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Date:   Fri, 12 Feb 2021 13:14:19 +0000
To:     Emil Velikov <emil.l.velikov@gmail.com>
From:   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>
Reply-To: Simon Ser <contact@emersion.fr>
Subject: Re: [PATCH v3] kcmp: Support selection of SYS_kcmp without CHECKPOINT_RESTORE
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On Friday, February 12th, 2021 at 1:57 PM, Emil Velikov <emil.l.velikov@gma=
il.com> wrote:

> On Fri, 5 Feb 2021 at 22:01, Chris Wilson <chris@chris-wilson.co.uk> wrot=
e:
> >
> > 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. devic=
e
> > 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.

[1]: https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/110