Re: [PATCH v3 07/11] perf: Add breakpoint information to siginfo on SIGTRAP

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Mar 24, 2021 at 12:24:59PM +0100, Marco Elver wrote:
> Encode information from breakpoint attributes into siginfo_t, which
> helps disambiguate which breakpoint fired.
> 
> Note, providing the event fd may be unreliable, since the event may have
> been modified (via PERF_EVENT_IOC_MODIFY_ATTRIBUTES) between the event
> triggering and the signal being delivered to user space.
> 
> Signed-off-by: Marco Elver <elver@google.com>
> ---
> v2:
> * Add comment about si_perf==0.
> ---
>  kernel/events/core.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 1e4c949bf75f..0316d39e8c8f 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6399,6 +6399,22 @@ static void perf_sigtrap(struct perf_event *event)
>  	info.si_signo = SIGTRAP;
>  	info.si_code = TRAP_PERF;
>  	info.si_errno = event->attr.type;
> +
> +	switch (event->attr.type) {
> +	case PERF_TYPE_BREAKPOINT:
> +		info.si_addr = (void *)(unsigned long)event->attr.bp_addr;
> +		info.si_perf = (event->attr.bp_len << 16) | (u64)event->attr.bp_type;

Ahh, here's the si_perf user. I wasn't really clear to me what was
supposed to be in that field at patch #5 where it was introduced.

Would it perhaps make sense to put the user address of struct
perf_event_attr in there instead? (Obviously we'd have to carry it from
the syscall to here, but it might be more useful than a random encoding
of some bits therefrom).

Then we can also clearly document that's in that field, and it might be
more useful for possible other uses.