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

From: Nadav Amit <namit@vmware.com>
To: Peter Xu <peterx@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"Dmitry V . Levin" <ldv@altlinux.org>,
	Gleb Fotengauer-Malinovskiy <glebfm@altlinux.org>,
	Hugh Dickins <hughd@google.com>, Jan Kara <jack@suse.cz>,
	Jonathan Corbet <corbet@lwn.net>,
	Mel Gorman <mgorman@techsingularity.net>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Mike Rapoport <rppt@kernel.org>, Shuah Khan <shuah@kernel.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Vlastimil Babka <vbabka@suse.cz>, zhangyi <yi.zhang@huawei.com>,
	"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux MM <linux-mm@kvack.org>,
	"linux-kselftest@vger.kernel.org"
	<linux-kselftest@vger.kernel.org>
Subject: Re: [PATCH v4 2/5] userfaultfd: add /dev/userfaultfd for fine grained access control
Date: Wed, 20 Jul 2022 17:42:51 +0000	[thread overview]
Message-ID: <3C93275E-B3B8-45CA-808E-0C163DBBB32F@vmware.com> (raw)
In-Reply-To: <YtdpQBrAGJwMnssj@xz-m1.local>

On Jul 19, 2022, at 7:32 PM, Peter Xu <peterx@redhat.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…