Re: [PATCH] VFS: filename_create(): fix incorrect intent.

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2022-03-28 at 11:56 +1100, NeilBrown wrote:
> 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 as 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.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
>  fs/namei.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 3f1829b3ab5b..3ffb42e56a8e 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3676,7 +3676,6 @@ static struct dentry *filename_create(int dfd, struct filename *name,
>  	int type;
>  	int err2;
>  	int error;
> -	bool is_dir = (lookup_flags & LOOKUP_DIRECTORY);
>  
>  	/*
>  	 * Note that only LOOKUP_REVAL and LOOKUP_DIRECTORY matter here. Any
> @@ -3698,9 +3697,11 @@ 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.  Request 'create' only if there is no trailing
> +	 * '/', or if directory is requested.
>  	 */
> -	lookup_flags |= LOOKUP_CREATE | LOOKUP_EXCL;
> +	if (!last.name[last.len] || (lookup_flags & LOOKUP_DIRECTORY))
> +		lookup_flags |= LOOKUP_CREATE | LOOKUP_EXCL;
>  	inode_lock_nested(path->dentry->d_inode, I_MUTEX_PARENT);
>  	dentry = __lookup_hash(&last, path->dentry, lookup_flags);
>  	if (IS_ERR(dentry))
> @@ -3716,7 +3717,7 @@ 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 (!likely(lookup_flags & LOOKUP_CREATE)) {
>  		error = -ENOENT;
>  		goto fail;
>  	}

Seems like a sane enough fix. Nice catch.

Reviewed-by: Jeff Layton <jlayton@kernel.org>