nss: Protect against errno changes in function lookup (bug 28953)

dlopen may clobber errno.  The nss_test_errno module uses an ELF
constructor to achieve that, but there could be internal errors
during dlopen that cause this, too.  Therefore, the NSS framework
has to guard against such errno clobbers.

__nss_module_get_function is currently the only function that calls
__nss_module_load, so it is sufficient to save and restore errno
around this call.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

nss/nss_module.c

@@ -330,8 +330,18 @@ name_search (const void *left, const void *right)
 void *
 __nss_module_get_function (struct nss_module *module, const char *name)
 {
+  /* A successful dlopen might clobber errno.   */
+  int saved_errno = errno;
+
   if (!__nss_module_load (module))
-    return NULL;
+    {
+      /* Reporting module load failure is currently inaccurate.  See
+	 bug 22041.  Not changing errno is the conservative choice.  */
+      __set_errno (saved_errno);
+      return NULL;
+    }
+
+  __set_errno (saved_errno);
 
   function_name *name_entry = bsearch (name, nss_function_name_array,
                                        array_length (nss_function_name_array),