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

Eric W. Biederman
2022-05-03T19:36:55Z

Oleg Nesterov <oleg@redhat.com> writes:

> 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.

I thought I remembered you suggesting fixing it in some other way.

I took at quick look in codesearch.debian.net and PTRACE_KILL is
definitely in use. I find uses in gcc-10, firefox-esr_91.8,
llvm_toolchain, qtwebengine. At which point I stopped looking.

>> --- 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.

Quite. The only failure I can find from send_sig_info is if
lock_task_sighand fails and PTRACE_KILL is deliberately ignoring errors
when the target task has exited.

case PTRACE_KILL:
send_sig_info(SIGKILL);
return 0;

I think that should suffice.

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

Good catch, thank you.

Eric