Arch Linux

joel10

Member

Registered: 2022-06-28

Posts: 1

'kernel.perf_event_paranoid' it not respected

I refer to this post on Unix / Linux SE but will tailor it here just for Arch:

When reading the Performance Counters (PMC) on CPUS, the kernel.perf_event_paranoid must be <=1 (see Kernel doc)

The program below reads the PMC and should exit early with 1, if it cannot open the counter i.e. kernel.perf_event_paranoid is >1 (check after `syscall`)

Now, by default, the parameter is 2. I run the program and it should not be able to open the perf counter. I can however, still do it.

My question is why?

This script could help to reproduce:

#!/bin/bash

cat >pmc.c <<'EOF'
#include <linux/perf_event.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>

static struct perf_event_attr attr;
static int fdperf = -1;
static struct perf_event_mmap_page *buf = 0;

long long cpucycles_amd64rdpmc(void) {
  long long result;
  unsigned int seq;
  long long index;
  long long offset;

  if (fdperf == -1) {
    attr.type = PERF_TYPE_HARDWARE;
    attr.config = PERF_COUNT_HW_CPU_CYCLES;
    attr.exclude_kernel = 1;
    fdperf = syscall(__NR_perf_event_open, &attr, 0, -1, -1, 0);
    if (fdperf == -1){
        fprintf(stderr, "\033[31m--> could not open perf counter. Check paranoid setting\033[0m\n");
        exit(1);
    }
    buf = mmap(NULL, sysconf(_SC_PAGESIZE), PROT_READ, MAP_SHARED, fdperf, 0);
  }

  do {
    seq = buf->lock;
    asm volatile("" ::: "memory");
    index = buf->index;
    offset = buf->offset;
    asm volatile("rdpmc;shlq $32,%%rdx;orq %%rdx,%%rax"
                 : "=a"(result)
                 : "c"(index - 1)
                 : "%rdx");
    asm volatile("" ::: "memory");
  } while (buf->lock != seq);

  result += offset;
  result &= 0xffffffffffff;
  return result;
}

int main() {

  long long c = cpucycles_amd64rdpmc();
  printf("counter: %llx\n", c);
  return 0;
}
EOF

param_name=kernel.perf_event_paranoid
read_pmc() {
  echo -n "--> reading, "
  sysctl "${param_name}"
}

set_pmc() {
  n=$1
  echo -e "--> setting to ${n}"
  sudo sysctl -w "${param_name}=${n}"

  read_pmc
  echo "should be ${n}"
}

run() {
  echo "--> running"
  ./pmc
}

#compile
gcc pmc.c -o pmc

read_pmc
run
echo -e "\n--> if ${param_name} as >1, that should have printed the error message\n\n"

# at point we could actually stop the test, as the number is 2
# but for completeness, I kept the rest.

set_pmc 1

run
echo -e "\n--> that should have worked and printed the counter.\n"

#re-set to 2
set_pmc 2
run
echo -e "\033[31m--> that should NOT have worked but it printed the counter\033[0m.\n"

rm pmc.c pmc

(rdpmc code is based on cpucycles/amd64rdpmc.c from SUPERCOP )
My system: Linux arch4 5.18.6-arch1-1 #1 SMP PREEMPT_DYNAMIC Wed, 22 Jun 2022 18:10:56 +0000 x86_64 GNU/Linux

Last edited by joel10 (2022-07-28 08:29:02)