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Biederman" , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: RFC: Use of user space handler vs. SIG_DFL on forced signals Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/2.1.4 (2021-12-11) Precedence: bulk List-ID: X-Mailing-List: linux-perf-users@vger.kernel.org On Tue, Mar 22, 2022 at 02:25PM +0100, Dmitry Vyukov wrote: > On Tue, 22 Mar 2022 at 11:42, Marco Elver wrote: > > > > Hello, > > > > Currently force_sig_info_to_task() will always unblock a blocked signal > > but deliver the signal to SIG_DFL: > > > > [...] > > * Note: If we unblock the signal, we always reset it to SIG_DFL, > > * since we do not want to have a signal handler that was blocked > > * be invoked when user space had explicitly blocked it. > > [...] > > > > Is this requirement part of the POSIX spec? Or is the intent simply to > > attempt to do the least-bad thing? > > > > The reason I'm asking is that we've encountered rare crashes with the > > new SIGTRAP on perf events, due to patterns like this: > > > > > > ... > > sigset_t s; > > sigemptyset(&s); > > sigaddset(&s, SIGTRAP | ); > > sigprocmask(SIG_BLOCK, &s, ...); > > ... > > > > > > When the perf event triggers, while SIGTRAP is blocked, force_sig_perf() > > will force the signal, but revert back to the default handler, thus > > terminating the task. > > > > For other types of signals, is the assumption here that if user space > > blocked the signal, it might not be able to handle it in the first > > place? > > > > For SIGTRAP on perf events we found this makes the situation worse, > > since the cause of the signal wasn't an error condition, but explicitly > > requested monitoring. In this case, we do in fact want delivery of the > > signal to user space even if the signal is blocked, i.e. > > force_sig_perf() should be an unblockable forced synchronous signal to > > user space! > > > > If there is no good reason to choose SIG_DFL, our preference would be to > > allow this kind of "unblockable forced" signal to the user space handler > > for force_sig_perf() -- with the caveat whoever requests SIGTRAP on perf > > events must be able to provide a handler that can always run safely. But > > we think that's better than crashing. > > > > The below patch would do what we want, but would like to first confirm > > if this is "within spec". > > > > Thoughts? > > > > Thanks, > > -- Marco > > > > ------ >8 ------ [...] > > @@ -1332,7 +1335,8 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, > > ignored = action->sa.sa_handler == SIG_IGN; > > blocked = sigismember(&t->blocked, sig); > > if (blocked || ignored || (handler != HANDLER_CURRENT)) { > > - action->sa.sa_handler = SIG_DFL; > > + if (handler != HANDLER_UNBLOCK) > > + action->sa.sa_handler = SIG_DFL; > > if (handler == HANDLER_EXIT) > > action->sa.sa_flags |= SA_IMMUTABLE; > > if (blocked) { > > @@ -1816,7 +1820,11 @@ int force_sig_perf(void __user *addr, u32 type, u64 sig_data) > > info.si_perf_data = sig_data; > > info.si_perf_type = type; > > > > - return force_sig_info(&info); > > + /* > > + * Delivering SIGTRAP on perf events must unblock delivery to not > > + * kill the task, but attempt delivery to the user space handler. > > + */ > > + return force_sig_info_to_task(&info, current, HANDLER_UNBLOCK); > > It seems that in this case we almost don't use any of the logic in > force_sig_info_to_task(). It effectively reduces to the call to > send_signal() protected by the lock. Maybe we should call something > like do_send_sig_info() directly? Unfortunately not -- without this: [...] blocked = sigismember(&t->blocked, sig); if (blocked || ignored || (handler != HANDLER_CURRENT)) { [...] if (blocked) { sigdelset(&t->blocked, sig); recalc_sigpending_and_wake(t); } } [...] , it doesn't work if blocked==true. The alternative is to introduce another helper, force_sig_info_unblockable() or something, but don't see the benefit. Having it all in force_sig_info_to_task() seems cleaner and we avoid replicating any unblock logic for forced signals. Thanks, -- Marco