perf, tools: Fix crash in 'perf trace'

From: Sukadev Bhattiprolu <[email protected]>
Date: Fri, 12 Jun 2015 01:28:36 -0400
Subject: [PATCH] perf, tools: Fix crash in perf trace

I get following crash on multiple systems and across several releases (at least since v3.18).

	Core was generated by `/tmp/perf trace sleep 0.2 '.
	Program terminated with signal SIGSEGV, Segmentation fault.
	#0  perf_mmap__read_head (mm=0x3fff9bf30070) at util/evlist.h:195
	195		u64 head = ACCESS_ONCE(pc->data_head);
	(gdb) bt
	#0  perf_mmap__read_head (mm=0x3fff9bf30070) at util/evlist.h:195
	#1  perf_evlist__mmap_read (evlist=0x10027f11910, idx=<optimized out>)
	    at util/evlist.c:637
	#2  0x000000001003ce4c in trace__run (argv=<optimized out>,
	    argc=<optimized out>, trace=0x3fffd7b28288) at builtin-trace.c:2259
	#3  cmd_trace (argc=<optimized out>, argv=<optimized out>,
	    prefix=<optimized out>) at builtin-trace.c:2799
	#4  0x00000000100657b8 in run_builtin (p=0x10176798 <commands+480>, argc=3,
	    argv=0x3fffd7b2b550) at perf.c:370
	#5  0x00000000100063e8 in handle_internal_command (argv=0x3fffd7b2b550, argc=3)
	    at perf.c:429
	#6  run_argv (argv=0x3fffd7b2af70, argcp=0x3fffd7b2af7c) at perf.c:473
	#7  main (argc=3, argv=0x3fffd7b2b550) at perf.c:588

The problem seems to be a race condition, when the application has just exited. Some/all fds associated with the perf-events (tracepoints) go into a POLLHUP/ POLLERR state and the mmap region associated with those events are unmapped (in perf_evlist__filter_pollfd()).

But we go back and do a perf_evlist__mmap_read() which assumes that the mmaps are still valid and we hit the crash.

If the mapping for an event is released, its refcnt is 0 (and ->base is NULL), so ensure we have non-zero refcount before accessing the map.

Note that perf-record has a similar logic but unlike perf-trace, the record__mmap_read_all() checks the evlist->mmap[i].base before accessing the map.

---
 tools/perf/util/evlist.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

index 080be93..084535b 100644
@@ -634,11 +634,18 @@ static struct perf_evsel *perf_evlist__event2evsel(struct perf_evlist *evlist,
 union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx)
 {
 	struct perf_mmap *md = &evlist->mmap[idx];
-	u64 head = perf_mmap__read_head(md);
+	u64 head;
 	u64 old = md->prev;
 	unsigned char *data = md->base + page_size;
 	union perf_event *event = NULL;
 
+	/*
+	 * Check if event was unmapped due to a POLLHUP/POLLERR.
+	 */
+	if (!md->refcnt)
+		return NULL;
+
+	head = perf_mmap__read_head(md);
 	if (evlist->overwrite) {
 		/*
 		 * If we're further behind than half the buffer, there's a chance

2.1.4

Em Thu, Jun 11, 2015 at 11:00:04PM -0700, Sukadev Bhattiprolu escreveu:

Trying it in perf/core I get:

util/evlist.c: In function ‘perf_evlist__mmap_read’:
util/evlist.c:645:6: error: wrong type argument to unary exclamation mark
if (!md->refcnt)

Trying after changing it to !atomic_read(&md->refcnt)

And it fixes the segfault that I reproduced, but this still looks strange, i.e. if it hit zero, then it should not have been used at this point anymore... Will look at it again in the weekend. :-\

- Arnaldo

Arnaldo Carvalho de Melo [[email protected]] wrote:

| Em Thu, Jun 11, 2015 at 11:00:04PM -0700, Sukadev Bhattiprolu escreveu:
| > >From 6669ed960a3ee4f9a02790f60b6a73ffc82fd6de Mon Sep 17 00:00:00 2001
| > From: Sukadev Bhattiprolu <[email protected]>
| > Date: Fri, 12 Jun 2015 01:28:36 -0400
| > Subject: [PATCH] perf, tools: Fix crash in perf trace 
| > 
| > I get following crash on multiple systems and across several releases
| > (at least since v3.18).
| 
| Trying it in perf/core I get:
| 

Ah, I should have based on perf/core.

| util/evlist.c: In function ‘perf_evlist__mmap_read’:
| util/evlist.c:645:6: error: wrong type argument to unary exclamation
| mark
|   if (!md->refcnt)
| 
| Trying after changing it to !atomic_read(&md->refcnt)
| 
| And it fixes the segfault that I reproduced, but this still looks
| strange, i.e. if it hit zero, then it should not have been used at this
| point anymore... Will look at it again in the weekend. :-\

I think its a small window - where application has started exiting, and set the PERF_EVENT_STATE_EXIT flag, but has not exited "enough" to issue a SIGCHLD. (Also we check the 'done' flag once - we could get SIGCHLD just after the check?)

Anyway, the poll() in this window returns with POLLHUP and we unmap the region.

Sukadev