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

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Date:   Wed, 1 Aug 2018 11:36:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        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 Tue, Jun 26, 2018 at 11:54:35AM -0600, Jason Gunthorpe wrote:

> What about more like this?
> 
>           check_shift_overflow(a, s, d) ({

Should that not be: check_shl_overflow() ? Just 'shift' lacks a
direction.

> 	      // Shift is always performed on the machine's largest unsigned
>               u64 _a = a;
> 	      typeof(s) _s = s;
>               typeof(d) _d = d;
> 
> 	      // Make s safe against UB
> 	      unsigned int _to_shift = _s >= 0 && _s < 8*sizeof(*d) : _s ? 0;

Should we not do a gcc-plugin or something that fixes that particular
UB? Shift acting all retarded like that is just annoying. I feel we
should eliminate UBs from the language where possible, like
-fno-strict-overflow mandates 2s complement.

>               *_d = (_a << _to_shift);
> 
> 	       // s is malformed
>               (_to_shift != _s ||

Not strictly an overflow though, just not expected behaviour.

> 	       // d is a signed type and became negative
> 	       *_d < 0 ||

Only a problem if it wasn't negative to start out with.

> 	       // a is a signed type and was negative
> 	       _a < 0 ||

Why would that be a problem? You can shift left negative values just
fine. The only problem is when you run out of sign bits.

> 	       // Not invertable means a was truncated during shifting
> 	       (*_d >> _to_shift) != a))
>           })

And I'm not exactly seeing the use case for this macro. What's the point
of a shift-left if you cannot truncate bits. I suppose it's in the name
_overflow, but still.