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>

2 files changed, 41 insertions, 0 deletions

diff --git a/resolv/Makefile b/resolv/Makefile
index dd0a98c74f..31d27454b4 100644
--- a/resolv/Makefile
+++ b/resolv/Makefile
@@ -48,6 +48,7 @@ routines := \
   ns_name_unpack \
   ns_samename \
   nsap_addr \
+  nss_dns_functions \
   res-close \
   res-name-checking \
   res-state \
diff --git a/resolv/nss_dns_functions.c b/resolv/nss_dns_functions.c
new file mode 100644
index 0000000000..158dafec90
--- /dev/null
+++ b/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));
+}