[PATCH] fs: don't leak the parent path when openat() fails after the lookup

[Ahmad Fatoum]

openat() jumps to out1 on three errors that happen after the lookup has
succeeded, and none of them drops what the lookup took:

- create() failing leaves both the negative dentry that filename_create()
  returned and the parent path it filled in.
- The -ENOENT branch dputs the dentry, which is path.dentry here, but not
  the vfsmount reference held alongside it.
- The -EISDIR branch drops nothing at all.

The leaked vfsmount reference keeps the file system busy for good:

  barebox:/ mount -t efivarfs none /efivarfs
  barebox:/ echo -o /efivarfs/Foo-8be4df61-93ca-11d2-aa0d-00e098032b8c x
  open: Operation not permitted
  barebox:/ umount /efivarfs
  umount: Device or resource busy

efivarfs refuses to create variables outside barebox' vendor GUID, so a
single mistyped GUID is enough to reach this; any file system whose
create() can fail - a full FAT, a read-only mount - gets there the same
way. Release the path on all three paths, as mknodat() already does for
the same filename_create() result.

Assisted-by: Claude:opus-5
Signed-off-by: Ahmad Fatoum <a.fatoum at pengutronix.de>
---
 fs/fs.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/fs.c b/fs/fs.c
index dc6c30802d89..ce41f23f880b 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -2748,8 +2748,11 @@ int openat(int dirfd, const char *pathname, int flags)
 	if (d_is_negative(dentry)) {
 		if (flags & O_CREAT) {
 			error = create(path.dentry, dentry);
-			if (error)
+			if (error) {
+				dput(dentry);
+				path_put(&path);
 				goto out1;
+			}
 			/* repoint path.dentry from parent to newly created entry.
 			 * path.mnt already points at the correct vfsmount, even
 			 * for a dirfd of the root directory, so that's fine.
@@ -2757,12 +2760,13 @@ int openat(int dirfd, const char *pathname, int flags)
 			dput(path.dentry);
 			path.dentry = dentry;
 		} else {
-			dput(dentry);
+			path_put(&path);
 			error = -ENOENT;
 			goto out1;
 		}
 	} else if (d_is_dir(dentry)) {
 		if (!(flags & (O_PATH | O_DIRECTORY)) && !dentry_is_tftp(dentry)) {
+			path_put(&path);
 			error = -EISDIR;
 			goto out1;
 		}
-- 
2.47.3