Re: [PATCH v8 02/19] fsnotify: opt-in for permission events at file open time

[Christian Brauner <brauner@kernel.org>]

On Wed, Nov 20, 2024 at 04:53:09PM +0100, Jan Kara wrote:
> On Fri 15-11-24 10:30:15, Josef Bacik wrote:
> > From: Amir Goldstein <amir73il@gmail.com>
> > 
> > Legacy inotify/fanotify listeners can add watches for events on inode,
> > parent or mount and expect to get events (e.g. FS_MODIFY) on files that
> > were already open at the time of setting up the watches.
> > 
> > fanotify permission events are typically used by Anti-malware sofware,
> > that is watching the entire mount and it is not common to have more that
> > one Anti-malware engine installed on a system.
> > 
> > To reduce the overhead of the fsnotify_file_perm() hooks on every file
> > access, relax the semantics of the legacy FAN_ACCESS_PERM event to generate
> > events only if there were *any* permission event listeners on the
> > filesystem at the time that the file was opened.
> > 
> > The new semantic is implemented by extending the FMODE_NONOTIFY bit into
> > two FMODE_NONOTIFY_* bits, that are used to store a mode for which of the
> > events types to report.
> > 
> > This is going to apply to the new fanotify pre-content events in order
> > to reduce the cost of the new pre-content event vfs hooks.
> > 
> > Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> > Link: https://lore.kernel.org/linux-fsdevel/CAHk-=wj8L=mtcRTi=NECHMGfZQgXOp_uix1YVh04fEmrKaMnXA@mail.gmail.com/
> > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> 
> FWIW I've ended up somewhat massaging this patch (see below).
> 
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 23bd058576b1..8e5c783013d2 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -173,13 +173,14 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
> >  
> >  #define	FMODE_NOREUSE		((__force fmode_t)(1 << 23))
> >  
> > -/* FMODE_* bit 24 */
> > -
> >  /* File is embedded in backing_file object */
> > -#define FMODE_BACKING		((__force fmode_t)(1 << 25))
> > +#define FMODE_BACKING		((__force fmode_t)(1 << 24))
> >  
> > -/* File was opened by fanotify and shouldn't generate fanotify events */
> > -#define FMODE_NONOTIFY		((__force fmode_t)(1 << 26))
> > +/* File shouldn't generate fanotify pre-content events */
> > +#define FMODE_NONOTIFY_HSM	((__force fmode_t)(1 << 25))
> > +
> > +/* File shouldn't generate fanotify permission events */
> > +#define FMODE_NONOTIFY_PERM	((__force fmode_t)(1 << 26))
> 
> Firstly, I've kept FMODE_NONOTIFY to stay a single bit instead of two bit
> constant. I've seen too many bugs caused by people expecting the constant
> has a single bit set when it actually had more in my life. So I've ended up
> with:
> 
> +/*
> + * Together with FMODE_NONOTIFY_PERM defines which fsnotify events shouldn't be
> + * generated (see below)
> + */
> +#define FMODE_NONOTIFY         ((__force fmode_t)(1 << 25))
> + 
> +/*
> + * Together with FMODE_NONOTIFY defines which fsnotify events shouldn't be
> + * generated (see below)
> + */
> +#define FMODE_NONOTIFY_PERM    ((__force fmode_t)(1 << 26))
> 
> and
> 
> +/*
> + * The two FMODE_NONOTIFY* define which fsnotify events should not be generated
> + * for a file. These are the possible values of (f->f_mode &
> + * FMODE_FSNOTIFY_MASK) and their meaning:
> + *
> + * FMODE_NONOTIFY - suppress all (incl. non-permission) events.
> + * FMODE_NONOTIFY_PERM - suppress permission (incl. pre-content) events.
> + * FMODE_NONOTIFY | FMODE_NONOTIFY_PERM - suppress only pre-content events.
> + */
> +#define FMODE_FSNOTIFY_MASK \
> +       (FMODE_NONOTIFY | FMODE_NONOTIFY_PERM)

This is fine by me. But I want to preemptively caution to please not
spread the disease of further defines based on such multi-bit defines
like fanotify does. I'm specifically worried about stuff like:

#define ALL_FSNOTIFY_PERM_EVENTS (FS_OPEN_PERM | FS_ACCESS_PERM | \
                                  FS_OPEN_EXEC_PERM)

#define FS_EVENTS_POSS_ON_CHILD   (ALL_FSNOTIFY_PERM_EVENTS | \
                                   FS_ACCESS | FS_MODIFY | FS_ATTRIB | \
                                   FS_CLOSE_WRITE | FS_CLOSE_NOWRITE | \
                                   FS_OPEN | FS_OPEN_EXEC)