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Re: endian bitshift defects [ was: staging: fusb302: don't bitshift __le16 type ]





On Fri, 16 Jun 2017, Joe Perches wrote:

> 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... ]

OK

> > > $ 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.

Is there some way to know from the code if it is compiled for a single cou
type?

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

If there are a lot of false positives, it could be a nuisance...

> 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

Partly.  You can define

binary operator bop = {<<,>>};

or

assignment operator aop = {<<=,>>=};

to make two rules instead of four.

julia