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

[Peter Zijlstra]

On Wed, Mar 24, 2021 at 02:01:56PM +0100, Peter Zijlstra wrote:
> 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...

Ok possibly something like so, which also gets the data address right
for more cases.

---
--- 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 kernel_siginfo 		siginfo;
 #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->siginfo.si_perf = (unsigned long)uattr;
+
 		return perf_event_modify_attr(event,  &new_attr);
 	}
 	default:
@@ -6394,13 +6398,7 @@ void perf_event_wakeup(struct perf_event
 
 static void perf_sigtrap(struct perf_event *event)
 {
-	struct kernel_siginfo info;
-
-	clear_siginfo(&info);
-	info.si_signo = SIGTRAP;
-	info.si_code = TRAP_PERF;
-	info.si_errno = event->attr.type;
-	force_sig_info(&info);
+	force_sig_info(&event->siginfo);
 }
 
 static void perf_pending_event_disable(struct perf_event *event)
@@ -6414,8 +6412,8 @@ static void perf_pending_event_disable(s
 		WRITE_ONCE(event->pending_disable, -1);
 
 		if (event->attr.sigtrap) {
-			atomic_set(&event->event_limit, 1); /* rearm event */
 			perf_sigtrap(event);
+			atomic_set_release(&event->event_limit, 1); /* rearm event */
 			return;
 		}
 
@@ -9121,6 +9119,7 @@ static int __perf_event_overflow(struct
 	if (events && atomic_dec_and_test(&event->event_limit)) {
 		ret = 1;
 		event->pending_kill = POLL_HUP;
+		event->siginfo.si_addr = (void *)data->addr;
 
 		perf_event_disable_inatomic(event);
 	}
@@ -12011,6 +12010,11 @@ SYSCALL_DEFINE5(perf_event_open,
 		goto err_task;
 	}
 
+	clear_siginfo(&event->siginfo);
+	event->siginfo.si_signo = SIGTRAP;
+	event->siginfo.si_code = TRAP_PERF;
+	event->siginfo.si_perf = (unsigned long)attr_uptr;
+
 	if (is_sampling_event(event)) {
 		if (event->pmu->capabilities & PERF_PMU_CAP_NO_INTERRUPT) {
 			err = -EOPNOTSUPP;