bpf_probe_read_user EFAULT

[Victor Laforet <victor.laforet@ip-paris.fr>]

Hi all,

I am trying to use bpf_probe_read_user to read a user space value from BPF. The issue is that I am getting -14 (-EFAULT) result from bpf_probe_read_user. I haven’t been able to make this function work reliably. Sometimes I get no error code then it goes back to EFAULT.

I am seeking your help to try and make this code work.
Thank you!

My goal is to read the variable pid on every bpf event.
Here is a full example:
(cat /sys/kernel/debug/tracing/trace_pipe to read the output).

sched_switch.bpf.c
```
#include "vmlinux.h"
#include <bpf/bpf_helpers.h>

int *input_pid;

char _license[4] SEC("license") = "GPL";

SEC("tp_btf/sched_switch")
int handle_sched_switch(u64 *ctx)
{
  int pid;
  int err;

  err = bpf_probe_read_user(&pid, sizeof(int), (void *)input_pid);
  if (err != 0)
  {
    bpf_printk("Error on bpf_probe_read_user(pid) -> %d.\n", err);
    return 0;
  }

  bpf_printk("pid %d.\n", pid);
  return 0;
}
```

sched_switch.c
```
#include <stdio.h>
#include <unistd.h>
#include <sys/resource.h>
#include <bpf/libbpf.h>
#include "sched_switch.skel.h"
#include <time.h>

static int libbpf_print_fn(enum libbpf_print_level level, const char *format, va_list args)
{
  return vfprintf(stderr, format, args);
}

int main(int argc, char **argv)
{
  struct sched_switch_bpf *skel;
  int err;
  int pid = getpid();

  libbpf_set_print(libbpf_print_fn);

  skel = sched_switch_bpf__open();
  if (!skel)
  {
    fprintf(stderr, "Failed to open BPF skeleton\n");
    return 1;
  }

  skel->bss->input_pid = &pid;

  err = sched_switch_bpf__load(skel);
  if (err)
  {
    fprintf(stderr, "Failed to load and verify BPF skeleton\n");
    goto cleanup;
  }

  err = sched_switch_bpf__attach(skel);
  if (err)
  {
    fprintf(stderr, "Failed to attach BPF skeleton\n");
    goto cleanup;
  }

  while (1);

cleanup:
  sched_switch_bpf__destroy(skel);
  return -err;
}
```