elf: Do not completely clear reused namespace in dlmopen (bug 29600)

The data in the _ns_debug member must be preserved, otherwise
_dl_debug_initialize enters an infinite loop.  To be conservative,
only clear the libc_map member for now, to fix bug 29528.

Fixes commit d0e357ff45a75553dee3b17ed7d303bfa544f6fe
("elf: Call __libc_early_init for reused namespaces (bug 29528)"),
by reverting most of it.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>
(cherry picked from commit 2c42257314536b94cc8d52edede86e94e98c1436)

3 files changed, 31 insertions, 12 deletions

diff --git a/NEWS b/NEWS
index de775ab116..a6da588c85 100644
--- a/NEWS
+++ b/NEWS
@@ -37,6 +37,7 @@ The following bugs are resolved with this release:
   [29537] libc: [2.34 regression]: Alignment issue on m68k when using
   [29539] libc: LD_TRACE_LOADED_OBJECTS changed how vDSO library are
   [29583] Use 64-bit interfaces in gconv_parseconfdir
+  [29600] Do not completely clear reused namespace in dlmopen
   [29607] nscd repeatably crashes calling __strlen_avx2 when hosts cache is
     enabled
   [29638] libc: stdlib: arc4random fallback is never used
diff --git a/elf/dl-open.c b/elf/dl-open.c
index 46e8066fd8..e7db5e9642 100644
--- a/elf/dl-open.c
+++ b/elf/dl-open.c
@@ -844,15 +844,13 @@ _dl_open (const char *file, int mode, const void *caller_dlopen, Lmid_t nsid,
 	  _dl_signal_error (EINVAL, file, NULL, N_("\
 no more namespaces available for dlmopen()"));
 	}
+      else if (nsid == GL(dl_nns))
+	{
+	  __rtld_lock_initialize (GL(dl_ns)[nsid]._ns_unique_sym_table.lock);
+	  ++GL(dl_nns);
+	}
 
-      if (nsid == GL(dl_nns))
-	++GL(dl_nns);
-
-      /* Initialize the new namespace.  Most members are
-	 zero-initialized, only the lock needs special treatment.  */
-      memset (&GL(dl_ns)[nsid], 0, sizeof (GL(dl_ns)[nsid]));
-      __rtld_lock_initialize (GL(dl_ns)[nsid]._ns_unique_sym_table.lock);
-
+      GL(dl_ns)[nsid].libc_map = NULL;
       _dl_debug_update (nsid)->r_state = RT_CONSISTENT;
     }
   /* Never allow loading a DSO in a namespace which is empty.  Such
diff --git a/elf/tst-dlmopen-twice.c b/elf/tst-dlmopen-twice.c
index 449f3c8fa9..70c71fe19c 100644
--- a/elf/tst-dlmopen-twice.c
+++ b/elf/tst-dlmopen-twice.c
@@ -16,18 +16,38 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#include <support/xdlfcn.h>
+#include <stdio.h>
 #include <support/check.h>
+#include <support/xdlfcn.h>
 
-static int
-do_test (void)
+/* Run the test multiple times, to check finding a new namespace while
+   another namespace is already in use.  This used to trigger bug 29600.  */
+static void
+recurse (int depth)
 {
-  void *handle = xdlmopen (LM_ID_NEWLM, "tst-dlmopen-twice-mod1.so", RTLD_NOW);
+  if (depth == 0)
+    return;
+
+  printf ("info: running at depth %d\n", depth);
+  void *handle = xdlmopen (LM_ID_NEWLM, "tst-dlmopen-twice-mod1.so",
+                           RTLD_NOW);
   xdlclose (handle);
   handle = xdlmopen (LM_ID_NEWLM, "tst-dlmopen-twice-mod2.so", RTLD_NOW);
   int (*run_check) (void) = xdlsym (handle, "run_check");
   TEST_COMPARE (run_check (), 0);
+  recurse (depth - 1);
   xdlclose (handle);
+}
+
+static int
+do_test (void)
+{
+  /* First run the test without nesting.  */
+  recurse (1);
+
+  /* Then with nesting.  The constant needs to be less than the
+     internal DL_NNS namespace constant.  */
+  recurse (10);
   return 0;
 }