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    SubjectRe: strace of io_uring events?
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    On 7/21/20 1:44 PM, Andy Lutomirski wrote:
    > On Tue, Jul 21, 2020 at 11:39 AM Jens Axboe <axboe@kernel.dk> wrote:
    >>
    >> On 7/21/20 11:44 AM, Andy Lutomirski wrote:
    >>> On Tue, Jul 21, 2020 at 10:30 AM Jens Axboe <axboe@kernel.dk> wrote:
    >>>>
    >>>> On 7/21/20 11:23 AM, Andy Lutomirski wrote:
    >>>>> On Tue, Jul 21, 2020 at 8:31 AM Jens Axboe <axboe@kernel.dk> wrote:
    >>>>>>
    >>>>>> On 7/21/20 9:27 AM, Andy Lutomirski wrote:
    >>>>>>> On Fri, Jul 17, 2020 at 1:02 AM Stefano Garzarella <sgarzare@redhat.com> wrote:
    >>>>>>>>
    >>>>>>>> On Thu, Jul 16, 2020 at 08:12:35AM -0700, Kees Cook wrote:
    >>>>>>>>> On Thu, Jul 16, 2020 at 03:14:04PM +0200, Stefano Garzarella wrote:
    >>>>>>>
    >>>>>>>>> access (IIUC) is possible without actually calling any of the io_uring
    >>>>>>>>> syscalls. Is that correct? A process would receive an fd (via SCM_RIGHTS,
    >>>>>>>>> pidfd_getfd, or soon seccomp addfd), and then call mmap() on it to gain
    >>>>>>>>> access to the SQ and CQ, and off it goes? (The only glitch I see is
    >>>>>>>>> waking up the worker thread?)
    >>>>>>>>
    >>>>>>>> It is true only if the io_uring istance is created with SQPOLL flag (not the
    >>>>>>>> default behaviour and it requires CAP_SYS_ADMIN). In this case the
    >>>>>>>> kthread is created and you can also set an higher idle time for it, so
    >>>>>>>> also the waking up syscall can be avoided.
    >>>>>>>
    >>>>>>> I stared at the io_uring code for a while, and I'm wondering if we're
    >>>>>>> approaching this the wrong way. It seems to me that most of the
    >>>>>>> complications here come from the fact that io_uring SQEs don't clearly
    >>>>>>> belong to any particular security principle. (We have struct creds,
    >>>>>>> but we don't really have a task or mm.) But I'm also not convinced
    >>>>>>> that io_uring actually supports cross-mm submission except by accident
    >>>>>>> -- as it stands, unless a user is very careful to only submit SQEs
    >>>>>>> that don't use user pointers, the results will be unpredictable.
    >>>>>>
    >>>>>> How so?
    >>>>>
    >>>>> Unless I've missed something, either current->mm or sqo_mm will be
    >>>>> used depending on which thread ends up doing the IO. (And there might
    >>>>> be similar issues with threads.) Having the user memory references
    >>>>> end up somewhere that is an implementation detail seems suboptimal.
    >>>>
    >>>> current->mm is always used from the entering task - obviously if done
    >>>> synchronously, but also if it needs to go async. The only exception is a
    >>>> setup with SQPOLL, in which case ctx->sqo_mm is the task that set up the
    >>>> ring. SQPOLL requires root privileges to setup, and there's no task
    >>>> entering the io_uring at all necessarily. It'll just submit sqes with
    >>>> the credentials that are registered with the ring.
    >>>
    >>> Really? I admit I haven't fully followed how the code works, but it
    >>> looks like anything that goes through the io_queue_async_work() path
    >>> will use sqo_mm, and can't most requests that end up blocking end up
    >>> there? It looks like, even if SQPOLL is not set, the mm used will
    >>> depend on whether the request ends up blocking and thus getting queued
    >>> for later completion.
    >>>
    >>> Or does some magic I missed make this a nonissue.
    >>
    >> No, you are wrong. The logic works as I described it.
    >
    > Can you enlighten me? I don't see any iov_iter_get_pages() calls or
    > equivalents. If an IO is punted, how does the data end up in the
    > io_uring_enter() caller's mm?

    If the SQE needs to be punted to an io-wq worker, then
    io_prep_async_work() is ultimately called before it's queued with the
    io-wq worker. That grabs anything we need to successfully process this
    request, user access and all. io-wq then assumes the right "context" to
    performn that request. As the async punt is always done on behalf of the
    task that is submitting the IO (via io_uring_enter()), that is the
    context that we grab and use for that particular request.

    You keep looking at ctx->sqo_mm, and I've told you several times that
    it's only related to the SQPOLL thread. If you don't use SQPOLL, no
    request will ever use it.

    --
    Jens Axboe

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