From: Kan Liang kan.liang@linux.intel.com
[ Upstream commit 24d3ae2f37d8bc3c14b31d353c5d27baf582b6a6 ]
The same task check in perf_event_set_output has some potential issues for some usages.
For the current perf code, there is a problem if using of perf_event_open() to have multiple samples getting into the same mmap’d memory when they are both attached to the same process. https://lore.kernel.org/all/92645262-D319-4068-9C44-2409EF44888E@gmail.com/ Because the event->ctx is not ready when the perf_event_set_output() is invoked in the perf_event_open().
Besides the above issue, before the commit bd2756811766 ("perf: Rewrite core context handling"), perf record can errors out when sampling with a hardware event and a software event as below. $ perf record -e cycles,dummy --per-thread ls failed to mmap with 22 (Invalid argument) That's because that prior to the commit a hardware event and a software event are from different task context.
The problem should be a long time issue since commit c3f00c70276d ("perk: Separate find_get_context() from event initialization").
The task struct is stored in the event->hw.target for each per-thread event. It is a more reliable way to determine whether two events are attached to the same task.
The event->hw.target was also introduced several years ago by the commit 50f16a8bf9d7 ("perf: Remove type specific target pointers"). It can not only be used to fix the issue with the current code, but also back port to fix the issues with an older kernel.
Note: The event->hw.target was introduced later than commit c3f00c70276d. The patch may cannot be applied between the commit c3f00c70276d and commit 50f16a8bf9d7. Anybody that wants to back-port this at that period may have to find other solutions.
Fixes: c3f00c70276d ("perf: Separate find_get_context() from event initialization") Signed-off-by: Kan Liang kan.liang@linux.intel.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Zhengjun Xing zhengjun.xing@linux.intel.com Link: https://lkml.kernel.org/r/20230322202449.512091-1-kan.liang@linux.intel.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/events/core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -12155,7 +12155,7 @@ perf_event_set_output(struct perf_event /* * If its not a per-cpu rb, it must be the same task. */ - if (output_event->cpu == -1 && output_event->ctx != event->ctx) + if (output_event->cpu == -1 && output_event->hw.target != event->hw.target) goto out;
/*