Re: [PATCH] perf/x86: Fix out of range data

[Liang, Kan]

On 2023-12-16 2:28 a.m., Namhyung Kim wrote:
> On x86 each cpu_hw_events maintains a table for counter assignment but
> it missed to update one for the deleted event in x86_pmu_del().  This
> can make perf_clear_dirty_counters() reset used counter if it's called
> before event scheduling or enabling.  Then it would return out of range
> data which doesn't make sense.
> 
> The following code can reproduce the problem.
> 
>   $ cat repro.c
>   #include <pthread.h>
>   #include <stdio.h>
>   #include <stdlib.h>
>   #include <unistd.h>
>   #include <linux/perf_event.h>
>   #include <sys/ioctl.h>
>   #include <sys/mman.h>
>   #include <sys/syscall.h>
> 
>   struct perf_event_attr attr = {
>   	.type = PERF_TYPE_HARDWARE,
>   	.config = PERF_COUNT_HW_CPU_CYCLES,
>   	.disabled = 1,
>   };
> 
>   void *worker(void *arg)
>   {
>   	int cpu = (long)arg;
>   	int fd1 = syscall(SYS_perf_event_open, &attr, -1, cpu, -1, 0);
>   	int fd2 = syscall(SYS_perf_event_open, &attr, -1, cpu, -1, 0);
>   	void *p;
> 
>   	do {
>   		ioctl(fd1, PERF_EVENT_IOC_ENABLE, 0);
>   		p = mmap(NULL, 4096, PROT_READ, MAP_SHARED, fd1, 0);
>   		ioctl(fd2, PERF_EVENT_IOC_ENABLE, 0);
> 
>   		ioctl(fd2, PERF_EVENT_IOC_DISABLE, 0);
>   		munmap(p, 4096);
>   		ioctl(fd1, PERF_EVENT_IOC_DISABLE, 0);
>   	} while (1);
> 
>   	return NULL;
>   }
> 
>   int main(void)
>   {
>   	int i;
>   	int n = sysconf(_SC_NPROCESSORS_ONLN);
>   	pthread_t *th = calloc(n, sizeof(*th));
> 
>   	for (i = 0; i < n; i++)
>   		pthread_create(&th[i], NULL, worker, (void *)(long)i);
>   	for (i = 0; i < n; i++)
>   		pthread_join(th[i], NULL);
> 
>   	free(th);
>   	return 0;
>   }
> 
> And you can see the out of range data using perf stat like this.
> Probably it'd be easier to see on a large machine.
> 
>   $ gcc -o repro repro.c -pthread
>   $ ./repro &
>   $ sudo perf stat -A -I 1000 2>&1 | awk '{ if (length($3) > 15) print }'
>        1.001028462 CPU6   196,719,295,683,763      cycles                           # 194290.996 GHz                       (71.54%)
>        1.001028462 CPU3   396,077,485,787,730      branch-misses                    # 15804359784.80% of all branches      (71.07%)
>        1.001028462 CPU17  197,608,350,727,877      branch-misses                    # 14594186554.56% of all branches      (71.22%)
>        2.020064073 CPU4   198,372,472,612,140      cycles                           # 194681.113 GHz                       (70.95%)
>        2.020064073 CPU6   199,419,277,896,696      cycles                           # 195720.007 GHz                       (70.57%)
>        2.020064073 CPU20  198,147,174,025,639      cycles                           # 194474.654 GHz                       (71.03%)
>        2.020064073 CPU20  198,421,240,580,145      stalled-cycles-frontend          #  100.14% frontend cycles idle        (70.93%)
>        3.037443155 CPU4   197,382,689,923,416      cycles                           # 194043.065 GHz                       (71.30%)
>        3.037443155 CPU20  196,324,797,879,414      cycles                           # 193003.773 GHz                       (71.69%)
>        3.037443155 CPU5   197,679,956,608,205      stalled-cycles-backend           # 1315606428.66% backend cycles idle   (71.19%)
>        3.037443155 CPU5   198,571,860,474,851      instructions                     # 13215422.58  insn per cycle
> 
> It should move the contents in the cpuc->assign as well.

Yes, the patch looks good to me.

Reviewed-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>

Thanks,
Kan
> 
> Fixes: 5471eea5d3bf ("perf/x86: Reset the dirty counter to prevent the leak for an RDPMC task")
> Cc: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
>  arch/x86/events/core.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 09050641ce5d..5b0dd07b1ef1 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -1644,6 +1644,7 @@ static void x86_pmu_del(struct perf_event *event, int flags)
>  	while (++i < cpuc->n_events) {
>  		cpuc->event_list[i-1] = cpuc->event_list[i];
>  		cpuc->event_constraint[i-1] = cpuc->event_constraint[i];
> +		cpuc->assign[i-1] = cpuc->assign[i];
>  	}
>  	cpuc->event_constraint[i-1] = NULL;
>  	--cpuc->n_events;