[PATCH AUTOSEL 5.17 29/34] VFS: filename_create(): fix incorrect intent.

Sasha Levin
2022-04-19T18:14:30Z

From: NeilBrown <neilb@suse.de>

[ Upstream commit b3d4650d82c71b9c9a8184de9e8bb656012b289e ]

When asked to create a path ending '/', but which is not to be a
directory (LOOKUP_DIRECTORY not set), filename_create() will never try
to create the file. If it doesn't exist, -ENOENT is reported.

However, it still passes LOOKUP_CREATE|LOOKUP_EXCL to the filesystems
->lookup() function, even though there is no intent to create. This is
misleading and can cause incorrect behaviour.

If you try

ln -s foo /path/dir/

where 'dir' is a directory on an NFS filesystem which is not currently
known in the dcache, this will fail with ENOENT.

But as the name is not in the dcache, nfs_lookup gets called with
LOOKUP_CREATE|LOOKUP_EXCL and so it returns NULL without performing any
lookup, with the expectation that a subsequent call to create the target
will be made, and the lookup can be combined with the creation. In the
case with a trailing '/' and no LOOKUP_DIRECTORY, that call is never
made. Instead filename_create() sees that the dentry is not (yet)
positive and returns -ENOENT - even though the directory actually
exists.

So only set LOOKUP_CREATE|LOOKUP_EXCL if there really is an intent to
create, and use the absence of these flags to decide if -ENOENT should
be returned.

Note that filename_parentat() is only interested in LOOKUP_REVAL, so we
split that out and store it in 'reval_flag'. __lookup_hash() then gets
reval_flag combined with whatever create flags were determined to be
needed.

Reviewed-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neilb@suse.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/namei.c | 22 ++++++++++------------
1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 3f1829b3ab5b..509657fdf4f5 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3673,18 +3673,14 @@ static struct dentry *filename_create(int dfd, struct filename *name,
{
struct dentry *dentry = ERR_PTR(-EEXIST);
struct qstr last;
+ bool want_dir = lookup_flags & LOOKUP_DIRECTORY;
+ unsigned int reval_flag = lookup_flags & LOOKUP_REVAL;
+ unsigned int create_flags = LOOKUP_CREATE | LOOKUP_EXCL;
int type;
int err2;
int error;
- bool is_dir = (lookup_flags & LOOKUP_DIRECTORY);

- /*
- * Note that only LOOKUP_REVAL and LOOKUP_DIRECTORY matter here. Any
- * other flags passed in are ignored!
- */
- lookup_flags &= LOOKUP_REVAL;
-
- error = filename_parentat(dfd, name, lookup_flags, path, &last, &type);
+ error = filename_parentat(dfd, name, reval_flag, path, &last, &type);
if (error)
return ERR_PTR(error);

@@ -3698,11 +3694,13 @@ static struct dentry *filename_create(int dfd, struct filename *name,
/* don't fail immediately if it's r/o, at least try to report other errors */
err2 = mnt_want_write(path->mnt);
/*
- * Do the final lookup.
+ * Do the final lookup. Suppress 'create' if there is a trailing
+ * '/', and a directory wasn't requested.
*/
- lookup_flags |= LOOKUP_CREATE | LOOKUP_EXCL;
+ if (last.name[last.len] && !want_dir)
+ create_flags = 0;
inode_lock_nested(path->dentry->d_inode, I_MUTEX_PARENT);
- dentry = __lookup_hash(&last, path->dentry, lookup_flags);
+ dentry = __lookup_hash(&last, path->dentry, reval_flag | create_flags);
if (IS_ERR(dentry))
goto unlock;

@@ -3716,7 +3714,7 @@ static struct dentry *filename_create(int dfd, struct filename *name,
* all is fine. Let's be bastards - you had / on the end, you've
* been asking for (non-existent) directory. -ENOENT for you.
*/
- if (unlikely(!is_dir && last.name[last.len])) {
+ if (unlikely(!create_flags)) {
error = -ENOENT;
goto fail;
}
--
2.35.1