linux-kernel - Re: [PATCH] Avoid that check_shl_overflow() triggers a compiler warning when building with W=1

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Message-ID: <20190308070258.GF32625@mtr-leonro.mtl.com>
Date:   Fri, 8 Mar 2019 07:03:02 +0000
From:   Leon Romanovsky <leonro@...lanox.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
CC:     Kees Cook <keescook@...omium.org>,
        Bart Van Assche <bvanassche@....org>,
        Jason Gunthorpe <jgg@...lanox.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH] Avoid that check_shl_overflow() triggers a compiler
 warning when building with W=1

On Thu, Mar 07, 2019 at 09:28:45PM +0100, Rasmus Villemoes wrote:
> On 07/03/2019 18.12, Kees Cook wrote:
> > On Thu, Mar 7, 2019 at 9:02 AM Leon Romanovsky <leonro@...lanox.com> wrote:
> >>
> >> On Thu, Mar 07, 2019 at 08:52:51AM -0800, Kees Cook wrote:
> >>> On Thu, Mar 7, 2019 at 7:40 AM Leon Romanovsky <leonro@...lanox.com> wrote:
> >>>>
> >>>> On Thu, Mar 07, 2019 at 06:53:54AM -0800, Bart Van Assche wrote:
> >>>>> On 3/6/19 11:24 PM, Leon Romanovsky wrote:
> >>>>>> My simple patch passes too :).
> >>>>>
> >>>>> Can you repost your patch?
> >>>>
> >>>> https://patchwork.kernel.org/patch/10841079/
> >>>>
> >>>> As Rasmus wrote, the thing is to avoid a < 0 check. In my patch,
> >>>> I converted a <= 0 to !(a > 0 || a == 0) expression.
> >>>
> >>> I'd be happy either way. Is there a larger benefit to having a safe
> >>> "is_non_negative()" helper, or should we go with the minimal change to
> >>> the shl macro?
> >>
> >> I personally prefer simplest possible solution.
>
> So, I played around with a few variants on godbolt.org, and it seems
> that gcc is smart enough to combine (a > 0 || a == 0) into (a >= 0) - in
> all the cases I tried Leon's patch resulted in the exact same generated
> code as the current version. Conversely, and rather surprising to me,
> Bart's patch seemed to cause worse code generation. So now I've changed
> my mind and also support Leon's version - however, I would _strongly_
> prefer if it introduced
>
> #define is_non_negative(a) (a > 0 || a == 0)
> #define is_negative(a) (!(is_non_negative(a))
>
> with appropriate comments and used that. check_shl_overflow is hard
> enough to read already.

What about if we call them is_normal(a) and is_negative(a)?

Thanks

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