[PATCH] elf: Assert range of ns argument in _dl_debug_initialize

[Florian Weimer <fweimer@redhat.com>]

This does not fix any bugs as such, but makes it more obvious
if _dl_debug_initialize is called with invalid arguments
(which would otherwise cause the function to clobber unrelated
data).

Tested on i686-linux-gnu and x86_64-linux-gnu.

---
 elf/dl-debug.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/elf/dl-debug.c b/elf/dl-debug.c
index 2cd5f09753..85b087455e 100644
--- a/elf/dl-debug.c
+++ b/elf/dl-debug.c
@@ -16,6 +16,8 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <array_length.h>
+#include <assert.h>
 #include <ldsodefs.h>
 
 
@@ -49,7 +51,11 @@ _dl_debug_initialize (ElfW(Addr) ldbase, Lmid_t ns)
   if (ns == LM_ID_BASE)
     r = &_r_debug;
   else
-    r = &GL(dl_ns)[ns]._ns_debug;
+    {
+      assert (ns >= 0);
+      assert (ns < array_length (GL (dl_ns)));
+      r = &GL(dl_ns)[ns]._ns_debug;
+    }
 
   if (r->r_map == NULL || ldbase != 0)
     {