linux-kernel - Re: [PATCH v4 0/5] userfaultfd: add /dev/userfaultfd for fine grained access control

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Date:   Mon, 1 Aug 2022 23:19:48 +0000
From:   Nadav Amit <namit@...are.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
CC:     "Schaufler, Casey" <casey.schaufler@...el.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>, Peter Xu <peterx@...hat.com>,
        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@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v4 0/5] userfaultfd: add /dev/userfaultfd for fine grained
 access control

On Aug 1, 2022, at 3:50 PM, Axel Rasmussen <axelrasmussen@...gle.com> wrote:

> ⚠ External Email
> 
> On Mon, Aug 1, 2022 at 12:53 PM Nadav Amit <namit@...are.com> wrote:
>> On Aug 1, 2022, at 10:13 AM, Axel Rasmussen <axelrasmussen@...gle.com> wrote:
> 
> Ah, that I think is more or less what my series already proposes, if I
> understand you correctly.
> 
> The usage is:
> 
> fd = open(/dev/userfaultfd) /* This FD is only useful for creating new
> userfaultfds */
> uffd = ioctl(fd, USERFAULTFD_IOC_NEW) /* Now you get a real uffd */
> close(fd); /* No longer needed now that we have a real uffd */
> 
> /* Use uffd to register, COPY, CONTINUE, whatever */
> 
> One thing we could do now or in the future is extend
> USERFAULTFD_IOC_NEW to take a pid as an argument, to support creating
> uffds for remote processes.
> 
> 
> 
> And then we get the benefit of permissions for /dev nodes working very
> naturally - they default to root, but can be configured by the
> sysadmin via chown/chmod, or udev rules, or whatever.

Oh. Stupid me. Then yes, using the /dev/userfaultfd is in line with other
usage models, such as KVM. And reading from each file descriptor is indeed
providing different output.