Re: [PATCH v2 3/3] pidfs: implement file handle support

Christian Brauner
2024-11-14T14:13:17Z

On Thu, Nov 14, 2024 at 02:13:06PM +0100, Erin Shepherd wrote:
> On 14/11/2024 13:52, Christian Brauner wrote:
>
> > I think you need at least something like the following completely
> > untested draft on top:
> >
> > - the pidfs_finish_open_by_handle_at() is somewhat of a clutch to handle
> > thread vs thread-group pidfds but it works.
> >
> > - In contrast to pidfd_open() that uses dentry_open() to create a pidfd
> > open_by_handle_at() uses file_open_root(). That's overall fine but
> > makes pidfds subject to security hooks which they aren't via
> > pidfd_open(). It also necessitats pidfs_finish_open_by_handle_at().
> > There's probably other solutions I'm not currently seeing.
>
> These two concerns combined with the special flag make me wonder if pidfs
> is so much of a special snowflake we should just special case it up front
> and skip all of the shared handle decode logic?

Care to try a patch and see what it looks like?

>
> > - The exportfs_decode_fh_raw() call that's used to decode the pidfd is
> > passed vfs_dentry_acceptable() as acceptability callback. For pidfds
> > we don't need any of that functionality and we don't need any of the
> > disconnected dentry handling logic. So the easiest way to fix that is
> > to rely on EXPORT_OP_UNRESTRICTED_OPEN to skip everything. That in
> > turns means the only acceptability we have is the nop->fh_to_dentry()
> > callback for pidfs.
>
> With the current logic we go exportfs_decode_fh_raw(...) ->
> find_acceptable_alias(result) -> vfs_dentry_acceptable(context, result).
> vfs_dentry_acceptable immediately returns 1 if ctx->flags is 0, which will
> always be the case if EXPORT_OP_UNRESTRICTED_OPEN was set, so we immediately
> fall back out of the call tree and return result.
>
> So I'm not 100% sure we actually need this special case but I'm not opposed.

Oh right, I completely forgot that find_acceptable_alias() is a no-op
for pidfs.