linux-kernel - Re: [PATCH v2 8/9] atomic,x86: Alternative atomic_*

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Date:   Mon, 13 Dec 2021 19:18:54 +0100
From:   Marco Elver <elver@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v2 8/9] atomic,x86: Alternative atomic_*_overflow() scheme

On Mon, Dec 13, 2021 at 10:11AM -0800, Linus Torvalds wrote:
> On Mon, Dec 13, 2021 at 8:43 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > So Marco was expressing doubt about this exact interface for the
> > atomic_*_overflow() functions, since it's extremely easy to get the
> > whole ATOMIC_OVERFLOW_OFFSET thing wrong.
> 
> I missed that discussion (maybe it was on irc? Or maybe I just get too
> much email).
> 
> Anyway, my preferred solution would simply be to make the ref-counting
> atomics use a different type.
> 
> Voilà, problem solved. You can't really misuse them by mistake,
> because you can't access it by mistake.
> 
> Sure, it could be a wrapper around 'atomic_t' on architectures that
> end up using the generic fallback, so it might be as simple as
> 
>    typedef atomic_t atomic_ref_t;
> 
> in some asm-generic implementation, although I suspect that you'd want
> type safety even there, and do
> 
>   typedef struct { atomic_t atomic_val; } atomic_ref_t;
> 
> But then on x86 - and other architectures that might prefer to use
> that offset trick because they have flags - I'm not sure it even makes
> sense to have anything to do with 'atomic_t' at all, since there would
> basically be zero overlap with the regular atomic operations (partly
> due to the offset, but partly simply because the 'ref' operations are
> simply different).
> 
> (Wrt naming: I do think this is more about the "ref" part than the
> "overflow" part - thus I'd suggest the "atomic_ref_t" rather than your
> ofl naming).

I'm still genuinely worried about this:

> 	2. Yet another potentially larger issue is if some code
> 	   kmalloc()s some structs containing refcount_t, and relies on
> 	   GFP_ZERO (kzalloc()) to initialize their data assuming that a
> 	   freshly initialized refcount_t contains 0.

Even with everything properly wrapped up in atomic_ref_t, it's not going
to prevent mis-initialization via kzalloc() and friends.

I think C won't let us design that misuse out of existence.

Thanks,
-- Marco