Re: [PATCH v2] vfs: shave work on failed file open

[Christian Brauner]

 On Fri, Sep 29, 2023 at 04:57:29PM -0700, Linus Torvalds wrote:
> On Fri, 29 Sept 2023 at 14:39, Mateusz Guzik <mjguzik@xxxxxxxxx> wrote:
> >
> > So to be clear, obtaining the initial count would require a dedicated
> > accessor.
> 
> Please, no.
> 
> Sequence numbers here are fundamentally broken, since getting that
> initial sequence number would involve either (a) making it something
> outside of 'struct file' itself or (b) require the same re-validation
> of the file pointer that the non-sequence number code needed in the
> first place.
> 
> We already have the right model in the only place that really matters
> (ie fd lookup). Using that same "validate file pointer after you got
> the ref to it" for the two or three other cases that didn't do it (and
> are simpler: the exec pointer in particular doesn't need the fdt
> re-validation at all).
> 
> The fact that we had some fd lookup that didn't do the full thing that
> a *real* fd lookup did is just bad. Let's fix it, not introduce a
> sequence counter that only adds more complexity.

I agree.

So I guess we're trying this. The appeneded patch now includes
documentation and renames *lookup_*_fd_rcu() to *lookup_*_fdget_rcu() to
reflect the refcount bump. It's now tentatively in vfs.misc (cf. [1])
and I've merged it into vfs.all to let -next chew on it. Please take a
close look and may the rcu gods be with us all...

[1]: git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git