On Mon, Nov 19, 2018 at 02:49:22PM -0800, Daniel Colascione wrote:

On Mon, Nov 19, 2018 at 2:40 PM Tycho Andersen [off-list ref] wrote:

quoted

Can I just register an objection here that I think using a syscall
just for this is silly?

Yes, you can argue that the bikeshed should be ioctl-colored and not
syscall-colored.

quoted

My understanding is that the concern is that some code might do:

unknown_fd = recv_fd();
ioctl(unknown_fd, SOME_IOCTL, NULL); // where SOME_IOCTL == PROC_FD_KILL
// whoops, unknown_fd was a procfd and we killed a task!

In my experience when writing fd sending/receiving code, the sender and
receiver are fairly tightly coupled. Has anyone ever actually fixed a
bug where they had an fd that they lost track of what "type" it was
and screwed up like this? It seems completely theoretical to me.

Yes, I have fixed bugs of this form.

quoted

The ioctl() approach has the benefit of being extensible.

The system call table is also extensible.

But not infinitely so. The x32 ABI starts at 512, and right now I see
334 syscalls on x86_64. So the next 178 people can say "let's just
define a syscall", and after that? I suppose we could move to setting
BIT(10), but how much userspace assumes > 512 => compat syscall and
blocks it via seccomp or whatever?

Contrast that with the ioctl space, which is an unsigned long and
fairly sparse still (Documentation/ioctl/ioctl-number.txt).

ioctl is for when a given piece of functionality *can't*
realistically get its own system call because it's separated from
the main kernel somehow. procfs is a core part of the kernel, so we
can and should expose interfaces to it using system calls.

I suppose it's obvious, but I disagree.

quoted

Adding a
syscall means that everyone has to do all the boilerplate for each new
pid op in the kernel, arches, libc, strace, etc.

These tools also care about ioctls. Adding a system call is a pain,
but the solution is to make adding system calls less of a pain, not to
permanently make the Linux ABI worse.

For user-defined values of "worse" :)

Tycho