Messages in this thread Patch in this message | | | Date | Mon, 8 Nov 2021 16:24:34 +0100 | From | Peter Zijlstra <> | Subject | Re: Using perf_event_open() to sample multiple events of a process |
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On Sat, Nov 06, 2021 at 01:57:23AM +0100, Peter Zijlstra wrote:
> The problem seems to be that we call perf_event_set_output() before we > set event->ctx, which is a bit of a problem. > > Now, afaict it's been broken since c3f00c70276d ("perf: Separate > find_get_context() from event initialization"), which is ages ago :/ > > It's waaay too late to try and fix it; I'll be likely to make an even > bigger mess if I tried. Perhaps tomorrow. > > Clearly FD_OUTPUT isn't much used :-(
The below seems to fix, it's a bit of a hack, but I couldn't really come up with anything saner.
--- diff --git a/kernel/events/core.c b/kernel/events/core.c index f2253ea729a2..dbe766663733 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -5595,6 +5595,7 @@ static inline int perf_fget_light(int fd, struct fd *p) } static int perf_event_set_output(struct perf_event *event, + struct perf_event_context *ctx, struct perf_event *output_event); static int perf_event_set_filter(struct perf_event *event, void __user *arg); static int perf_copy_attr(struct perf_event_attr __user *uattr, @@ -5647,10 +5648,10 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon if (ret) return ret; output_event = output.file->private_data; - ret = perf_event_set_output(event, output_event); + ret = perf_event_set_output(event, event->ctx, output_event); fdput(output); } else { - ret = perf_event_set_output(event, NULL); + ret = perf_event_set_output(event, event->ctx, NULL); } return ret; } @@ -11830,7 +11831,9 @@ static int perf_copy_attr(struct perf_event_attr __user *uattr, } static int -perf_event_set_output(struct perf_event *event, struct perf_event *output_event) +perf_event_set_output(struct perf_event *event, + struct perf_event_context *event_ctx, + struct perf_event *output_event) { struct perf_buffer *rb = NULL; int ret = -EINVAL; @@ -11851,7 +11854,7 @@ perf_event_set_output(struct perf_event *event, struct perf_event *output_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->ctx != event_ctx) goto out; /* @@ -12232,7 +12235,7 @@ SYSCALL_DEFINE5(perf_event_open, } if (output_event) { - err = perf_event_set_output(event, output_event); + err = perf_event_set_output(event, ctx, output_event); if (err) goto err_context; }
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