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From:   Emil Velikov <emil.l.velikov@gmail.com>
Date:   Fri, 12 Feb 2021 14:07:23 +0000
Message-ID: <CACvgo50Bki0GKW0SNjzn9zieWqHO=curbVhkL0ALpA_d2E5bfw@mail.gmail.com>
Subject: Re: [PATCH v3] kcmp: Support selection of SYS_kcmp without CHECKPOINT_RESTORE
To:     Simon Ser <contact@emersion.fr>
Cc:     Chris Wilson <chris@chris-wilson.co.uk>,
        Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        "Linux-Kernel@Vger. Kernel. Org" <linux-kernel@vger.kernel.org>,
        ML dri-devel <dri-devel@lists.freedesktop.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        "# 3.13+" <stable@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
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On Fri, 12 Feb 2021 at 13:14, Simon Ser <contact@emersion.fr> wrote:
>
> On Friday, February 12th, 2021 at 1:57 PM, Emil Velikov <emil.l.velikov@gmail.com> wrote:
>
> > On Fri, 5 Feb 2021 at 22:01, Chris Wilson <chris@chris-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