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

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Date:   Wed, 27 Jun 2018 20:39:11 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Jason Gunthorpe <jgg@...lanox.com>,
        Doug Ledford <dledford@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        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 Wed, Jun 27, 2018 at 11:36:03AM +0200, Rasmus Villemoes wrote:
> On 26 June 2018 at 19:54, Jason Gunthorpe <jgg@...lanox.com> wrote:
>
> > On Tue, Jun 26, 2018 at 10:07:07AM +0200, Rasmus Villemoes wrote:
> > >    On 25 June 2018 at 19:11, Jason Gunthorpe <[1]jgg@...lanox.com>
> > wrote:
> > >
> >
> > When thinking about signed cases.. The explicit u64 cast, and
> > implict promotion to typeof(d), produce something counter intuitive,
> > eg:
> >
> >   (u64)(s32)-1 == 0xffffffffffffffff
> >
> > Which would result in a shift oucome that is not what anyone would
> > expect, IMHO... So the first version isn't what I'd expect either..
> >
>
> Wouldn't check_shift_overflow(-1, 4, &someint) just put -16 in someint and
> report no overflow? That's what I'd expect, if negative values are to be
> supported at all.

Most if not all the times we don't do shifts on negative values, so I
don't think that we should support them.

Thanks

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