From: Mike Rapoport <rppt@kernel.org>
To: Nadav Amit <nadav.amit@gmail.com>
Cc: David Hildenbrand <david@redhat.com>,
Mike Rapoport <rppt@linux.vnet.ibm.com>,
Andrea Arcangeli <aarcange@redhat.com>,
Peter Xu <peterx@redhat.com>, Linux-MM <linux-mm@kvack.org>
Subject: Re: userfaultfd: usability issue due to lack of UFFD events ordering
Date: Mon, 31 Jan 2022 12:42:45 +0200 [thread overview]
Message-ID: <Yfe9JS47vCQv6R1l@kernel.org> (raw)
In-Reply-To: <BF076F95-B5B3-41A7-8302-6F5D00E3AEC5@gmail.com>
Hi Nadav,
On Sat, Jan 29, 2022 at 10:23:55PM -0800, Nadav Amit wrote:
> Using userfautlfd and looking at the kernel code, I encountered a usability
> issue that complicates userspace UFFD-monitor implementation. I obviosuly
> might be wrong, so I would appreciate a (polite?) feedback. I do have a
> userspace workaround, but I thought it is worthy to share and to hear your
> opinion, as well as feedback from other UFFD users.
>
> The issue I encountered regards the ordering of UFFD events tbat might not
> reflect the actual order in which events took place.
>
> In more detail, UFFD events (e.g., unmap, fork) are not ordered against
> themselves [*]. The mm-lock is dropped before notifying the userspace
> UFFD-monitor, and therefore there is no guarantee as to whether the order of
> the events actually reflects the order in which the events took place.
> This can prevent a UFFD-monitor from using the events to track which
> ranges are mapped. Specifically, UFFD_EVENT_FORK message and a
> UFFD_EVENT_UNMAP message (which reflects unmap in the parent process) can
> be reordered, if the events are triggered by two different threads. In
> this case the UFFD-monitor cannot figure from the events whether the
> child process has the unmapped memory range still mapped (because fork
> happened first) or not.
Yeah, it seems that something like this is possible:
fork() munmap()
mmap_write_unlock();
mmap_write_lock_killable();
do_things();
mmap_{read,write}_unlock();
userfaultfd_unmap_complete();
dup_userfaultfd_complete();
A solution could be to split uffd_*_complete() to two parts: one that
queues up the event message and the second one that waits for it to be read
by the monitor. The first part then can run befor mm-lock is released.
If you can think of something nicer, it'll be really great!
> Obviously, it does not make sense to keep holding mm-lock while notifying the
> user, as it can even lead to deadlocks. Userspace UFFD-monitors can
> workaround this issue by using seccomp+ptrace instead of UFFD-events to
> obtain order of the events or examine /proc/[pid]/smaps. Yet, this introduces
> overheads, is complicated, and I doubt anyone does so. I wonder if the API is
> reasonable, or whether I am missing something.
>
> Thanks,
> Nadav
>
> [*] Note that I do not discuss UFFD-monitor issued ioctl's, but the order
> between UFFD-events.
>
--
Sincerely yours,
Mike.
next prev parent reply other threads:[~2022-01-31 10:42 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-30 6:23 userfaultfd: usability issue due to lack of UFFD events ordering Nadav Amit
2022-01-31 10:42 ` Mike Rapoport [this message]
2022-01-31 10:48 ` David Hildenbrand
2022-01-31 14:05 ` Mike Rapoport
2022-01-31 14:12 ` David Hildenbrand
2022-01-31 14:28 ` Mike Rapoport
2022-01-31 14:41 ` David Hildenbrand
2022-01-31 18:47 ` Mike Rapoport
2022-01-31 22:39 ` Nadav Amit
2022-02-01 9:10 ` Mike Rapoport
2022-02-10 7:48 ` Peter Xu
2022-02-10 18:42 ` Nadav Amit
2022-02-14 4:02 ` Peter Xu
2022-02-15 22:35 ` Nadav Amit
2022-02-16 8:27 ` Peter Xu
2022-02-17 21:15 ` Mike Rapoport
2022-01-31 17:23 ` Nadav Amit
2022-01-31 17:28 ` David Hildenbrand
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