From: Jens Axboe <[email protected]>
To: "Eric W. Biederman" <[email protected]>,
Linus Torvalds <[email protected]>
Cc: io-uring <[email protected]>,
Linux Kernel Mailing List <[email protected]>,
Oleg Nesterov <[email protected]>,
Stefan Metzmacher <[email protected]>
Subject: Re: [PATCH 1/2] signal: don't allow sending any signals to PF_IO_WORKER threads
Date: Sat, 20 Mar 2021 16:42:09 -0600 [thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
On 3/20/21 3:38 PM, Eric W. Biederman wrote:
> Linus Torvalds <[email protected]> writes:
>
>> On Sat, Mar 20, 2021 at 9:19 AM Eric W. Biederman <[email protected]> wrote:
>>>
>>> The creds should be reasonably in-sync with the rest of the threads.
>>
>> It's not about credentials (despite the -EPERM).
>>
>> It's about the fact that kernel threads cannot handle signals, and
>> then get caught in endless loops of "if (sigpending()) return
>> -EAGAIN".
>>
>> For a normal user thread, that "return -EAGAIN" (or whatever) will end
>> up returning an error to user space - and before it does that, it will
>> go through the "oh, returning to user space, so handle signal" path.
>> Which will clear sigpending etc.
>>
>> A thread that never returns to user space fundamentally cannot handle
>> this. The sigpending() stays on forever, the signal never gets
>> handled, the thread can't do anything.
>>
>> So delivering a signal to a kernel thread fundamentally cannot work
>> (although we do have some threads that explicitly see "oh, if I was
>> killed, I will exit" - think things like in-kernel nfsd etc).
>
> I agree that getting a kernel thread to receive a signal is quite
> tricky. But that is not what the patch affects.
>
> The patch covers the case when instead of specifying the pid of the
> process to kill(2) someone specifies the tid of a thread. Which implies
> that type is PIDTYPE_TGID, and in turn the signal is being placed on the
> t->signal->shared_pending queue. Not the thread specific t->pending
> queue.
>
> So my question is since the signal is delivered to the process as a
> whole why do we care if someone specifies the tid of a kernel thread,
> rather than the tid of a userspace thread?
Right, that's what this first patch does, and in all honesty, it's not
required like the 2/2 patch is. I do think it makes it more consistent,
though - the threads don't take signals, period. Allowing delivery from
eg kill(2) and then pass it to the owning task of the io_uring is
somewhat counterintuitive, and differs from earlier kernels where there
was no relationsship between that owning task and the async worker
thread.
That's why I think the patch DOES make sense. These threads may share a
personality with the owning task, but I don't think we should be able to
manipulate them from userspace at all. That includes SIGSTOP, of course,
but also regular signals.
Hence I do think we should do something like this.
--
Jens Axboe
next prev parent reply other threads:[~2021-03-20 22:42 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-20 15:38 [PATCHSET 0/2] PF_IO_WORKER signal tweaks Jens Axboe
2021-03-20 15:38 ` [PATCH 1/2] signal: don't allow sending any signals to PF_IO_WORKER threads Jens Axboe
2021-03-20 16:18 ` Eric W. Biederman
2021-03-20 17:56 ` Linus Torvalds
2021-03-20 21:38 ` Eric W. Biederman
2021-03-20 22:42 ` Jens Axboe [this message]
2021-03-21 14:54 ` Eric W. Biederman
2021-03-21 15:40 ` Jens Axboe
2021-03-20 15:38 ` [PATCH 2/2] signal: don't allow STOP on " Jens Axboe
2021-03-20 16:21 ` Eric W. Biederman
2021-03-22 16:18 ` Oleg Nesterov
2021-03-22 16:15 ` Oleg Nesterov
2021-03-20 16:26 ` [PATCHSET 0/2] PF_IO_WORKER signal tweaks Eric W. Biederman
2021-03-20 17:51 ` Linus Torvalds
2021-03-20 19:18 ` Linus Torvalds
2021-03-20 22:08 ` Eric W. Biederman
2021-03-20 22:53 ` Jens Axboe
2021-03-21 15:18 ` Eric W. Biederman
2021-03-21 15:42 ` Jens Axboe
2021-03-20 22:56 ` Jens Axboe
2021-03-20 17:05 ` kernel test robot
2021-03-20 17:05 ` kernel test robot
2021-03-20 19:10 ` kernel test robot
2021-03-22 16:05 ` Oleg Nesterov
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