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

On Sun, Nov 18, 2018 at 12:28 PM, Andy Lutomirski [off-list ref] wrote:

quoted

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.

quoted

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 can’t 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.