[tip: perf/urgent] perf/x86: Fix out of range data

[tip-bot2 for Namhyung Kim]

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     dec8ced871e17eea46f097542dd074d022be4bd1
Gitweb:        https://git.kernel.org/tip/dec8ced871e17eea46f097542dd074d022be4bd1
Author:        Namhyung Kim <namhyung@xxxxxxxxxx>
AuthorDate:    Tue, 05 Mar 2024 22:10:03 -08:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Wed, 10 Apr 2024 06:12:01 +02:00

perf/x86: Fix out of range data

On x86 each struct 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.

Fixes: 5471eea5d3bf ("perf/x86: Reset the dirty counter to prevent the leak for an RDPMC task")
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Reviewed-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/20240306061003.1894224-1-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 0905064..5b0dd07 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;