On Mon, Apr 4, 2022 at 1:30 PM Marco Elver <elver@xxxxxxxxxx> wrote: > With SIGTRAP on perf events, we have encountered termination of > processes due to user space attempting to block delivery of SIGTRAP. > Consider this case: > > <set up SIGTRAP on a perf event> > ... > sigset_t s; > sigemptyset(&s); > sigaddset(&s, SIGTRAP | <and others>); > sigprocmask(SIG_BLOCK, &s, ...); > ... > <perf event triggers> > > 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. > > This makes sense for error conditions, but not so much for explicitly > requested monitoring. However, the expectation is still that signals > generated by perf events are synchronous, which will no longer be the > case if the signal is blocked and delivered later. > > To give user space the ability to clearly distinguish synchronous from > asynchronous signals, introduce siginfo_t::si_perf_flags and > TRAP_PERF_FLAG_ASYNC (opted for flags in case more binary information is > required in future). > > The resolution to the problem is then to (a) no longer force the signal > (avoiding the terminations), but (b) tell user space via si_perf_flags > if the signal was synchronous or not, so that such signals can be > handled differently (e.g. let user space decide to ignore or consider > the data imprecise). > > The alternative of making the kernel ignore SIGTRAP on perf events if > the signal is blocked may work for some usecases, but likely causes > issues in others that then have to revert back to interception of > sigprocmask() (which we want to avoid). [ A concrete example: when using > breakpoint perf events to track data-flow, in a region of code where > signals are blocked, data-flow can no longer be tracked accurately. > When a relevant asynchronous signal is received after unblocking the > signal, the data-flow tracking logic needs to know its state is > imprecise. ] > > Link: https://lore.kernel.org/all/Yjmn%2FkVblV3TdoAq@xxxxxxxxxxxxxxxx/ > Fixes: 97ba62b27867 ("perf: Add support for SIGTRAP on perf events") > Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx> > Signed-off-by: Marco Elver <elver@xxxxxxxxxx> > arch/m68k/kernel/signal.c | 1 + Acked-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
