On Fri 23-09-16 11:50:32, Oleg Nesterov wrote:
> 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.
Rework them to not rely on the cred_guard_mutex? Is there any way to
workaround this in the strace code?
> 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);
> }
>
> sleep(1);
> // or anything else which needs ->cred_guard_mutex,
> // say open(/proc/$pid/mem)
> ptrace(PTRACE_ATTACH, pid, 0,0);
> kill(pid, SIGCONT);
>
> return 0;
> }
>
> Oleg.
--
Michal Hocko
SUSE Labs
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