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

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Message-ID: <20181122074807.GB15484@mail.hallyn.com>
Date:   Thu, 22 Nov 2018 01:48:07 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Tycho Andersen <tycho@...ho.ws>
Cc:     Christian Brauner <christian@...uner.io>, 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 03:39:54PM -0700, Tycho Andersen wrote:
> 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!

This could just be my own mental model, but for something like "kill a
task", an ioctl just seems wrong.  Syscall seems more natural.

I'd ack either method.

-serge