Date: Mon, 17 Jun 2024 15:07:39 +0200
From: Jan Kara <jack@...e.cz>
To: Amir Goldstein <amir73il@...il.com>
Cc: Jan Kara <jack@...e.cz>, 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 17-06-24 15:09:09, Amir Goldstein wrote:
> 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;
>         }

First I was somewhat nervous about this as it results in returning O_PATH
fd with __FMODE_NONOTIFY to userspace and I was afraid it may influence
generation of events *somewhere*. But checking a bit, we use 'file' for
generating only open, access, modify, and close events so yes, this should
be safe. Alternatively we could add explicit checks for !O_PATH when
generating open / close events.

> > @@ -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).

I don't think so. AFAICT that case will generate the event in vfs_open()
anyway and not in open_last_lookups() / do_open(). Am I missing something?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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