perf cs-etm: Always allocate memory for cs_etm_queue::prev_packet [Linux 5.1]

perf cs-etm: Always allocate memory for cs_etm_queue::prev_packet [Linux 5.1]

This Linux kernel change "perf cs-etm: Always allocate memory for cs_etm_queue::prev_packet" is included in the Linux 5.1 release. This change is authored by Leo Yan <leo.yan [at]> on Sun Apr 28 16:32:27 2019 +0800. The commit for this change in Linux stable tree is 35bb59c (patch).

perf cs-etm: Always allocate memory for cs_etm_queue::prev_packet

Robert Walker reported a segmentation fault is observed when process
CoreSight trace data; this issue can be easily reproduced by the command
'perf report --itrace=i1000i' for decoding tracing data.

If neither the 'b' flag (synthesize branches events) nor 'l' flag
(synthesize last branch entries) are specified to option '--itrace',
cs_etm_queue::prev_packet will not been initialised.  After merging the
code to support exception packets and sample flags, there introduced a
number of uses of cs_etm_queue::prev_packet without checking whether it
is valid, for these cases any accessing to uninitialised prev_packet
will cause crash.

As cs_etm_queue::prev_packet is used more widely now and it's already
hard to follow which functions have been called in a context where the
validity of cs_etm_queue::prev_packet has been checked, this patch
always allocates memory for cs_etm_queue::prev_packet.

Reported-by: Robert Walker <>
Suggested-by: Robert Walker <>
Signed-off-by: Leo Yan <>
Tested-by: Robert Walker <>
Cc: Alexander Shishkin <>
Cc: Jiri Olsa <>
Cc: Mathieu Poirier <>
Cc: Mike Leach <>
Cc: Namhyung Kim <>
Cc: Suzuki K Poulouse <>
Fixes: 7100b12cf474 ("perf cs-etm: Generate branch sample for exception packet")
Fixes: 24fff5eb2b93 ("perf cs-etm: Avoid stale branch samples when flush packet")
Signed-off-by: Arnaldo Carvalho de Melo <>

There are 8 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 tools/perf/util/cs-etm.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
index 7777cfc..de488b4 100644
--- a/tools/perf/util/cs-etm.c
+++ b/tools/perf/util/cs-etm.c
@@ -422,11 +422,9 @@ static struct cs_etm_queue *cs_etm__alloc_queue(struct cs_etm_auxtrace *etm)
    if (!etmq->packet)
        goto out_free;

-   if (etm->synth_opts.last_branch || etm->sample_branches) {
-       etmq->prev_packet = zalloc(szp);
-       if (!etmq->prev_packet)
-           goto out_free;
-   }
+   etmq->prev_packet = zalloc(szp);
+   if (!etmq->prev_packet)
+       goto out_free;

    if (etm->synth_opts.last_branch) {
        size_t sz = sizeof(struct branch_stack);

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