Re: [PATCH v1 2/2] signal: add procfd_signal() syscall

[Daniel Colascione <dancol@google.com>]

On Mon, Nov 19, 2018 at 2:40 PM Tycho Andersen <tycho@tycho.ws> wrote:
> 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.

> 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.

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

The system call table is also extensible. 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.

> 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.