Re: [PATCH] proc: allow killing processes via file descriptors

[Daniel Colascione]

On Sun, Nov 18, 2018 at 12:43 PM, Christian Brauner
<christian@xxxxxxxxxx> wrote:
> On Sun, Nov 18, 2018 at 01:28:41PM -0700, Andy Lutomirski wrote:
>>
>>
>> > On Nov 18, 2018, at 12:44 PM, Daniel Colascione <dancol@xxxxxxxxxx> wrote:
>> >
>>
>> >
>> > That is, I'm proposing an API that looks like this:
>> >
>> > int process_kill(int procfs_dfd, int signo, const union sigval value)
>> >
>> > If, later, process_kill were to *also* accept process-capability FDs,
>> > nothing would break.
>>
>> Except that this makes it ambiguous to the caller as to whether their current creds are considered.  So it would need to be a different syscall or at least a flag.  Otherwise a lot of those nice theoretical properties go away.
>
> I can add a flag argument
> int process_signal(int procfs_dfd, int signo, siginfo_t *info, int flags)
> The way I see it process_signal() should be equivalent to kill(pid, signal) for now.
> That is siginfo_t is cleared and set to:
>
> info.si_signo = sig;
> info.si_errno = 0;
> info.si_code = SI_USER;
> info.si_pid = task_tgid_vnr(current);
> info.si_uid = from_kuid_munged(current_user_ns(), current_uid());

That makes sense. I just don't want to get into a situation where
callers feel that they *have* to use the PID-based APIs to send a
signal because process_kill doesn't offer some bit of functionality.

Are you imagining something like requiring info t be NULL unless flags
contains some "I have a siginfo_t" value?

BTW: passing SI_USER to rt_sigqueueinfo *should* as long as the
passed-in si_pid and si_uid match what the kernel would set them to in
the kill(2) case. The whole point of SI_USER is that the recipient
knows that it can trust the origin information embedded in the
siginfo_t in the signal handler. If the kernel verifies that a signal
sender isn't actually lying, why not let people send SI_USER with
rt_sigqueueinfo?