On Tue, Jul 12, 2016 at 08:57:53AM +0300, Konstantin Belousov wrote: > On Mon, Jul 11, 2016 at 06:19:38PM -0700, Mark Johnston wrote: > > Hi, > > > > It seems to be possible for ptrace(PT_ATTACH) to race with the delivery > > of a signal to the same process. ptrace(PT_ATTACH) sets P_TRACED and > > sends SIGSTOP to a thread in the target process. Consider the case where > > a signal is delivered to a second thread, and both threads are executing > > ast() concurrently. The two threads will both call issignal() and from > > there call ptracestop() because P_TRACED is set, though they will be > > serialized by the proc lock. If the thread receiving SIGSTOP wins the > > race, it will suspend first and set p->p_xthread. The second thread will > > also suspend in ptracestop(), overwriting the p_xthread field set by the > > first thread. Later, ptrace(PT_DETACH) will unsuspend the threads, but > > it will set td->td_xsig only in the second thread. This means that the > > first thread will return SIGSTOP from ptracestop() and subsequently > > suspend the process, which seems rather incorrect. > Why ? In particular, why delivering STOP after attach, in the described > situation, is perceived as incorrect ? Parallel STOPs, one from attach, > and other from kill(2), must result in two stops.
I suppose it is not strictly incorrect. I find it surprising that a PT_ATTACH followed by a PT_DETACH may leave the process in a different state than it was in before the attach. This means that it is not possible to gcore a process without potentially leaving it stopped, for instance. This result may occur in a single-threaded process as well, since a signal may already be queued when the PT_ATTACH handler sends SIGSTOP. To me it just seems a bit strange that ptrace's mechanism for stopping the target - sending SIGSTOP - interacts this way with ptrace's handling of signals - ptracestop()). Specifically, PT_ATTACH does not rely on the SA_STOP property of SIGSTOP to stop the process, but rather on the special signal handling in ptracestop(). > > The bit about overwriting p_xsig/p_xthread indeed initially sound worrysome, > but probably not too much. The only consequence of reassigning p_xthread > is the selection of the 'lead' thread in sys_process.c, it seems. > > > > > The above is just a theory to explain an unexpectedly-stopped > > multi-threaded process that I've observed. Is there some mechanism I'm > > missing that prevents multiple threads from suspending in ptracestop() > > at the same time? If not, then I think that's the root of the problem, > > since p_xthread is pretty clearly not meant to be overwritten this way. > Again, why ? > > Note the comment > * Just make wait() to work, the last stopped thread > * will win. > which seems to point to the situation. Indeed, I somehow missed that. I had assumed that the leaked TDB_XSIG represented a bug in ptracestop(). > > > Moreover, in my scenario I see a thread with TDB_XSIG set even after > > ptrace(PT_DETACH) was called (P_TRACED is cleared). > This is interesting, we indeed do not clear the flag consistently. > But again, the only consequence seems to be a possible invalid reporting > of events. _______________________________________________ email@example.com mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"