nss: Directly load nss_dns, without going through dlsym/dlopen

This partially fixes static-only NSS support (bug 27959): The dns
module no longer needs dlopen.  Support for disabling dlopen altogher
remains to be added.

This commit introduces module_load_builtin into nss/nss_module.c, which
handles the common parts of loading the built-in nss_files and nss_dns
modules.

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

include/nss_dns.h

@@ -24,13 +24,16 @@
 NSS_DECLARE_MODULE_FUNCTIONS (dns)
 
 libc_hidden_proto (_nss_dns_getcanonname_r)
-libc_hidden_proto (_nss_dns_gethostbyname3_r)
-libc_hidden_proto (_nss_dns_gethostbyname2_r)
-libc_hidden_proto (_nss_dns_gethostbyname_r)
-libc_hidden_proto (_nss_dns_gethostbyname4_r)
 libc_hidden_proto (_nss_dns_gethostbyaddr2_r)
 libc_hidden_proto (_nss_dns_gethostbyaddr_r)
-libc_hidden_proto (_nss_dns_getnetbyname_r)
+libc_hidden_proto (_nss_dns_gethostbyname2_r)
+libc_hidden_proto (_nss_dns_gethostbyname3_r)
+libc_hidden_proto (_nss_dns_gethostbyname4_r)
+libc_hidden_proto (_nss_dns_gethostbyname_r)
 libc_hidden_proto (_nss_dns_getnetbyaddr_r)
+libc_hidden_proto (_nss_dns_getnetbyname_r)
+
+void __nss_dns_functions (nss_module_functions_untyped pointers)
+  attribute_hidden;
 
 #endif

nss/nss_files_functions.c

@@ -34,10 +34,4 @@ __nss_files_functions (nss_module_functions_untyped pointers)
 #undef DEFINE_NSS_FUNCTION
 #define DEFINE_NSS_FUNCTION(x) *fptr++ = _nss_files_##x;
 #include "function.def"
-
-#ifdef PTR_MANGLE
-  void **end = fptr;
-  for (fptr = pointers; fptr != end; ++fptr)
-    PTR_MANGLE (*fptr);
-#endif
 }

nss/nss_module.c

@@ -26,11 +26,13 @@
 #include <dlfcn.h>
 #include <gnu/lib-names.h>
 #include <libc-lock.h>
+#include <nss_dns.h>
+#include <nss_files.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <nss_files.h>
+#include <sysdep.h>
 
 /* Suffix after .so of NSS service modules.  This is a bit of magic,
    but we assume LIBNSS_FILES_SO looks like "libnss_files.so.2" and we
@@ -111,6 +113,39 @@ static const function_name nss_function_name_array[] =
 #include "function.def"
   };
 
+/* Loads a built-in module, binding the symbols using the supplied
+   callback function.  Always returns true.  */
+static bool
+module_load_builtin (struct nss_module *module,
+		     void (*bind) (nss_module_functions_untyped))
+{
+  /* Initialize the function pointers, following the double-checked
+     locking idiom.  */
+  __libc_lock_lock (nss_module_list_lock);
+  switch ((enum nss_module_state) atomic_load_acquire (&module->state))
+    {
+    case nss_module_uninitialized:
+    case nss_module_failed:
+      bind (module->functions.untyped);
+
+#ifdef PTR_MANGLE
+      for (int i = 0; i < nss_module_functions_count; ++i)
+	PTR_MANGLE (module->functions.untyped[i]);
+#endif
+
+      module->handle = NULL;
+      /* Synchronizes with unlocked __nss_module_load atomic_load_acquire.  */
+      atomic_store_release (&module->state, nss_module_loaded);
+      break;
+    case nss_module_loaded:
+      /* Nothing to clean up.  */
+      break;
+    }
+  __libc_lock_unlock (nss_module_list_lock);
+  return true;
+}
+
+/* Loads the built-in nss_files module.  */
 static bool
 module_load_nss_files (struct nss_module *module)
 {
@@ -124,25 +159,14 @@ module_load_nss_files (struct nss_module *module)
       _nss_files_init (cb);
     }
 #endif
+  return module_load_builtin (module, __nss_files_functions);
+}
 
-  /* Initialize the function pointers, following the double-checked
-     locking idiom.  */
-  __libc_lock_lock (nss_module_list_lock);
-  switch ((enum nss_module_state) atomic_load_acquire (&module->state))
-    {
-    case nss_module_uninitialized:
-    case nss_module_failed:
-      __nss_files_functions (module->functions.untyped);
-      module->handle = NULL;
-      /* Synchronizes with unlocked __nss_module_load atomic_load_acquire.  */
-      atomic_store_release (&module->state, nss_module_loaded);
-      break;
-    case nss_module_loaded:
-      /* Nothing to clean up.  */
-      break;
-    }
-  __libc_lock_unlock (nss_module_list_lock);
-  return true;
+/* Loads the built-in nss_dns module.  */
+static bool
+module_load_nss_dns (struct nss_module *module)
+{
+  return module_load_builtin (module, __nss_dns_functions);
 }
 
 /* Internal implementation of __nss_module_load.  */
@@ -151,6 +175,8 @@ module_load (struct nss_module *module)
 {
   if (strcmp (module->name, "files") == 0)
     return module_load_nss_files (module);
+  if (strcmp (module->name, "dns") == 0)
+    return module_load_nss_dns (module);
 
   void *handle;
   {
@@ -398,7 +424,9 @@ __nss_module_freeres (void)
   struct nss_module *current = nss_module_list;
   while (current != NULL)
     {
-      if (current->state == nss_module_loaded && current->handle != NULL)
+      /* Ignore built-in modules (which have a NULL handle).  */
+      if (current->state == nss_module_loaded
+	  && current->handle != NULL)
         __libc_dlclose (current->handle);
 
       struct nss_module *next = current->next;

nss/nss_module.h

@@ -33,10 +33,16 @@ struct nss_module_functions
 #include "function.def"
 };
 
+/* Number of elements of the nss_module_functions_untyped array.  */
+enum
+  {
+    nss_module_functions_count = (sizeof (struct nss_module_functions)
+                                  / sizeof (void *))
+  };
+
 /* Untyped version of struct nss_module_functions, for consistent
    processing purposes.  */
-typedef void *nss_module_functions_untyped[sizeof (struct nss_module_functions)
-                                           / sizeof (void *)];
+typedef void *nss_module_functions_untyped[nss_module_functions_count];
 
 /* Locate the nss_files functions, as if by dlopen/dlsym.  */
 void __nss_files_functions (nss_module_functions_untyped pointers)

resolv/Makefile

@@ -48,6 +48,7 @@ routines := \
   ns_name_unpack \
   ns_samename \
   nsap_addr \
+  nss_dns_functions \
   res-close \
   res-name-checking \
   res-state \

resolv/nss_dns_functions.c

@@ -0,0 +1,40 @@
+/* Direct access for nss_dns functions for NSS module loading.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <nss/nss_module.h>
+#include <nss_dns.h>
+#include <string.h>
+
+void
+__nss_dns_functions (nss_module_functions_untyped pointers)
+{
+  struct nss_module_functions typed =
+    {
+      .getcanonname_r = &_nss_dns_getcanonname_r,
+      .gethostbyname3_r = &_nss_dns_gethostbyname3_r,
+      .gethostbyname2_r = &_nss_dns_gethostbyname2_r,
+      .gethostbyname_r = &_nss_dns_gethostbyname_r,
+      .gethostbyname4_r = &_nss_dns_gethostbyname4_r,
+      .gethostbyaddr2_r = &_nss_dns_gethostbyaddr2_r,
+      .gethostbyaddr_r = &_nss_dns_gethostbyaddr_r,
+      .getnetbyname_r = &_nss_dns_getnetbyname_r,
+      .getnetbyaddr_r = &_nss_dns_getnetbyaddr_r,
+    };
+
+  memcpy (pointers, &typed, sizeof (nss_module_functions_untyped));
+}