[PATCH v4 1/8] server: Actually set initial status in set_async_direct_result handler.

[Jin-oh Kang]

On Sat, Mar 5, 2022, 4:05 AM Jinoh Kang <jinoh.kang.kr at gmail.com> wrote:

> On 3/5/22 02:47, Zebediah Figura wrote:
> > Right, but I think it's possible to trigger this assert now. For a
> rather pathological case, consider a client which calls
> set_async_direct_result on an async which is currently being serviced by a
> device driver and has not yet reached get_next_device_request.
>
> For that to be possible, something has to terminate the async so that
> set_async_direct_result does not reject it.
> The easiest way to do so is to cancel it, or make it timeout somehow.
>
> In fact, I think we have discovered an _existing_ bug: the client can use
> set_async_direct_result to interfere with cancelled asyncs (device-backed
> or not).
>
> Further investigation revealed some other bugs, which is tangentially
> related:
>
> 1. An async may time out before the client calls set_async_direct_result
> to report back the status of initial I/O.  In this case, the timeout is
> ignored.
> 2. There is a time window where alerted asyncs ignore IoCancel/IoCancelEx
> requests.  If async_terminate( async, STATUS_ALERTED ) is called, the async
> will ignore any cancellation requests until the APC_ASYNC_IO routine
> returns.
>    Note that this bug has existed *before* set_async_direct_result, and I
> think this one may need to be addressed first.
>
> So what I think we need here is a mechanism to tell alerted asyncs apart
> from irreversibly-terminated asyncs.
>

To make it clear, it should be "restartable" vs. "non-restartable" asyncs.
See below...

This will solve all of the problems above:
>
> - It's no longer possible to call set_async_direct_result on a device
> async in the first place, since it will now reject irreversibly-terminated
> asyncs while still accepting alerted asyncs (which is not possible for
> device asyncs).
>

Actually it's possible for devive asyncs to be in alerted state, but note
that it is *not* supposed to be restartable.

- Timeout and cancellation works as expected: set_async_direct_result will
> treat timed out asyncs as irreversibly-terminated.  (the client needs to be
> modified to handle this case explicitly, though.)
>
> Any thoughts?
>
> >
> > (And as mentioned, we should probably be calling
> async_set_initial_status() in async_set_result, in which case this could be
> triggered much less pathologically by the controlling process dying.)
>
> That has been split off as patch 2/8.
>
>
> --
> Sincerely,
> Jinoh Kang
>
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