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

[Daniel Colascione]

On Sun, Nov 18, 2018 at 9:13 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> On Sun, Nov 18, 2018 at 8:29 AM Daniel Colascione <dancol@xxxxxxxxxx> wrote:
>>
>> On Sun, Nov 18, 2018 at 8:17 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> > On Sun, Nov 18, 2018 at 7:53 AM Daniel Colascione <dancol@xxxxxxxxxx> wrote:
>> >>
>> >> On Sun, Nov 18, 2018 at 7:38 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> >> > I fully agree that a more comprehensive, less expensive API for
>> >> > managing processes would be nice.  But I also think that this patch
>> >> > (using the directory fd and ioctl) is better from a security
>> >> > perspective than using a new file in /proc.
>> >>
>> >> That's an assertion, not an argument. And I'm not opposed to an
>> >> operation on the directory FD, now that it's clear Linus has banned
>> >> "write(2)-as-a-command" APIs. I just insist that we implement the API
>> >> with a system call instead of a less-reliable ioctl due to the
>> >> inherent namespace collision issues in ioctl command names.
>> >
>> > Linus banned it because of bugs iike the ones in the patch.
>>
>> Maybe: he didn't provide a reason. What's your point?
>
> My point is that an API that involves a file like /proc/PID/kill is
> very tricky to get right.  Here are some considerations:

Moot. write(2) for this interface is off the table anyway. The right
approach here is a system call that accepts a /proc/pid directory file
descriptor, a signal number, and a signal information field (as in
sigqueue(2)).

> Now if we had an ioctlat() API, maybe it would make sense.  But we
> don't, and I think it would be a bit crazy to add one.

A process is not a driver. Why won't this idea of using an ioctl for
the kill-process-by-dfd thing just won't die? An ioctl has *zero*
advantage in this context.