>From 6669ed960a3ee4f9a02790f60b6a73ffc82fd6de Mon Sep 17 00:00:00 2001
From: Sukadev Bhattiprolu <suka...@linux.vnet.ibm.com>
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.
Signed-off-by: Sukadev Bhattiprolu <suka...@linux.vnet.ibm.com>
---
tools/perf/util/evlist.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index 080be93..084535b 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -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
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