Re: [PATCH RFC 4/4] perf/core: Add breakpoint information to siginfo on SIGTRAP

On Tue, 23 Feb 2021 at 16:16, Dmitry Vyukov [off-list ref] wrote:

On Tue, Feb 23, 2021 at 4:10 PM 'Marco Elver' via kasan-dev
[off-list ref] wrote:

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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>
---
 kernel/events/core.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 8718763045fd..d7908322d796 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c

@@ -6296,6 +6296,17 @@ 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;
+               break;
+       default:
+               /* No additional info set. */

Should we prohibit using attr.sigtrap for !PERF_TYPE_BREAKPOINT if we
don't know what info to pass yet?

I don't think it's necessary. This way, by default we get support for
other perf events. If user space observes si_perf==0, then there's no
information available. That would require that any event type that
sets si_perf in future, must ensure that it sets si_perf!=0.

I can add a comment to document the requirement here (and user space
facing documentation should get a copy of how the info is encoded,
too).

Alternatively, we could set si_errno to 0 if no info is available, at
the cost of losing the type information for events not explicitly
listed here.

Note that PERF_TYPE_HARDWARE == 0, so setting si_errno to 0 does not
work. Which leaves us with:

1. Ensure si_perf==0 (or some other magic value) if no info is
available and !=0 otherwise.

2. Return error for events where we do not officially support
requesting sigtrap.

I'm currently leaning towards (1).

quoted

What do you prefer?

Ah, I see.
Let's wait for the opinions of other people. There are a number of
options for how to approach this.