On 3/2/20 1:28 PM, Oleg Nesterov wrote:
On 03/01, Bernd Edlinger wrote:
This fixes a deadlock in the tracer when tracing a multi-threaded application that calls execve while more than one thread are running.
Heh. Yes, known problem. See my attempt to fix it: https://lore.kernel.org/lkml/20170213141452.GA30203@redhat.com/
@@ -1224,7 +1224,7 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode) struct mm_struct *mm; int err;
- err = mutex_lock_killable(&task->signal->cred_guard_mutex);
- err = mutex_lock_killable(&task->signal->cred_change_mutex);
So if I understand correctly your patch doesn't fix other problems with debugger waiting for cred_guard_mutex.
No, but I see this just as a first step.
I too do not think this can justify the new mutex in signal_struct...
I think for the vm_access the semantic of this mutex is clear, that it prevents the credentials to change while it is held by vm_access, and probably other places can take advantage of this mutex as well.
While on the other hand, the cred_guard_mutex is needed to avoid two threads calling execve at the same time. So that is needed as well.
What remains is probably making PTHREAD_ATTACH detect that the process is currently in execve, and make that call fail in that situation. I have not thought in depth about that problem, but it will probably just need the right mutex to access current->in_execve.
That's at least how I see it.
Thanks Bernd.