Re: [PATCH] signal: Deliver SIGTRAP on perf event asynchronously if blocked

[Geert Uytterhoeven]

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