[RFC/PATCH v2 bpf-next fanotify 7/7] selftests/bpf: Add test for BPF based fanotify fastpath handler

[Alexei Starovoitov]

On Mon, Nov 18, 2024 at 12:51 PM Song Liu <songliubraving at meta.com> wrote:
>
>
>
> > On Nov 15, 2024, at 1:05 PM, Song Liu <songliubraving at meta.com> wrote:
>
> [...]
> >
> >>
> >> fsnotify_open_perm->fsnotify->send_to_group->fanotify_handle_event.
> >>
> >> is a pretty long path to call bpf prog and
> >> preparing a giant 'struct fanotify_fastpath_event'
> >> is not going to fast either.
> >>
> >> If we want to accelerate that with bpf it needs to be done
> >> sooner with negligible overhead.
> >
> > Agreed. This is actually something I have been thinking
> > since the beginning of this work: Shall it be fanotify-bpf
> > or fsnotify-bpf. Given we have more materials, this is a
> > good time to have broader discussions on this.
> >
> > @all, please chime in whether we should redo this as
> > fsnotify-bpf. AFAICT:
> >
> > Pros of fanotify-bpf:
> > - There is existing user space that we can leverage/reuse.
> >
> > Pros of fsnotify-bpf:
> > - Faster fast path.
> >
> > Another major pros/cons did I miss?
>
> Adding more thoughts on this: I think it makes more sense to
> go with fanotify-bpf. This is because one of the benefits of
> fsnotify/fanotify over LSM solutions is the built-in event
> filtering of events. While this call chain is a bit long:
>
> fsnotify_open_perm->fsnotify->send_to_group->fanotify_handle_event.
>
> There are built-in filtering in fsnotify() and
> send_to_group(), so logics in the call chain are useful.

fsnotify_marks based filtering happens in fsnotify.
No need to do more indirect calls to get to fanotify.

I would add the bpf struct_ops hook right before send_to_group
or inside of it.
Not sure whether fsnotify_group concept should be reused
or avoided.
Per inode mark/mask filter should stay.

> struct fanotify_fastpath_event is indeed big. But I think
> we need to pass these information to the fastpath handler
> either way.

Disagree.
That was the old way of hooking bpf bits in.
uapi/bpf.h is full of such "context" structs.
xpd_md, bpf_tcp_sock, etc.
They pack fields into one struct only because
old style bpf has one input argument: ctx.
struct_ops doesn't have this limitation.
Pass things like path/dentry/inode/whatever pointers directly.
No need to pack into fanotify_fastpath_event.

> Overall, I think current fastpath design makes sense,
> though there are things we need to fix (as Amir and Alexei
> pointed out). Please let me know comments and suggestions
> on this.

On one side you're arguing that extra indirection for
inode local storage due to inode->i_secruity is needed
for performance,
but on the other side you're not worried about the deep
call stack of fsnotify->fanotify and argument packing
which add way more overhead than i_security hop.