linux-kernel - Re: [PATCH v4 2/5] userfaultfd: add /dev/userfaultfd for fine grained access control

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Date:   Tue, 19 Jul 2022 22:32:32 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Nadav Amit <namit@...are.com>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Dmitry V . Levin" <ldv@...linux.org>,
        Gleb Fotengauer-Malinovskiy <glebfm@...linux.org>,
        Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>, Shuah Khan <shuah@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        zhangyi <yi.zhang@...wei.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v4 2/5] userfaultfd: add /dev/userfaultfd for fine
 grained access control

On Tue, Jul 19, 2022 at 11:55:21PM +0000, Nadav Amit wrote:
> Anyhow, I do want to clarify a bit about the “cross-process support”
> userfaultfd situation. Basically, you can already get cross-process support
> today, by using calling userfaultfd() on the controlled process and calling
> pidfd_open() from another process. It does work and I do not remember any
> issues that it introduced (in contrast, for instance, to io-uring, that
> would break if you use userfaultfd+iouring+fork today).

Do you mean to base it on pidof_getfd()?

Just want to mention that this will still need collaboration of the target
process as userfaultfd needs to be created explicitly there.  From that POV
it's still more similar to general SCM_RIGHTS trick to pass over the fd but
just to pass it in a different way.

IMHO the core change about having /proc/pid/userfaultfd is skipping that
only last step to create the handle.

-- 
Peter Xu