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

From: Daniel Colascione
Date: Sun Nov 18 2018 - 15:33:02 EST

On Sun, Nov 18, 2018 at 12:28 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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.

Sure. A flag might make for better ergonomics.

>> Yes, that's what I have in mind. A siginfo_t is small enough that we
>> could just store it as a blob allocated off the procfs inode or
>> something like that without bothering with a shmfs file. You'd be able
>> to read(2) the exit status as many times as you wanted.
> I think that, if the syscall in question is read(2), then it should work *once* per struct file. Otherwise running cat on the file would behave very oddly.

Why? The file pointer would work normally.

> Read and poll have the same problem as write: we canât check caps in read or poll either.

Why not? Reading /proc/pid/stat does an access check today and
conditionally replaces the exit status with zero.