Re: [PATCH v3 12/14] vfs: short-circuit MAY_WRITE access for O_DIRECTORY opens

[NeilBrown]

On Sun, 05 Jul 2026, Jori Koolstra wrote:
> Requesting write access on a directory can never succeed. Rather
> than performing a path-walk to determine whether the target is
> actually a directory (-EISDIR) or not (-ENOTDIR), or does not exist
> (-ENOENT), etc., we short-circuit to -ENOTDIR.
> 
> Currently O_WRONLY for directories is only blocked in may_open(),
> which happens after we have the inode for the target, so after any
> create via O_CREAT|O_DIRECTORY.
> 
> The advantage of short-circuiting is that we don't have to add even
> more logic to lookup_open() to differentiate -EISDIR/-ENOTDIR. Also,
> for filesystems that define atomic_open(), handling this cannot even be
> done at the VFS level, as we can't know ahead of calling
> ->atomic_open() what the result of the lookup is.
> 
> Suggested-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>
> Signed-off-by: Jori Koolstra <jkoolstra@xxxxxxxxx>
> ---
>  fs/open.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/open.c b/fs/open.c
> index 9121aece78bc..87e8864d9480 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -1270,9 +1270,16 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
>  
>  	op->intent = flags & O_PATH ? 0 : LOOKUP_OPEN;
>  
> +	/*
> +	 * Requesting write access on a directory can never succeed. Rather
> +	 * than performing a path-walk to determine whether the target is
> +	 * actually a directory (-EISDIR) or not (-ENOTDIR), we short-circuit
> +	 * to -ENOTDIR.
> +	 */
> +	if ((flags & O_DIRECTORY) && !(flags & __O_TMPFILE) && (acc_mode & MAY_WRITE))
> +		return -ENOTDIR;
> +

Nice simple approach.

Reviewed-by: NeilBrown <neil@xxxxxxxxxx>

NeilBrown


>  	if (flags & O_CREAT) {
> -		if ((flags & O_DIRECTORY) && (acc_mode & MAY_WRITE))
> -			return -EISDIR;
>  		op->intent |= LOOKUP_CREATE;
>  		if (flags & O_EXCL) {
>  			op->intent |= LOOKUP_EXCL;
> -- 
> 2.55.0
> 
>