[PATCH nfs-utils 16/17] exportfs: use netlink to probe kernel support, skip export_test

[Jeff Layton <jlayton@kernel.org>]

Refactor can_test() to first try resolving the sunrpc genl family via
netlink. If successful, the kernel supports the new netlink-based cache
interfaces and we can skip the advisory export_test() probe -- mountd
and exportd will validate exports at cache-fill time via check_export().

can_test() now returns:
  2 = netlink available (skip export_test)
  1 = /proc available (keep export_test as before)
  0 = neither available

Update validate_export() to check this return value and skip the
export_test() calls when netlink is available.

Link exportfs against libnl3/libnl-genl-3 (already a mandatory build
dependency for nfs-utils).

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 utils/exportfs/Makefile.am |  6 ++++--
 utils/exportfs/exportfs.c  | 42 +++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 45 insertions(+), 3 deletions(-)

diff --git a/utils/exportfs/Makefile.am b/utils/exportfs/Makefile.am
index 7f8ce9faf2b469c8560f5fbee40b81e3443eab78..8db5fdae1741e1c79639bbdfd5d63f8571363e63 100644
--- a/utils/exportfs/Makefile.am
+++ b/utils/exportfs/Makefile.am
@@ -11,8 +11,10 @@ exportfs_LDADD = ../../support/export/libexport.a \
 	       	 ../../support/nfs/libnfs.la \
 		 ../../support/misc/libmisc.a \
 		 ../../support/reexport/libreexport.a \
-		 $(LIBWRAP) $(LIBNSL) $(LIBPTHREAD)
+		 $(LIBWRAP) $(LIBNSL) $(LIBPTHREAD) \
+		 $(LIBNL3_LIBS) $(LIBNLGENL3_LIBS)
 
-exportfs_CPPFLAGS = $(AM_CPPFLAGS) $(CPPFLAGS) -I$(top_srcdir)/support/reexport
+exportfs_CPPFLAGS = $(AM_CPPFLAGS) $(CPPFLAGS) -I$(top_srcdir)/support/reexport \
+		    $(LIBNL3_CFLAGS) $(LIBNLGENL3_CFLAGS)
 
 MAINTAINERCLEANFILES = Makefile.in
diff --git a/utils/exportfs/exportfs.c b/utils/exportfs/exportfs.c
index 748c38e3e966703c7a870a6aa9589d32708edcd2..04753fa169f97c6b07893613197739ff36e0d09b 100644
--- a/utils/exportfs/exportfs.c
+++ b/utils/exportfs/exportfs.c
@@ -40,6 +40,15 @@
 #include "conffile.h"
 #include "reexport.h"
 
+#include <netlink/genl/genl.h>
+#include <netlink/genl/ctrl.h>
+
+#ifdef USE_SYSTEM_SUNRPC_NETLINK_H
+#include <linux/sunrpc_netlink.h>
+#else
+#include "sunrpc_netlink.h"
+#endif
+
 static void	export_all(int verbose);
 static void	exportfs(char *arg, char *options, int verbose);
 static void	unexportfs(char *arg, int verbose);
@@ -476,13 +485,34 @@ unexportfs(char *arg, int verbose)
 		xlog(L_ERROR, "Invalid export syntax: %s", arg);
 }
 
+/* Return values:
+ *   2 = netlink available (skip export_test)
+ *   1 = /proc available (keep export_test)
+ *   0 = neither available
+ */
 static int can_test(void)
 {
+	struct nl_sock *sock;
+	int family;
 	char buf[1024] = { 0 };
 	int fd;
 	int n;
 	size_t bufsiz = sizeof(buf);
 
+	/* Try netlink first: resolve sunrpc genl family */
+	sock = nl_socket_alloc();
+	if (sock) {
+		if (genl_connect(sock) == 0) {
+			family = genl_ctrl_resolve(sock, SUNRPC_FAMILY_NAME);
+			nl_socket_free(sock);
+			if (family >= 0)
+				return 2;
+		} else {
+			nl_socket_free(sock);
+		}
+	}
+
+	/* Fallback: /proc probe */
 	fd = open("/proc/net/rpc/auth.unix.ip/channel", O_WRONLY);
 	if (fd < 0)
 		return 0;
@@ -522,6 +552,7 @@ validate_export(nfs_export *exp)
 	char *path = exportent_realpath(&exp->m_export);
 	struct statfs stf;
 	int fs_has_fsid = 0;
+	int test_result;
 
 	if (stat(path, &stb) < 0) {
 		xlog(L_ERROR, "Failed to stat %s: %m", path);
@@ -532,7 +563,16 @@ validate_export(nfs_export *exp)
 			"Remote access will fail", path);
 		return;
 	}
-	if (!can_test())
+
+	test_result = can_test();
+	if (!test_result)
+		return;
+
+	/*
+	 * When netlink is available, skip the export_test() probe.
+	 * mountd/exportd will validate exports at cache-fill time.
+	 */
+	if (test_result == 2)
 		return;
 
 	if (!statfs(path, &stf) &&

-- 
2.53.0