linux-kernel - Re: [PATCH v1 2/2] signal: add procfd

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Date:   Mon, 19 Nov 2018 16:07:09 -0700
From:   Tycho Andersen <tycho@...ho.ws>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Christian Brauner <christian@...uner.io>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, Jann Horn <jannh@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        linux-man <linux-man@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v1 2/2] signal: add procfd_signal() syscall

On Mon, Nov 19, 2018 at 02:49:22PM -0800, Daniel Colascione wrote:
> On Mon, Nov 19, 2018 at 2:40 PM Tycho Andersen <tycho@...ho.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.

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.

> > 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