linux-kernel - Re: [PATCH] scripts/coccinelle/misc/shift.cocci: detect

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Date:   Thu, 8 Feb 2018 14:58:56 +0100 (CET)
From:   Julia Lawall <julia.lawall@...6.fr>
To:     Dan Carpenter <dan.carpenter@...cle.com>
cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        kernel-janitors@...r.kernel.org,
        Gilles Muller <Gilles.Muller@...6.fr>,
        Nicolas Palix <nicolas.palix@...g.fr>,
        Michal Marek <michal.lkml@...kovi.net>, cocci@...teme.lip6.fr,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scripts/coccinelle/misc/shift.cocci: detect < or > that
 should be a shift



On Thu, 8 Feb 2018, Dan Carpenter wrote:

> On Thu, Feb 08, 2018 at 01:53:54PM +0100, Julia Lawall wrote:
> > This checks for a comparison using < or > between two constants,
> > considering both explicit constants (1, 2, etc) and macros defined
> > with #define.  False positives are possible in the latter case, when
> > a macro may have multiple possible definitions and it is indeed
> > necessary to check the value.  There are currently two such false
> > positives, in drivers/net/ethernet/chelsio/cxgb3/sge.c:
> >
> >        q->fl[0].use_pages = FL0_PG_CHUNK_SIZE > 0;
> >        q->fl[1].use_pages = FL1_PG_CHUNK_SIZE > 0;
> >
>
> We could eliminate both these false postives by ignoring >> vs >.  Did
> searching for > actually find any bugs?  I think you were right that
> right shifting a constant is way less common than left shifting and I
> have some smatch scripts where I ignore right shifting bugs.
>
> On the other hand, two false positives are not a big deal.

I found no bugs with > at the moment.  I figured that the person could
have just as easily written 0 < FL0_PG_CHUNK_SIZE, so it was not worth
making a special case.  But if anyone wants me to drop the case id > cst,
then I can do that.

thanks,
julia