Re: [PATCH] backing-file: inherit FMODE_NONOTIFY from the user file

[Amir Goldstein]

On Tue, Oct 27, 2026 at 10:32 AM Edvard Conrad <edvard@xxxxxxx> wrote:
>
> On Tue, Oct 27, 2026 at 12:53 AM Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> > Any chance you can post this as an LTP test?
>
> Yes. Written and verified, I will post it to the LTP list shortly as
> syscalls/fanotify26. Two cases: a mark on the overlay only, which passes
> on every kernel and keeps the test from passing for the wrong reason,
> and a mark on the overlay plus the backing filesystem, which is the
> shape that deadlocks.
>

Great!

> > Urgh I hate how special case this is.
> >
> > How about something like below (untested)
>
> Agreed on the shape, and it covers backing_tmpfile_open() as well, which
> mine did not.

There is no code that adds NONOTIFY to backing_tmpfile_open()
the only code that could add NONOTIFY to backing file is from
dentry_open_nonotify()

> As posted it does not fix the deadlock though:
>
> > -       f->f_mode       = OPEN_FMODE(flags);
> > +       f->f_mode       = OPEN_FMODE(flags) | fmode;
>
> Five lines further down init_file() calls
>
>         file_set_fsnotify_mode(f, FMODE_NONOTIFY_PERM);
>
> and that helper clears FMODE_FSNOTIFY_MASK before it sets its argument,
> so the inherited FMODE_NONOTIFY is dropped again. The backing file ends
> up at FMODE_NONOTIFY_PERM, which is not FMODE_FSNOTIFY_NONE(), so
> fsnotify_open_perm_and_set_mode() does not take its early return and
> still raises FS_OPEN_PERM on the real path.
>
> Measured in a VM on a defconfig + OVERLAY_FS + FANOTIFY +
> FANOTIFY_ACCESS_PERMISSIONS build of this tree, with the reproducer from
> the patch and with the LTP test:
>
>   kernel                        reproducer    fanotify26
>   mainline                      hangs         1 pass 1 fail
>   v1 (fs/backing-file.c)        ok            2 pass
>   your diff as posted           hangs         1 pass 1 fail
>   v2                            ok            2 pass
>
> v2 keeps your structure and routes the fsnotify part through
> file_set_fsnotify_mode() instead:
>
>         fmode_t fsnotify_mode = fmode & FMODE_FSNOTIFY_MASK;
>         ...
>         f->f_mode       = OPEN_FMODE(flags) | fmode;
>         file_set_fsnotify_mode(f, fsnotify_mode ? fsnotify_mode :
>                                                   FMODE_NONOTIFY_PERM);
>
> One deliberate difference from your version: it inherits only the "no
> events at all" mode rather than masking FMODE_NONOTIFY out of whatever
> the user file carries. A user file in the HSM mode (FMODE_NONOTIFY |
> FMODE_NONOTIFY_PERM) would otherwise leave its backing file fully
> silent, suppressing the events on real files that bc2473c90fca
> deliberately enabled.

The problem is that I wanted something more generic and v2 is
still very much special casing NONOTIFY and much bigger than
v1, so there is no gain.

Let's admit that I gave you a bad advise and a wrong sketch patch
and feel free to add to v1:

Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>

BTW, pls remove Assisted-by it is not needed and remove the
newline after cc: stable

Thanks,
Amir.