linux-kernel - Re: endian bitshift defects [ was: staging: fusb302: don't bitshift _

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Date:   Fri, 16 Jun 2017 22:50:01 -0700
From:   Joe Perches <joe@...ches.com>
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     Frans Klaver <fransklaver@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        kernel-janitors <kernel-janitors@...r.kernel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Yueyao Zhu <yueyao.zhu@...il.com>,
        Rui Miguel Silva <rmfrfs@...il.com>,
        Guru Das Srinagesh <gurooodas@...il.com>,
        Javier Martinez Canillas <javier@...hile0.org>,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: endian bitshift defects [ was: staging: fusb302: don't bitshift
 __le16 type ]

On Sat, 2017-06-17 at 07:23 +0200, Julia Lawall wrote:
> On Fri, 16 Jun 2017, Joe Perches wrote:
> > On Fri, 2017-06-16 at 19:45 +0200, Frans Klaver wrote:
> > > The header field in struct pd_message is declared as an __le16 type. The
> > > data in the message is supposed to be little endian. This means we don't
> > > have to go and shift the individual bytes into position when we're
> > > filling the buffer, we can just copy the contents right away. As an
> > > added benefit we don't get fishy results on big endian systems anymore.
> > 
> > Thanks for pointing this out.
> > 
> > There are several instances of this class of error.
> > 
> > Here's a cocci script to find them.
> > 
> > This is best used with cocci's --all-includes option like:
> > 
> > $ spatch --all-includes --very-quiet --sp-file lebe_bitshifts.cocci .
> > [ many defects...]

Probably would have been better as [ many possible defects... ]

> > $ cat lebe_bitshifts.cocci
> > @@
> > typedef __le16, __le32, __le64,  __be16, __be32, __be64;
> > { __le16, __le32, __le64,  __be16, __be32, __be64 } a;
> > expression b;
> > @@
> > 
> > *	a << b

[etc...]

> Is this always a problem?

No, not always.

If the CPU is the equivalent endian, the bitshift is fine.
It can't be known if the code is only compiled on a
single cpu type.  It is rather odd though to use endian
notation if the code is compiled for a single cpu type.

> Would it be useful to add this to the scripts
> in the kernel?

Maybe.

btw: is there a way for the operators to be surrounded by
some \( \| \) or some other bracket style so it could
be written with a single test?

Something like:

@@
typedef __le16, __le32, __le64,  __be16, __be32, __be64;
{ __le16, __le32, __le64,  __be16, __be32, __be64 } a;
expression b;
@@

*	a [<<|<<=|>>|>>=] b