linux-kernel - Re: [PATCH v2] VFS: generate FS_CREATE before FS_OPEN when ->atomic

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Date: Mon, 17 Jun 2024 15:09:09 +0300
From: Amir Goldstein <amir73il@...il.com>
To: Jan Kara <jack@...e.cz>
Cc: Christian Brauner <brauner@...nel.org>, NeilBrown <neilb@...e.de>, James Clark <james.clark@....com>, 
	ltp@...ts.linux.it, linux-nfs@...r.kernel.org, 
	LKML <linux-kernel@...r.kernel.org>, linux-fsdevel@...r.kernel.org, 
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v2] VFS: generate FS_CREATE before FS_OPEN when
 ->atomic_open used.

On Mon, Jun 17, 2024 at 12:37 PM Jan Kara <jack@...e.cz> wrote:
>
> On Sat 15-06-24 07:35:42, Christian Brauner wrote:
> > On Wed, 12 Jun 2024 17:09:55 +1000, NeilBrown wrote:
> > > When a file is opened and created with open(..., O_CREAT) we get
> > > both the CREATE and OPEN fsnotify events and would expect them in that
> > > order.   For most filesystems we get them in that order because
> > > open_last_lookups() calls fsnofify_create() and then do_open() (from
> > > path_openat()) calls vfs_open()->do_dentry_open() which calls
> > > fsnotify_open().
> > >
> > > [...]
> >
> > Applied to the vfs.fixes branch of the vfs/vfs.git tree.
> > Patches in the vfs.fixes branch should appear in linux-next soon.
> >
> > Please report any outstanding bugs that were missed during review in a
> > new review to the original patch series allowing us to drop it.
> >
> > It's encouraged to provide Acked-bys and Reviewed-bys even though the
> > patch has now been applied. If possible patch trailers will be updated.
> >
> > Note that commit hashes shown below are subject to change due to rebase,
> > trailer updates or similar. If in doubt, please check the listed branch.
> >
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
> > branch: vfs.fixes
> >
> > [1/1] VFS: generate FS_CREATE before FS_OPEN when ->atomic_open used.
> >       https://git.kernel.org/vfs/vfs/c/7536b2f06724
>
> I have reviewed the patch you've committed since I wasn't quite sure which
> changes you're going to apply after your discussion with Amir. And I have
> two comments:
>
> @@ -1085,8 +1080,17 @@ EXPORT_SYMBOL(file_path);
>   */
>  int vfs_open(const struct path *path, struct file *file)
>  {
> +       int ret;
> +
>         file->f_path = *path;
> -       return do_dentry_open(file, NULL);
> +       ret = do_dentry_open(file, NULL);
> +       if (!ret)
> +               /*
> +                * Once we return a file with FMODE_OPENED, __fput() will call
> +                * fsnotify_close(), so we need fsnotify_open() here for symmetry.
> +                */
> +               fsnotify_open(file);

Please add { } around multi line indented text.

> +       return ret;
>  }
>
> AFAICT this will have a side-effect that now fsnotify_open() will be
> generated even for O_PATH open. It is true that fsnotify_close() is getting
> generated for them already and we should strive for symmetry. Conceptually
> it doesn't make sense to me to generate fsnotify events for O_PATH
> opens/closes but maybe I miss something. Amir, any opinion here?

Good catch!

I agree that we do not need OPEN nor CLOSE events for O_PATH.
I suggest to solve it with:

@@ -915,7 +929,7 @@ static int do_dentry_open(struct file *f,
        f->f_sb_err = file_sample_sb_err(f);

        if (unlikely(f->f_flags & O_PATH)) {
-               f->f_mode = FMODE_PATH | FMODE_OPENED;
+               f->f_mode = FMODE_PATH | FMODE_OPENED | __FMODE_NONOTIFY;
                f->f_op = &empty_fops;
                return 0;
        }

>
> @@ -3612,6 +3612,9 @@ static int do_open(struct nameidata *nd,
>         int acc_mode;
>         int error;
>
> +       if (file->f_mode & FMODE_OPENED)
> +               fsnotify_open(file);
> +
>         if (!(file->f_mode & (FMODE_OPENED | FMODE_CREATED))) {
>                 error = complete_walk(nd);
>                 if (error)
>
> Frankly, this works but looks as an odd place to put this notification to.
> Why not just placing it just next to where fsnotify_create() is generated
> in open_last_lookups()? Like:
>
>         if (open_flag & O_CREAT)
>                 inode_lock(dir->d_inode);
>         else
>                 inode_lock_shared(dir->d_inode);
>         dentry = lookup_open(nd, file, op, got_write);
> -       if (!IS_ERR(dentry) && (file->f_mode & FMODE_CREATED))
> -               fsnotify_create(dir->d_inode, dentry);
> +       if (!IS_ERR(dentry)) {
> +               if (file->f_mode & FMODE_CREATED)
> +                       fsnotify_create(dir->d_inode, dentry);
> +               if (file->f_mode & FMODE_OPENED)
> +                       fsnotify_open(file);
> +       }
>         if (open_flag & O_CREAT)
>                 inode_unlock(dir->d_inode);
>         else
>                 inode_unlock_shared(dir->d_inode);
>
> That looks like a place where it is much more obvious this is for
> atomic_open() handling? Now I admit I'm not really closely familiar with
> the atomic_open() paths so maybe I miss something and do_open() is better.

It looks nice, but I think it is missing the fast lookup case without O_CREAT
(i.e. goto finish_lookup).

Thanks,
Amir.