linux-kernel - Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount

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Message-ID: <87k23gsn4u.fsf@xmission.com>
Date:   Mon, 10 Jul 2017 03:37:53 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     "Reshetova\, Elena" <elena.reshetova@...el.com>
Cc:     "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz\@infradead.org" <peterz@...radead.org>,
        "gregkh\@linuxfoundation.org" <gregkh@...uxfoundation.org>,
        "akpm\@linux-foundation.org" <akpm@...ux-foundation.org>,
        "mingo\@redhat.com" <mingo@...hat.com>,
        "adobriyan\@gmail.com" <adobriyan@...il.com>,
        "serge\@hallyn.com" <serge@...lyn.com>,
        "arozansk\@redhat.com" <arozansk@...hat.com>,
        "dave\@stgolabs.net" <dave@...olabs.net>,
        "keescook\@chromium.org" <keescook@...omium.org>,
        Hans Liljestrand <ishkamiel@...il.com>,
        "David Windsor" <dwindsor@...il.com>
Subject: Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

"Reshetova, Elena" <elena.reshetova@...el.com> writes:

2>> Elena Reshetova <elena.reshetova@...el.com> writes:
>> 
>> > refcount_t type and corresponding API should be
>> > used instead of atomic_t when the variable is used as
>> > a reference counter. This allows to avoid accidental
>> > refcounter overflows that might lead to use-after-free
>> > situations.
>> 
>> In this patch you can see all of the uses of the count.
>> What accidental refcount overflows are possible?
>
> Even if one can guarantee and prove that in the current implementation
> there are no overflows possible, we can't say that for
> sure for any future implementation. Bugs might always happen
> unfortunately, but if we convert the refcounter to a safer
> type we can be sure that overflows are not possible. 
>
> Does it make sense to you?

Not for code that is likely to remain unchanged for a decade no.

This looks like a large set of unautomated changes without any real
thought put into it.  That almost always results in a typo somewhere
that breaks things.

So there is no benefit to the code, and a non-zero chance that there
will be a typo breaking the code.

All to harden the code for an unlikely future when the code is
updated with a full test cycle and people paying attention.

Introduce a bug now to avoid a bug in the future.  That seems like a
very poor engineering trade off.

Eric