linux-kernel - Re: [PATCH v3] kcmp: Support selection of SYS_kcmp without CHECKPOINT

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Message-ID: <CACvgo50Bki0GKW0SNjzn9zieWqHO=curbVhkL0ALpA_d2E5bfw@mail.gmail.com>
Date:   Fri, 12 Feb 2021 14:07:23 +0000
From:   Emil Velikov <emil.l.velikov@...il.com>
To:     Simon Ser <contact@...rsion.fr>
Cc:     Chris Wilson <chris@...is-wilson.co.uk>,
        Will Drewry <wad@...omium.org>,
        Kees Cook <keescook@...omium.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        ML dri-devel <dri-devel@...ts.freedesktop.org>,
        Andy Lutomirski <luto@...capital.net>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        "# 3.13+" <stable@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3] kcmp: Support selection of SYS_kcmp without CHECKPOINT_RESTORE

On Fri, 12 Feb 2021 at 13:14, Simon Ser <contact@...rsion.fr> wrote:
>
> On Friday, February 12th, 2021 at 1:57 PM, Emil Velikov <emil.l.velikov@...il.com> wrote:
>
> > On Fri, 5 Feb 2021 at 22:01, Chris Wilson <chris@...is-wilson.co.uk> wrote:
> > >
> > > Userspace has discovered the functionality offered by SYS_kcmp and has
> > > started to depend upon it. In particular, Mesa uses SYS_kcmp for
> > > os_same_file_description() in order to identify when two fd (e.g. device
> > > or dmabuf)
> >
> > As you rightfully point out, SYS_kcmp is a bit of a two edged sword.
> > While you mention the CONFIG issue, there is also a portability aspect
> > (mesa runs on more than just linux) and as well as sandbox filtering
> > of the extra syscall.
> >
> > Last time I looked, the latter was still an issue and mesa was using
> > SYS_kcmp to compare device node fds.
> > A far shorter and more portable solution is possible, so let me
> > prepare a Mesa patch.
>
> Comparing two DMA-BUFs can be done with their inode number, I think.
>
> Comparing two device FDs is more subtle, because of GEM handle
> ref'counting. You sometimes really want to check whether two FDs are
> backed by the same file *description*. See [1] for details.
>
Thanks for the correction and the reference.
Seems like I've short circuited file description table vs file descriptor.

Emil