On 3/21/21 8:54 AM, Eric W. Biederman wrote: > Jens Axboe <axboe@xxxxxxxxx> writes: > >> On 3/20/21 3:38 PM, Eric W. Biederman wrote: >>> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes: >>> >>>> On Sat, Mar 20, 2021 at 9:19 AM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> 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. > > I agree about signals. Especially because being able to use kill(2) > with the tid of thread is a linuxism and a backwards compatibility thing > from before we had CLONE_THREAD. > > I think for kill(2) we should just return -ESRCH. > > Thank you for providing the reasoning that is what I really saw missing > in the patches. The why. And software is difficult to maintain without > the why. Thanks Eric, I'll change that patch to -ESRCH and augment the commit message a bit. -- Jens Axboe