Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT_RT

[Eric W. Biederman]

Oleg Nesterov <oleg@xxxxxxxxxx> writes:

> On 04/21, Peter Zijlstra wrote:
>>
>> @@ -1329,8 +1337,7 @@ SYSCALL_DEFINE4(ptrace, long, request, l
>>  		goto out_put_task_struct;
>>  
>>  	ret = arch_ptrace(child, request, addr, data);
>> -	if (ret || request != PTRACE_DETACH)
>> -		ptrace_unfreeze_traced(child);
>> +	ptrace_unfreeze_traced(child);
>
> Forgot to mention... whatever we do this doesn't look right.
>
> ptrace_unfreeze_traced() must not be called if the tracee was untraced,
> anothet debugger can come after that. I agree, the current code looks
> a bit confusing, perhaps it makes sense to re-write it:
>
> 	if (request == PTRACE_DETACH && ret == 0)
> 		; /* nothing to do, no longer traced by us */
> 	else
> 		ptrace_unfreeze_traced(child);

This was a bug in my original JOBCTL_DELAY_WAITKILL patch and it was
just cut and pasted here.  I thought it made sense when I was throwing
things together but when I looked more closely I realized that it is
not safe.

Eric