[BUG] perf stat: events inheritance can break task targets

[Alexander Yarygin <yarygin@linux.vnet.ibm.com>]

perf stat can block pthread_create() for a multithreaded userspace
process (i.e. qemu) when:
- process is running with non-root privileges
- perf stat is running as root with trace events in -e option
- it is attached to the process's pid.

Here is a simple test scenario:

~$ cat test.c
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <string.h>

#define THREADS 50

static pthread_t threads[THREADS];

void *loop()
{
    while(1);
}

int main()
{
    for (int i = 0; i < THREADS; i++) {

        int err = pthread_create(&threads[i], NULL, loop, 0);

        if (!err)
            printf("thread created: %i\n", i);
        else
            printf("couldn't create thread %i: %s\n", i, strerror(err));

        sleep(1);
    }

    return 0;
}
~$ gcc test.c -lpthread -std=c99 -o test
~$ ./test
thread created: 0
thread created: 1
# now perf is running:
# ~$ sudo perf stat -e "kvm:*" -p `pidof test`
couldn't create thread 2: Operation not permitted
couldn't create thread 3: Operation not permitted
couldn't create thread 4: Operation not permitted
# here is perf was stopped
thread created: 5
thread created: 6
^C

When perf is running, every invoke of pthread_create() returns -EPERM.

On the kernel side, copy_process() creates a task, scheduled it,
than perf_event_init_task() (kernel/events/core.c) returns an error,
and the kernel cleans task's resources.

It looks like child process doesn't have access to trace events,
so perf_trace_event_perm() (kernel/trace/trace_event_perf.c)
returns -EPERM:

static int perf_trace_event_perm(struct ftrace_event_call *tp_event,
				 struct perf_event *p_event)
{
...
    /*
     * ...otherwise raw tracepoint data can be a severe data leak,
     * only allow root to have these.
     */
    if (perf_paranoid_tracepoint_raw() && !capable(CAP_SYS_ADMIN))
        return -EPERM;
...
}

If we explicitly use the --no-inherit option, payload wouldn't die.