[PATCH v2 06/12] ptrace: Reimplement PTRACE_KILL by always sending SIGKILL

[Oleg Nesterov]

On 04/29, Eric W. Biederman wrote:
>
> Call send_sig_info in PTRACE_KILL instead of ptrace_resume.  Calling
> ptrace_resume is not safe to call if the task has not been stopped
> with ptrace_freeze_traced.

Oh, I was never, never able to understand why do we have PTRACE_KILL
and what should it actually do.

I suggested many times to simply remove it but OK, we probably can't
do this.

> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -1238,7 +1238,7 @@ int ptrace_request(struct task_struct *child, long request,
>  	case PTRACE_KILL:
>  		if (child->exit_state)	/* already dead */
>  			return 0;
> -		return ptrace_resume(child, request, SIGKILL);
> +		return send_sig_info(SIGKILL, SEND_SIG_NOINFO, child);

Note that currently ptrace(PTRACE_KILL) can never fail (yes, yes, it
is unsafe), but send_sig_info() can. If we do not remove PTRACE_KILL,
then I'd suggest

	case PTRACE_KILL:
		if (!child->exit_state)
			send_sig_info(SIGKILL);
		return 0;

to make this change a bit more compatible.

Also, please remove the note about PTRACE_KILL in set_task_blockstep().

Oleg.