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From:   Jeff Layton <jlayton@kernel.org>
To:     steved@redhat.com
Cc:     linux-nfs@vger.kernel.org, Chuck Lever <chuck.lever@oracle.com>,
        JianHong Yin <jiyin@redhat.com>
Subject: [nfs-utils PATCH] Don't allow junction tests to trigger automounts
Date:   Tue, 13 Dec 2022 11:01:04 -0500
Message-Id: <20221213160104.198237-1-jlayton@kernel.org>
MIME-Version: 1.0
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JianHong reported some strange behavior with automounts on an nfs server
without an explicit pseudoroot. When clients issued a readdir in the
pseudoroot, automounted directories that were not yet mounted would show
up even if they weren't exported, though the clients wouldn't be able to
do anything with them.

The issue was that triggering the automount on a directory would cause
the mountd upcall to time out, which would cause nfsd to include the
automounted dentry in the readdir response. Eventually, the automount
would work and report that it wasn't exported and subsequent attempts to
access the dentry would (properly) fail.

We never want mountd to trigger an automount. The kernel should do that
if it wants to use it. Change the junction checks to do an O_PATH open
and use fstatat with AT_NO_AUTOMOUNT.

Cc: Chuck Lever <chuck.lever@oracle.com>
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2148353
Reported-by: JianHong Yin <jiyin@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 support/junction/junction.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/support/junction/junction.c b/support/junction/junction.c
index 41cce261cb52..0628bb0ffffb 100644
--- a/support/junction/junction.c
+++ b/support/junction/junction.c
@@ -63,7 +63,7 @@ junction_open_path(const char *pathname, int *fd)
 	if (pathname == NULL || fd == NULL)
 		return FEDFS_ERR_INVAL;
 
-	tmp = open(pathname, O_DIRECTORY);
+	tmp = open(pathname, O_PATH|O_DIRECTORY);
 	if (tmp == -1) {
 		switch (errno) {
 		case EPERM:
@@ -93,7 +93,7 @@ junction_is_directory(int fd, const char *path)
 {
 	struct stat stb;
 
-	if (fstat(fd, &stb) == -1) {
+	if (fstatat(fd, "", &stb, AT_NO_AUTOMOUNT|AT_EMPTY_PATH) == -1) {
 		xlog(D_GENERAL, "%s: failed to stat %s: %m",
 				__func__, path);
 		return FEDFS_ERR_ACCESS;
@@ -121,7 +121,7 @@ junction_is_sticky_bit_set(int fd, const char *path)
 {
 	struct stat stb;
 
-	if (fstat(fd, &stb) == -1) {
+	if (fstatat(fd, "", &stb, AT_NO_AUTOMOUNT|AT_EMPTY_PATH) == -1) {
 		xlog(D_GENERAL, "%s: failed to stat %s: %m",
 				__func__, path);
 		return FEDFS_ERR_ACCESS;
@@ -155,7 +155,7 @@ junction_set_sticky_bit(int fd, const char *path)
 {
 	struct stat stb;
 
-	if (fstat(fd, &stb) == -1) {
+	if (fstatat(fd, "", &stb, AT_NO_AUTOMOUNT|AT_EMPTY_PATH) == -1) {
 		xlog(D_GENERAL, "%s: failed to stat %s: %m",
 			__func__, path);
 		return FEDFS_ERR_ACCESS;
@@ -393,7 +393,7 @@ junction_get_mode(const char *pathname, mode_t *mode)
 	if (retval != FEDFS_OK)
 		return retval;
 
-	if (fstat(fd, &stb) == -1) {
+	if (fstatat(fd, "", &stb, AT_NO_AUTOMOUNT|AT_EMPTY_PATH) == -1) {
 		xlog(D_GENERAL, "%s: failed to stat %s: %m",
 			__func__, pathname);
 		(void)close(fd);
-- 
2.38.1