Re: strace lockup when tracing exec in go

From: Oleg Nesterov
Date: Fri Sep 23 2016 - 05:51:38 EST

Sorry for delay, I was offline. I'll try to return to this problem next
week, currently I can't even read this thread but at first glance the
proposed patch(es) do not look right...

On 09/21, Michal Hocko wrote:
> The further investigation shown that the tracer (strace) is stuck
> waiting for cred_guard_mutex
> [<0000000000000000>] mm_access+0x22/0xa0
> [<0000000000000000>] process_vm_rw_core.isra.1+0x112/0x6c0
> [<0000000000000000>] process_vm_rw+0xab/0x110
> [<0000000000000000>] SyS_process_vm_readv+0x15/0x20
> [<0000000000000000>] system_call_fastpath+0x16/0x1b
> [<0000000000000000>] 0x7f186f031c3a
> [<0000000000000000>] 0xffffffffffffffff
> this however doesn't happen because both threads which are dead
> are zombies waiting to be reaped by the parent and to call their
> release_task->__exit_signal.

Yes, I know, this was already reported. And so far I do not see any
reasonable fix. I _think_ that the "real" fix should rework the
security_bprm_* helpers, but unlikely this is possible.

The trivial test-case:

void *thread(void *arg)
ptrace(PTRACE_TRACEME, 0,0,0);
return NULL;

int main(void)
int pid = fork();

if (!pid) {
pthread_t pt;
pthread_create(&pt, NULL, thread, NULL);
pthread_join(pt, NULL);
execlp("echo", "echo", "passed", NULL);

// or anything else which needs ->cred_guard_mutex,
// say open(/proc/$pid/mem)
ptrace(PTRACE_ATTACH, pid, 0,0);
kill(pid, SIGCONT);

return 0;