Re: [PATCH RFC v2] fhandle: expose u64 mount id to name_to_handle_at(2)

[Trond Myklebust <trondmy@hammerspace.com>]

On Sun, 2024-05-26 at 02:25 -0700, Christoph Hellwig wrote:
> On Thu, May 23, 2024 at 01:57:32PM -0700, Aleksa Sarai wrote:
> > Now that we provide a 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.
> 
> file handles are not tied to mounts, they are tied to super_blocks,
> and they can survive reboots or (less relevant) remounts.  This thus
> seems like a very confusing if not wrong interfaces.

I assume the reason is to give the caller a race free way to figure out
which submount the path resolves to. The problem is that nothing stops
another process from calling umount() before you're done parsing
/proc/mountinfo and have resolved the mount id.

If we're looking to change the API, then perhaps returning a file
descriptor might be a better alternative?
Most userland NFS servers are in any case going to follow up obtaining
the filehandle with a stat() or even a full blown open() in order to
get file attributes, set up file state, etc. By returning an open file
descriptor to the resolved file (even if it is only an O_PATH
descriptor) we could accelerate those operations in addition to solving
the umount() race.

Alternatively, just remove the path argument altogether, and require
the descriptor argument to be an O_PATH or regular open file descriptor
that resolves to the file we want to get a filehandle for. However this
would require a userland NFS server to generally do a
open_by_handle_at() to resolve the parent directory handle, then do an
openat(O_PATH) to get the file to look up, before being able to call
the name_to_handle_at() replacement.
i.e. there would be 1 extra syscall.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com