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From: Amir Goldstein <amir73il@gmail.com>
Date: Wed, 12 Jun 2024 13:34:53 +0300
Message-ID: <CAOQ4uxidUYY02xry+y5VpRWfBjCmAt0CnmJ3JbgLTLkZ6nn1sA@mail.gmail.com>
Subject: Re: [PATCH v2] VFS: generate FS_CREATE before FS_OPEN when
 ->atomic_open used.
To: NeilBrown <neilb@suse.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>, Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>, 
	James Clark <james.clark@arm.com>, ltp@lists.linux.it, linux-nfs@vger.kernel.org, 
	LKML <linux-kernel@vger.kernel.org>, linux-fsdevel@vger.kernel.org, 
	Miklos Szeredi <miklos@szeredi.hu>
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On Wed, Jun 12, 2024 at 10:10=E2=80=AFAM NeilBrown <neilb@suse.de> 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().
>
> However when ->atomic_open is used, the
>    do_dentry_open() -> fsnotify_open()
> call happens from finish_open() which is called from the ->atomic_open
> handler in lookup_open() which is called *before* open_last_lookups()
> calls fsnotify_create.  So we get the "open" notification before
> "create" - which is backwards.  ltp testcase inotify02 tests this and
> reports the inconsistency.
>
> This patch lifts the fsnotify_open() call out of do_dentry_open() and
> places it higher up the call stack.  There are three callers of
> do_dentry_open().
>
> For vfs_open() and kernel_file_open() the fsnotify_open() is placed
> directly in that caller so there should be no behavioural change.
>
> For finish_open() there are two cases:
>  - finish_open is used in ->atomic_open handlers.  For these we add a
>    call to fsnotify_open() at the top of do_open() if FMODE_OPENED is
>    set - which means do_dentry_open() has been called.
>  - finish_open is used in ->tmpfile() handlers.  For these a similar
>    call to fsnotify_open() is added to vfs_tmpfile()

Any handlers other than ovl_tmpfile()?
This one is a very recent and pretty special case.
Did open(O_TMPFILE) used to emit an OPEN event before that change?

>
> With this patch NFSv3 is restored to its previous behaviour (before
> ->atomic_open support was added) of generating CREATE notifications
> before OPEN, and NFSv4 now has that same correct ordering that is has
> not had before.  I haven't tested other filesystems.
>
> Fixes: 7c6c5249f061 ("NFS: add atomic_open for NFSv3 to handle O_TRUNC co=
rrectly.")
> Reported-by: James Clark <james.clark@arm.com>
> Closes: https://lore.kernel.org/all/01c3bf2e-eb1f-4b7f-a54f-d2a05dd3d8c8@=
arm.com
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
>  fs/namei.c |  5 +++++
>  fs/open.c  | 19 ++++++++++++-------
>  2 files changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 37fb0a8aa09a..057afacc4b60 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -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 =3D complete_walk(nd);
>                 if (error)
> @@ -3700,6 +3703,8 @@ int vfs_tmpfile(struct mnt_idmap *idmap,
>         mode =3D vfs_prepare_mode(idmap, dir, mode, mode, mode);
>         error =3D dir->i_op->tmpfile(idmap, dir, file, mode);
>         dput(child);
> +       if (file->f_mode & FMODE_OPENED)
> +               fsnotify_open(file);
>         if (error)
>                 return error;
>         /* Don't check for other permissions, the inode was just created =
*/
> diff --git a/fs/open.c b/fs/open.c
> index 89cafb572061..970f299c0e77 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -1004,11 +1004,6 @@ static int do_dentry_open(struct file *f,
>                 }
>         }
>
> -       /*
> -        * Once we return a file with FMODE_OPENED, __fput() will call
> -        * fsnotify_close(), so we need fsnotify_open() here for symmetry=
.
> -        */
> -       fsnotify_open(f);
>         return 0;
>
>  cleanup_all:
> @@ -1085,8 +1080,17 @@ EXPORT_SYMBOL(file_path);
>   */
>  int vfs_open(const struct path *path, struct file *file)
>  {
> +       int ret;
> +
>         file->f_path =3D *path;
> -       return do_dentry_open(file, NULL);
> +       ret =3D 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);

I agree that this change preserves the logic, but (my own) comment
above is inconsistent with the case of:

        if ((f->f_flags & O_DIRECT) && !(f->f_mode & FMODE_CAN_ODIRECT))
                return -EINVAL;

Which does set FMODE_OPENED, but does not emit an OPEN event.

I have a feeling that the comment is correct about the CLOSE event in
that case, but honestly, I don't think this corner case is that important,
just maybe the comment needs to be slightly clarified?

Thanks,
Amir.

> +       return ret;
>  }
>
>  struct file *dentry_open(const struct path *path, int flags,
> @@ -1178,7 +1182,8 @@ struct file *kernel_file_open(const struct path *pa=
th, int flags,
>         if (error) {
>                 fput(f);
>                 f =3D ERR_PTR(error);
> -       }
> +       } else
> +               fsnotify_open(f);
>         return f;
>  }
>  EXPORT_SYMBOL_GPL(kernel_file_open);
> --
> 2.44.0
>