Re: [PATCH] fuse: skip lookup during atomic_open() when O_CREAT is set

[Miklos Szeredi]

On Fri, 20 Feb 2026 at 21:41, Jim Harris <jim.harris@xxxxxxxxxx> wrote:
>
> From: Jim Harris <jim.harris@xxxxxxxxxx>
>
> When O_CREAT is set, we don't need the lookup. The lookup doesn't
> harm anything, but it's an extra FUSE operation that's not required.
>
> Signed-off-by: Jim Harris <jim.harris@xxxxxxxxxx>
> ---
>  fs/fuse/dir.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> index f25ee47822ad..35f65d49ed2a 100644
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -895,7 +895,8 @@ static int fuse_create_open(struct mnt_idmap *idmap, struct inode *dir,
>                 goto out_err;
>         }
>         kfree(forget);
> -       d_instantiate(entry, inode);
> +       d_drop(entry);
> +       d_splice_alias(inode, entry);
>         entry->d_time = epoch;
>         fuse_change_entry_timeout(entry, &outentry);
>         fuse_dir_changed(dir);
> @@ -936,14 +937,15 @@ static int fuse_atomic_open(struct inode *dir, struct dentry *entry,
>         if (fuse_is_bad(dir))
>                 return -EIO;
>
> -       if (d_in_lookup(entry)) {
> -               struct dentry *res = fuse_lookup(dir, entry, 0);
> -               if (res || d_really_is_positive(entry))
> -                       return finish_no_open(file, res);
> -       }
> +       if (!(flags & O_CREAT)) {
> +               if (d_in_lookup(entry)) {
> +                       struct dentry *res = fuse_lookup(dir, entry, 0);
>
> -       if (!(flags & O_CREAT))
> +                       if (res || d_really_is_positive(entry))
> +                               return finish_no_open(file, res);
> +               }
>                 return finish_no_open(file, NULL);
> +       }
>
>         /* Only creates */
>         file->f_mode |= FMODE_CREATED;

Now this is not necessarily true.  Setting FMODE_CREATED in case of an
existing file will have the effect of not checking permissions to open
(think open(path, O_CREAT | O_RDWR, 0444)).

Sad fact is that the same thing could happen with current code: a file
created on a remote client during the window between LOOKUP and
CREATE.

To prevent that with the current protocol we'd need to add O_EXCL and
retry on -EEXIST.

Better fix would be to add a flag to the CREATE reply indicating if
the file was created or not.

Thanks,
Miklos