linux-kernel - Re: [PATCH 1/1] userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT

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Message-Id: <20191107083902.GB3247@linux.ibm.com>
Date:   Thu, 7 Nov 2019 10:39:03 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Nick Kralevich <nnk@...gle.com>,
        Nosh Minwalla <nosh@...gle.com>,
        Pavel Emelyanov <ovzxemul@...il.com>,
        Tim Murray <timmurray@...gle.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 1/1] userfaultfd: require CAP_SYS_PTRACE for
 UFFD_FEATURE_EVENT_FORK

Hi Daniel,

On Tue, Nov 05, 2019 at 08:41:18AM -0800, Daniel Colascione wrote:
> On Tue, Nov 5, 2019 at 8:24 AM Andrea Arcangeli <aarcange@...hat.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.

I don't follow you here. Can you elaborate on how recvmsg would be used in
this case?

-- 
Sincerely yours,
Mike.