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

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Date:   Mon, 26 Jun 2017 22:57:48 +0200
From:   Frans Klaver <fransklaver@...il.com>
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     Joe Perches <joe@...ches.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" <linux-kernel@...r.kernel.org>
Subject: Re: endian bitshift defects [ was: staging: fusb302: don't bitshift
 __le16 type ]

On Fri, Jun 23, 2017 at 07:37:28PM -0400, Julia Lawall wrote:
> 
> 
> On Sat, 24 Jun 2017, Frans Klaver wrote:
> 
> > Hm. For some reason the great mail filtering scheme decided to push
> > this past my inbox :-/
> >
> > On Sat, Jun 17, 2017 at 12:44 AM, Joe Perches <joe@...ches.com> 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.
> >
> > There are other smells around __(le|be) types that show up in staging
> > that might be worth checking in the rest of the kernel as well. e.g.
> > converting to cpu and storing it back into itself (possibly with its
> > bytes reversed), direct assignments without conversion and what else
> > you might have. sparse obviously already flags anything fishy going on
> > with these types, but cannot distinguish between the classes of
> > errors. I'll need to acquaint myself with spatch a bit more to be able
> > to track that down.
> 
> If you have concrete code examples, even fake ones, illustrating a class
> of problem, then that would be great.

Alright, I'll describe two fairly simple cases for starters.

One class of issue that I have on top of mind is simply

	__le16 val;

	val = le16_to_cpu(val);

The problem there obviously being that val is supposed to be guaranteed
little endian. Sparse will throw a warning at this. It may also appear
as (or be 'fixed' as)

	__le16 val;

	le16_to_cpus(val);

Sparse doesn't flag this second version as an issue, while it causes the
same problem. It is especially a potential problem when the value is
stored in driver data.

Another smell that is prevalent, at least in staging, is

	u16 in;
	u16 out;

	out = cpu_to_le16(in);

or in one instance (drivers/staging/fbtft/fbtft-io.c) I saw

	u64 tmp;

	*(u64*)dst = cpu_to_be64(tmp);

Now these aren't necessarily problematic. Usually this typo of code is
preparing the data to be sent out in a specific byte ordering, but again
issues may arise if this specifically ordered data is stored somewhere.

I'll leave it at that for now. 

Frans