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

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Message-ID: <3C93275E-B3B8-45CA-808E-0C163DBBB32F@vmware.com>
Date:   Wed, 20 Jul 2022 17:42:51 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Xu <peterx@...hat.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 Jul 19, 2022, at 7:32 PM, Peter Xu <peterx@...hat.com> wrote:

> ⚠ External Email
> 
> 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()?

autocorrect? :)

I did refer to pidfd_getfd() as a syscall that can be used today by one
process to control the address space of another process. I did not intend to
use it for the actual implementation.

> 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.

There are also some tricks you can do with ptrace in order not to need the
collaboration, but they are admittedly fragile.

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

Yes. The point that I was trying to make is that there are no open issues
with adding support for remote process control through
/proc/pid/userfaultfd. This is in contrast, for example, for using io-uring
with userfaultfd. For instance, if you try to use io-uring TODAY with
userfaultfd (without the async support that I need to add), and you try to
monitor the fork event, things would break (the new userfaultfd file
descriptor after fork would be installed on the io-worker thread).

This is all to say that it is really simple to add support for one process
monitoring userfaultfd of another process, since I understood that Axel had
concerned that this might be utterly broken…