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

Oleg Nesterov
2022-05-02T14:37:51Z

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.