Re: [PATCH RFC] fhandle: expose u64 mount id to name_to_handle_at(2)

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2024-05-21 at 16:11 +0200, Christian Brauner wrote:
> On Tue, May 21, 2024 at 03:46:06PM +0200, Christian Brauner wrote:
> > On Mon, May 20, 2024 at 05:35:49PM -0400, Aleksa Sarai wrote:
> > > Now that we have stabilised the unique 64-bit mount ID interface in
> > > statx, we can now provide a race-free way for name_to_handle_at(2) to
> > > provide a file handle and corresponding mount without needing to worry
> > > about racing with /proc/mountinfo parsing.
> > > 
> > > As with AT_HANDLE_FID, AT_HANDLE_UNIQUE_MNT_ID reuses a statx AT_* bit
> > > that doesn't make sense for name_to_handle_at(2).
> > > 
> > > Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> > > ---
> > 
> > So I think overall this is probably fine (famous last words). If it's
> > just about being able to retrieve the new mount id without having to
> > take the hit of another statx system call it's indeed a bit much to
> > add a revised system call for this. Althoug I did say earlier that I
> > wouldn't rule that out.
> > 
> > But if we'd that then it'll be a long discussion on the form of the new
> > system call and the information it exposes.
> > 
> > For example, I lack the grey hair needed to understand why
> > name_to_handle_at() returns a mount id at all. The pitch in commit
> > 990d6c2d7aee ("vfs: Add name to file handle conversion support") is that
> > the (old) mount id can be used to "lookup file system specific
> > information [...] in /proc/<pid>/mountinfo".
> > 
> > Granted, that's doable but it'll mean a lot of careful checking to avoid
> > races for mount id recycling because they're not even allocated
> > cyclically. With lots of containers it becomes even more of an issue. So
> > it's doubtful whether exposing the mount id through name_to_handle_at()
> > would be something that we'd still do.
> > 
> > So really, if this is just about a use-case where you want to spare the
> > additional system call for statx() and you need the mnt_id then
> > overloading is probably ok.
> > 
> > But it remains an unpleasant thing to look at.
> 
> And I'd like an ok from Jeff and Amir if we're going to try this. :)

I don't have strong feelings about it other than "it looks sort of
ugly", so I'm OK with doing this.

I suspect we will eventually need name_to_handle_at2, or something
similar, as it seems like we're starting to grow some new use-cases for
filehandles, and hitting the limits of the old syscall. I don't have a
good feel for what that should look like though, so I'm happy to put
that off for a while.
-- 
Jeff Layton <jlayton@kernel.org>