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

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Date:   Mon, 19 Nov 2018 20:31:31 +0100
From:   Christian Brauner <christian@...uner.io>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Daniel Colascione <dancol@...gle.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 01:02:06PM -0600, Eric W. Biederman wrote:
> Christian Brauner <christian@...uner.io> writes:
> 
> > On Mon, Nov 19, 2018 at 07:59:24AM -0800, Daniel Colascione wrote:
> >> You never addressed my comment on the previous patch about your use of
> >
> > Sorry, that thread exploded so quickly that I might have missed it.
> >
> >> private_data here. Why can't you use the struct pid reference that's
> >> already in the inode?
> >
> > If that's what people prefer we can probably use that. There was
> > precedent for stashing away such data in fs/proc/base.c already for
> > various other things including user namespaces and struct mm so I
> > followed this model. A prior version of my patch (I didn't send out) did
> > retrive the inode via proc_pid() in .open() took an additional reference
> > via get_pid() and dropped it in .release(). Do we prefer that?
> 
> If you are using proc/<pid>/ directories as your file descriptors, you
> don't need to add an open or a release method at all.  The existing file
> descriptors hold a reference to the inode which holds a reference the
> the struct pid.
> 
> The only time you need to get a reference is when you need a task
> and kill_pid_info already performs that work for you.

Oh, I see what you and Andy are saying now. Sweet, that means we can
trim down the patch even more. Less code, less headache.

Thanks!

> 
> So using proc_pid is all you need to do to get the pid from the existing
> file descriptors.
> 
> Eric
>