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

[Marco Elver]

On Tue, 5 Apr 2022 at 15:30, Dmitry Vyukov <dvyukov at google.com> wrote:
> On Mon, 4 Apr 2022 at 13:12, Marco Elver <elver at google.com> 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@elver.google.com/
> > Fixes: 97ba62b27867 ("perf: Add support for SIGTRAP on perf events")
> > Reported-by: Dmitry Vyukov <dvyukov at google.com>
> > Signed-off-by: Marco Elver <elver at google.com>
>
> Tested-by: Dmitry Vyukov <dvyukov at google.com>
>
> I've tested delivery of SIGTRAPs when it's blocked with sigprocmask,
> it does not kill the process now.
>
> And tested the case where previously I was getting infinite recursion
> and stack overflow (SIGTRAP handler causes another SIGTRAP recursively
> before being able to detect recursion and return). With this patch it
> can be handled by blocking recursive SIGTRAPs (!SA_NODEFER).

Thanks!


Should there be any further comments, please shout.

Thanks,
-- Marco