From 78ed93d72ded679e3caf0758357209887bda885f Mon Sep 17 00:00:00 2001 From: Marco Elver Date: Mon, 4 Apr 2022 13:12:04 +0200 Subject: signal: Deliver SIGTRAP on perf event asynchronously if blocked With SIGTRAP on perf events, we have encountered termination of processes due to user space attempting to block delivery of SIGTRAP. Consider this case: ... 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. 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. ] Fixes: 97ba62b27867 ("perf: Add support for SIGTRAP on perf events") Reported-by: Dmitry Vyukov Signed-off-by: Marco Elver Signed-off-by: Peter Zijlstra (Intel) Acked-by: Geert Uytterhoeven Tested-by: Dmitry Vyukov Link: https://lore.kernel.org/r/20220404111204.935357-1-elver@google.com --- arch/arm64/kernel/signal.c | 1 + arch/arm64/kernel/signal32.c | 1 + 2 files changed, 2 insertions(+) (limited to 'arch/arm64/kernel') diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c index 4a4122ef6f39..41b5d9d3672a 100644 --- a/arch/arm64/kernel/signal.c +++ b/arch/arm64/kernel/signal.c @@ -1011,6 +1011,7 @@ static_assert(offsetof(siginfo_t, si_upper) == 0x28); static_assert(offsetof(siginfo_t, si_pkey) == 0x20); static_assert(offsetof(siginfo_t, si_perf_data) == 0x18); static_assert(offsetof(siginfo_t, si_perf_type) == 0x20); +static_assert(offsetof(siginfo_t, si_perf_flags) == 0x24); static_assert(offsetof(siginfo_t, si_band) == 0x10); static_assert(offsetof(siginfo_t, si_fd) == 0x18); static_assert(offsetof(siginfo_t, si_call_addr) == 0x10); diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c index d984282b979f..4700f8522d27 100644 --- a/arch/arm64/kernel/signal32.c +++ b/arch/arm64/kernel/signal32.c @@ -487,6 +487,7 @@ static_assert(offsetof(compat_siginfo_t, si_upper) == 0x18); static_assert(offsetof(compat_siginfo_t, si_pkey) == 0x14); static_assert(offsetof(compat_siginfo_t, si_perf_data) == 0x10); static_assert(offsetof(compat_siginfo_t, si_perf_type) == 0x14); +static_assert(offsetof(compat_siginfo_t, si_perf_flags) == 0x18); static_assert(offsetof(compat_siginfo_t, si_band) == 0x0c); static_assert(offsetof(compat_siginfo_t, si_fd) == 0x10); static_assert(offsetof(compat_siginfo_t, si_call_addr) == 0x0c); -- cgit v1.2.3