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

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Date:   Mon, 19 Nov 2018 14:49:22 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     Tycho Andersen <tycho@...ho.ws>
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 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. 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.