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References: <20210205163752.11932-1-chris@chris-wilson.co.uk> <20210205220012.1983-1-chris@chris-wilson.co.uk>
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From:   Emil Velikov <emil.l.velikov@gmail.com>
Date:   Fri, 12 Feb 2021 12:57:59 +0000
Message-ID: <CACvgo52u1ASWXOuWuDwoXvbZhoq+RHn_GTxD5y9k+kO_dzmT7w@mail.gmail.com>
Subject: Re: [PATCH v3] kcmp: Support selection of SYS_kcmp without CHECKPOINT_RESTORE
To:     Chris Wilson <chris@chris-wilson.co.uk>
Cc:     "Linux-Kernel@Vger. Kernel. Org" <linux-kernel@vger.kernel.org>,
        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>,
        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, 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.

-Emil