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. Eric
