[PATCH v2 4/4] openat2: don't trigger automounts with RESOLVE_NO_XDEV

openat2 had a bug: if we pass RESOLVE_NO_XDEV, then openat2
doesn't traverse through automounts, but may still trigger them.
(See the link for full bug report with reproducer.)

This commit fixes this bug.

Link: https://lore.kernel.org/linux-fsdevel/[email protected]/
Fixes: fddb5d430ad9fa91b49b1 ("open: introduce openat2(2) syscall")
Reviewed-by: Aleksa Sarai <[email protected]>
Cc: <[email protected]>
Signed-off-by: Askar Safin <[email protected]>
---
 fs/namei.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index c23e5a076ab3..bfa8232b94dc 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1449,6 +1449,10 @@ static int follow_automount(struct path *path, int *count, unsigned lookup_flags
 	    dentry->d_inode)
 		return -EISDIR;
 
+	/* No need to trigger automounts if mountpoint crossing is disabled. */
+	if (lookup_flags & LOOKUP_NO_XDEV)
+		return -EXDEV;
+
 	if (count && (*count)++ >= MAXSYMLINKS)
 		return -ELOOP;
 
@@ -1472,6 +1476,10 @@ static int __traverse_mounts(struct path *path, unsigned flags, bool *jumped,
 		/* Allow the filesystem to manage the transit without i_rwsem
 		 * being held. */
 		if (flags & DCACHE_MANAGE_TRANSIT) {
+			if (lookup_flags & LOOKUP_NO_XDEV) {
+				ret = -EXDEV;
+				break;
+			}
 			ret = path->dentry->d_op->d_manage(path, false);
 			flags = smp_load_acquire(&path->dentry->d_flags);
 			if (ret < 0)
-- 
2.47.2