Re: [PATCH v2] vfs: Tighten up linkat(..., AT_EMPTY_PATH)

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, Aug 21, 2013 at 12:14 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>
> So let's be careful for now: only allow linkat(..., AT_EMPTY_PATH)
> if the target is I_LINKABLE.

So I really detest this just because it's such a special case. Now
this is only useful for that one special case, and the thing very
fundamentally checks that one special case in a place that is
impossible to check for the /proc case, so the proc case remains
totally separate.

Which just bothers me.

I think we could easily at least allow the "file->f_creds ==
current->creds" case (and yes, I literally mean comparing the pointers
- not only is it cheaper, but it literally means "nothing odd has
happened in between opening and the lookup").

And I'm wondering if we shouldn't actually do that at "path_init"
time. Right now the code says:

                /* Caller must check execute permissions on the
starting path component */
                struct fd f = fdget_raw(dfd);

and then uses the struct file mindlessly.

I'm wondering if we should just do some validation in that place, and say:

 - for directories, we require exec permissions here
 - for everything else, we require that f->f_cred == current->cred check.

I dunno. But that I_LINKABLE thing just bothers me. It screams "I'm
hacky" to me.

               Linus