linux-kernel - [PATCH v4] VFS: filename_create(): fix incorrect intent.

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Message-id: <164878611050.25542.6758961460499392000@noble.neil.brown.name>
Date:   Fri, 01 Apr 2022 15:08:30 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Al Viro" <viro@...iv.linux.org.uk>
Cc:     linux-nfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        "LKML" <linux-kernel@...r.kernel.org>,
        "Jeff Layton" <jlayton@...nel.org>,
        "David Disseldorp" <ddiss@...e.de>
Subject: [PATCH v4] VFS: filename_create(): fix incorrect intent.

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.
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'.
__looku_hash() then gets reval_flag combined with whatever create flags
were determined to be needed.

Signed-off-by: NeilBrown <neilb@...e.de>
---
 fs/namei.c | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

ARG - v3 had a missing semi-colon.  Sorry.

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