugger (GDB) can traverse r_debug linked-list to
retrieve information for all loaded namespaces.
---
 elf/dl-debug.c | 13 ++++++++++---
 elf/link.h     |  4 ++++
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/elf/dl-debug.c b/elf/dl-debug.c
index 4b3d3ad6ba..7eba477fd3 100644
--- a/elf/dl-debug.c
+++ b/elf/dl-debug.c
@@ -44,20 +44,27 @@ struct r_debug _r_debug;
 struct r_debug *
 _dl_debug_initialize (ElfW(Addr) ldbase, Lmid_t ns)
 {
-  struct r_debug *r;
+  struct r_debug *r, *rp;
 
   if (ns == LM_ID_BASE)
     r = &_r_debug;
   else
-    r = &GL(dl_ns)[ns]._ns_debug;
+    {
+      r = &GL(dl_ns)[ns]._ns_debug;
+      rp = &GL(dl_ns)[ns - 1]._ns_debug;
+      rp->next = r;
+      if (ns - 1 == LM_ID_BASE)
+        _r_debug.next = r;
+    }
 
   if (r->r_map == NULL || ldbase != 0)
     {
       /* Tell the debugger where to find the map of loaded objects.  */
-      r->r_version = 1	/* R_DEBUG_VERSION XXX */;
+      r->r_version = 3  /* R_DEBUG_VERSION XXX */;
       r->r_ldbase = ldbase ?: _r_debug.r_ldbase;
       r->r_map = (void *) GL(dl_ns)[ns]._ns_loaded;
       r->r_brk = (ElfW(Addr)) &_dl_debug_state;
+      r->next = NULL;
     }
 
   return r;
diff --git a/elf/link.h b/elf/link.h
index 0048ad5d4d..5a42511636 100644
--- a/elf/link.h
+++ b/elf/link.h
@@ -61,6 +61,10 @@ struct r_debug
       } r_state;
 
     ElfW(Addr) r_ldbase;	/* Base address the linker is loaded at.  */
+
+    /* Link to next r_debug struct. Each r_debug struct represents a
+       different namespace.  The first r_debug struct is the default.  */
+    struct r_debug *next;
   };
 
 /* This is the instance of that structure used by the dynamic linker.  */
-- 
2.17.1