Bug 214611 - UM: stdout output ceases under certain conditions

[Anton Ivanov]

On 06/10/2021 20:48, Glenn Washburn wrote:
> On Wed, 6 Oct 2021 19:53:48 +0100
> Anton Ivanov <anton.ivanov at kot-begemot.co.uk> wrote:
>
>> On 06/10/2021 19:05, Glenn Washburn wrote:
>>> On Wed, 6 Oct 2021 17:44:14 +0100
>>> Anton Ivanov <anton.ivanov at kot-begemot.co.uk> wrote:
>>>
>>>> On 06/10/2021 16:57, Anton Ivanov wrote:
>>>>> On 04/10/2021 17:54, Glenn Washburn wrote:
>>>>>> On Mon, 04 Oct 2021 14:48:34 +0200
>>>>>> Johannes Berg <johannes at sipsolutions.net> wrote:
>>>>>>
>>>>>>> On Sat, 2021-10-02 at 21:00 -0500, Glenn Washburn wrote:
>>>>>>>> Hi list,
>>>>>>>>
>>>>>>>> I'm notifying the list of a bug report[1] I created in the kernel
>>>>>>>> bugzilla. I'm not subscribed to this list, so please add this my email
>>>>>>>> in any replies to this email.
>>>>>>>> [1] https://bugzilla.kernel.org/show_bug.cgi?id=214611
>>>>>>> This really has nothing to do with UBD or something. What's happening is
>>>>>>> that you're using the command line badly.
>>>>>>>
>>>>>>> What do you expect this:
>>>>>>>
>>>>>>>      ... < <(cat /dev/null)
>>>>>>>
>>>>>>> to do?
>>>>>> This was just a way to trigger the issue I was seeing. I have a bash
>>>>>> script which was doing something like the following:
>>>>>>
>>>>>> grep "search" /path/to/file |
>>>>>> while read VAR; do
>>>>>>      run_some_script_which_eventually_runs_uml $VAR;
>>>>>> done
>>>>>>
>>>>>> I was confused why running this script caused UML to lose output always
>>>>>> when mounting the ubd in the UML mount script. And it didn't happen
>>>>>> when I ran "run_some_script_which_eventually_runs_uml" alone. Since the
>>>>>> amount of data returned by the grep was small, this issue was triggered
>>>>>> all the time. If the output were a lot of data, I might have noticed
>>>>>> that early runs of run_some_script_which_eventually_runs_uml would not
>>>>>> have output disappear after mounting. Thanks for debugging this.
>>>>>>
>>>>>>> What happens is that the shell creates a pipe. This pipe is connected on
>>>>>>> the one side to fd:1 in UML (stdin) and on the other to stdout of 'cat'.
>>>>>>>
>>>>>>> Now this is all fine, but 'cat' will *quit immediately* since it cannot
>>>>>>> read anything from /dev/null (it's write-only!).
>>>>>>>
>>>>>>> Therefore, the fd:1 in UML will be invalidated pretty much immediately,
>>>>>>> receiving EPOLLHUP.
>>>>>>>
>>>>>>> This is detected by the epoll code, raising an interrupt into the line
>>>>>>> level code, and the line code then closes the stdio console channel
>>>>>>> entirely, including stdout.
>>>>>> This seems like it could be a bug. Couldn't the console not be closed,
>>>>>> but the console handling code internally mark stdin as closed? Perhaps
>>>>>> there could even be logic to detect if stdin and stdout are from the
>>>>>> same fd, then close the console, otherwise don't. From a user
>>>>>> perspective, thinking of UML as a normal process, it doesn't make sense
>>>>>> that closing stdin would close stdout as well.
>>>>> There is an even more convoluted case where the stdin is a socket (which
>>>>> is possible - you pass it to UML as a fd:N). That can be half-closed.
>>>>>
>>>>> Looking at it at the moment, but to be honest, separating the logic for in
>>>>> and out if the fd is the same is going to be quite difficult (if at all
>>>>> possible). It all ends as EPOLL events at the bottom. Even if you handle IN
>>>>> and OUT separately in the upper layers, the kernel will handle them as the
>>>>> same fd and any event (f.e. closure) will show up on both.
>>>> Further to this, the same holds even if we start playing games with multiple
>>>> EPOLL descriptors, dup-ing fds, etc, the event will still show up on all of
>>>> them.
>>> Thanks for looking into this. If I'm understanding correctly, you're
>>> looking at the case where the UML process has STDIN and STDOUT to the
>>> same file descriptor. However, the situation is when STDIN is to a pipe
>>> that gets closed and STDOUT is to something else (pty, tty, file,
>>> different pipe, etc..). Does your logic still hold true in this case?
>> No. They should be on different IRQs.
>>
>> Question:
>>
>> Have you tried using con0=null,fd:1 ?
>>
>> Assign null explicitly to the input instead of a fd which is closed?
>>
>> A
> I just tried that and it does not trigger the bug, which I'd expect.
> This would be another work around, but I think it would be good to fix
> the bug. What if you want to pipe some data to stdin? Then when program
> on the write side of the pipe exits because its done sending data, the
> UML will stop sending data to stdout because the pipe gets closed.  How
> hard do you think this would be to fix?

I will see if we can do something about it.

I'd rather have this as an option instead of always enabled, because 
having it always on will break error handling elsewhere.

>
> Glenn
>
>>> Glenn
>>>
>>>> I frankly do not have any ideas at present to solve this. This is just
>>>> the way it is - a file close event for one fd will show up on all instances
>>>> where it is mentioned.
>>>>
>>>> A.
>>>>
>>>>> A.
>>>>>
>>>>>>> If anything, the bug is that when you're not causing enough interrupts
>>>>>>> by using ubd, somehow this situation doesn't get detected, and the
>>>>>>> console remains open, so you still see the output... I think this might
>>>>>>> be if closing the FD didn't generate a SIGIO?
>>>>>> This leads to strange behavior. But for this issue, I think the
>>>>>> suggestion above would obviate the need to do anything about this.
>>>>>>
>>>>>>> In fact, if you generate SIGIO in *any* other way, including pressing
>>>>>>> enter while the script is running even if stdin is redirected from your
>>>>>>> dead cat [1], you still get the same behaviour of the channel getting
>>>>>>> closed.
>>>>>> Ok, I've confirmed that. I think that's another reason to fix this in
>>>>>> a manner that doesn't tie stdin to stdout, just hitting enter can make
>>>>>> outut disappear (non-intuitive). In my case, stdout was going to a file.
>>>>>> So my intuition would say that stdin (from a pipe) and stdout (to a
>>>>>> file) shouldn't be connected (yes a program can create any kind of
>>>>>> connection it wants, but it's not intuitive). It would be like rsync
>>>>>> -av dir1 dir2 < <(cat /dev/null) having its output disappear if you
>>>>>> pressed enter in the middle of the run.
>>>>>>
>>>>>>> johannes
>>>>>>>
>>>>>>> [1] did I really just write that? heh.
>>>>>> Thanks, got a chuckle out of that.
>>>>>>
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>>>>>>
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-- 
Anton R. Ivanov
https://www.kot-begemot.co.uk/