Re: [PATCH 02/10] fsnotify: introduce pre-content permission event

[Jan Kara <jack@suse.cz>]

On Fri 25-10-24 09:55:21, Amir Goldstein wrote:
> On Sat, Aug 3, 2024 at 6:52 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > On Thu, Aug 1, 2024 at 6:31 PM Jan Kara <jack@suse.cz> wrote:
> > > On Thu 25-07-24 14:19:39, Josef Bacik wrote:
> > > > From: Amir Goldstein <amir73il@gmail.com>
> > > >
> > > > The new FS_PRE_ACCESS permission event is similar to FS_ACCESS_PERM,
> > > > but it meant for a different use case of filling file content before
> > > > access to a file range, so it has slightly different semantics.
> > > >
> > > > Generate FS_PRE_ACCESS/FS_ACCESS_PERM as two seperate events, same as
> > > > we did for FS_OPEN_PERM/FS_OPEN_EXEC_PERM.
> > > >
> > > > FS_PRE_MODIFY is a new permission event, with similar semantics as
> > > > FS_PRE_ACCESS, which is called before a file is modified.
> > > >
> > > > FS_ACCESS_PERM is reported also on blockdev and pipes, but the new
> > > > pre-content events are only reported for regular files and dirs.
> > > >
> > > > The pre-content events are meant to be used by hierarchical storage
> > > > managers that want to fill the content of files on first access.
> > > >
> > > > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> > >
> > > The patch looks good. Just out of curiosity:
> > >
> > > > diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
> > > > index 8be029bc50b1..21e72b837ec5 100644
> > > > --- a/include/linux/fsnotify_backend.h
> > > > +++ b/include/linux/fsnotify_backend.h
> > > > @@ -56,6 +56,9 @@
> > > >  #define FS_ACCESS_PERM               0x00020000      /* access event in a permissions hook */
> > > >  #define FS_OPEN_EXEC_PERM    0x00040000      /* open/exec event in a permission hook */
> > > >
> > > > +#define FS_PRE_ACCESS                0x00100000      /* Pre-content access hook */
> > > > +#define FS_PRE_MODIFY                0x00200000      /* Pre-content modify hook */
> > >
> > > Why is a hole left here in the flag space?
> >
> > Can't remember.
> >
> > Currently we have a draft design for two more events
> > FS_PATH_ACCESS, FS_PATH_MODIFY
> > https://github.com/amir73il/man-pages/commits/fan_pre_path
> >
> > So might have been a desire to keep the pre-events group on the nibble.
> 
> Funny story.
> 
> I straced a program with latest FS_PRE_ACCESS (0x00080000) and
> see what I got:
> 
> fanotify_mark(3, FAN_MARK_ADD|FAN_MARK_MOUNT,
> FAN_CLOSE_WRITE|FAN_OPEN_PERM|FAN_ACCESS_PERM|FAN_DIR_MODIFY|FAN_ONDIR,
> AT_FDCWD, "/vdd") = 0
> 
> "FAN_DIR_MODIFY"! a blast from the past [1]
> 
> It would have been nice if we reserved 0x00080000 for FAN_PATH_MODIFY [2]
> to be a bit less confusing for users with old strace.
> 
> WDYT?

Yeah, reusing that bit for something semantically close would reduce some
confusion. But realistically I don't think FAN_DIR_MODIFY go wide use when
it was never supported in a released upstream kernel.

								Honza

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