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

[Peter Zijlstra]

On Wed, Mar 24, 2021 at 01:53:48PM +0100, Peter Zijlstra wrote:
> 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@xxxxxxxxxx>
> > ---
> > 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.

Something like so...

---

--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -778,6 +778,8 @@ struct perf_event {
 	void *security;
 #endif
 	struct list_head		sb_list;
+
+	struct perf_event_attr		__user *uattr;
 #endif /* CONFIG_PERF_EVENTS */
 };
 
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5652,13 +5652,17 @@ static long _perf_ioctl(struct perf_even
 		return perf_event_query_prog_array(event, (void __user *)arg);
 
 	case PERF_EVENT_IOC_MODIFY_ATTRIBUTES: {
+		struct perf_event_attr __user *uattr;
 		struct perf_event_attr new_attr;
-		int err = perf_copy_attr((struct perf_event_attr __user *)arg,
-					 &new_attr);
+		int err;
 
+		uattr = (struct perf_event_attr __user *)arg;
+		err = perf_copy_attr(uattr, &new_attr);
 		if (err)
 			return err;
 
+		event->uattr = uattr;
+
 		return perf_event_modify_attr(event,  &new_attr);
 	}
 	default:
@@ -6400,6 +6404,8 @@ static void perf_sigtrap(struct perf_eve
 	info.si_signo = SIGTRAP;
 	info.si_code = TRAP_PERF;
 	info.si_errno = event->attr.type;
+	info.si_perf = (unsigned long)event->uattr;
+
 	force_sig_info(&info);
 }
 
@@ -12011,6 +12017,8 @@ SYSCALL_DEFINE5(perf_event_open,
 		goto err_task;
 	}
 
+	event->uattr = attr_uptr;
+
 	if (is_sampling_event(event)) {
 		if (event->pmu->capabilities & PERF_PMU_CAP_NO_INTERRUPT) {
 			err = -EOPNOTSUPP;