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. -- Jens Axboe
