linux-kernel - Re: [PATCH rdma-next 08/12] overflow.h: Add arithmetic shift helper

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Date:   Mon, 25 Jun 2018 11:11:57 -0600
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Leon Romanovsky <leon@...nel.org>,
        Doug Ledford <dledford@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Leon Romanovsky <leonro@...lanox.com>,
        RDMA mailing list <linux-rdma@...r.kernel.org>,
        Hadar Hen Zion <hadarh@...lanox.com>,
        Matan Barak <matanb@...lanox.com>,
        Michael J Ruhl <michael.j.ruhl@...el.com>,
        Noa Osherovich <noaos@...lanox.com>,
        Raed Salem <raeds@...lanox.com>,
        Yishai Hadas <yishaih@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        linux-netdev <netdev@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH rdma-next 08/12] overflow.h: Add arithmetic shift helper

On Mon, Jun 25, 2018 at 11:26:05AM +0200, Rasmus Villemoes wrote:

>    check_shift_overflow(a, s, d) {
>        unsigned _nbits = 8*sizeof(a);
>        typeof(a) _a = (a);
>        typeof(s) _s = (s);
>        typeof(d) _d = (d);
> 
>        *_d = ((u64)(_a) << (_s & (_nbits-1)));
>        _s >= _nbits || (_s > 0 && (_a >> (_nbits - _s -
>    is_signed_type(a))) != 0);
>    }

Those types are not quite right.. What about this?

    check_shift_overflow(a, s, d) ({
        unsigned int _nbits = 8*sizeof(d) - is_signed_type(d);
        typeof(d) _a = a;  // Shift is always performed on type 'd'
        typeof(s) _s = s;
        typeof(d) _d = d;
 
        *_d = (_a << (_s & (_nbits-1)));

	(((*_d) >> (_s & (_nbits-1)) != _a);
    })

And can we use mathamatcial invertability to prove no overlow and
bound _a ? As above.

Jason