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

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Message-ID: <20181119223954.GA4992@cisco>
Date:   Mon, 19 Nov 2018 15:39:54 -0700
From:   Tycho Andersen <tycho@...ho.ws>
To:     Christian Brauner <christian@...uner.io>
Cc:     ebiederm@...ssion.com, linux-kernel@...r.kernel.org,
        serge@...lyn.com, jannh@...gle.com, luto@...nel.org,
        akpm@...ux-foundation.org, oleg@...hat.com, cyphar@...har.com,
        viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, dancol@...gle.com, timmurray@...gle.com,
        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 11:32:39AM +0100, Christian Brauner wrote:
>
> +/**
> + *  sys_procfd_signal - send a signal to a process through a process file
> + *                      descriptor
> + *  @fd: the file descriptor of the process
> + *  @sig: signal to be sent
> + *  @info: the signal info
> + *  @flags: future flags to be passed
> + */
> +SYSCALL_DEFINE4(procfd_signal, int, fd, int, sig, siginfo_t __user *, info,
> +		int, flags)
> +{

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

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.

The ioctl() approach has the benefit of being extensible. Adding a
syscall means that everyone has to do all the boilerplate for each new
pid op in the kernel, arches, libc, strace, etc.

Tycho