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From:   Daniel Colascione <dancol@google.com>
Date:   Tue, 5 Nov 2019 08:41:18 -0800
Message-ID: <CAKOZuet=g++G+biSP5bU-Rppu6fykU1TVUDj20NapqAYQY4r9A@mail.gmail.com>
Subject: Re: [PATCH 1/1] userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT_FORK
To:     Andrea Arcangeli <aarcange@redhat.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jann Horn <jannh@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Lokesh Gidra <lokeshgidra@google.com>,
        Nick Kralevich <nnk@google.com>,
        Nosh Minwalla <nosh@google.com>,
        Pavel Emelyanov <ovzxemul@gmail.com>,
        Tim Murray <timmurray@google.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>
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On Tue, Nov 5, 2019 at 8:24 AM Andrea Arcangeli <aarcange@redhat.com> wrote:
> The long term plan is to introduce UFFD_FEATURE_EVENT_FORK2 feature
> flag that uses the ioctl to receive the child uffd, it'll consume more
> CPU, but it wouldn't require the PTRACE privilege anymore.

Why not just have callers retrieve FDs using recvmsg? This way, you
retrieve the message packet and the file descriptor at the same time
and you don't need any appreciable extra CPU use.