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

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Message-ID: <87o9ak28nl.fsf@xmission.com>
Date:   Mon, 19 Nov 2018 13:02:06 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Christian Brauner <christian@...uner.io>
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

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.

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

Eric