linux-kernel - Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer

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Date:   Fri, 31 May 2019 18:44:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Jann Horn <jannh@...gle.com>, Greg KH <gregkh@...uxfoundation.org>,
        Al Viro <viro@...iv.linux.org.uk>, raven@...maw.net,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-block@...r.kernel.org, keyrings@...r.kernel.org,
        linux-security-module <linux-security-module@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 1/7] General notification queue with user mmap()'able
 ring buffer

On Fri, May 31, 2019 at 03:20:12PM +0100, David Howells wrote:
> Peter Zijlstra <peterz@...radead.org> wrote:

> > (and it has already been established that refcount_t doesn't work for
> > usage count scenarios)
> 
> ?
> 
> Does that mean struct kref doesn't either?

Indeed, since kref is just a pointless wrapper around refcount_t it does
not either.

The main distinction between a reference count and a usage count is that
0 means different things. For a refcount 0 means dead. For a usage count
0 is merely unused but valid.

Incrementing a 0 refcount is a serious bug -- use-after-free (and hence
refcount_t will refuse this and splat), for a usage count this is no
problem.

Now, it is sort-of possible to merge the two, by basically stating
something like: usage = refcount - 1. But that can get tricky and people
have not really liked the result much for the few times I tried.