linux-kernel - Re: [PATCH 11/16] byteorder: provide a linux/byteorder.h with {be, le}_to_cpu() and cpu_to

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Date:	Wed, 28 May 2014 18:26:57 -0500
From:	Cody P Schafer <dev@...yps.com>
To:	Joe Perches <joe@...ches.com>
Cc:	David Laight <David.Laight@...lab.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PPC <linuxppc-dev@...ts.ozlabs.org>,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Subject: Re: [PATCH 11/16] byteorder: provide a linux/byteorder.h with {be,
 le}_to_cpu() and cpu_to_{be, le}() macros

On Wed, May 28, 2014 at 6:00 PM, Joe Perches <joe@...ches.com> wrote:
> On Wed, 2014-05-28 at 17:11 -0500, Cody P Schafer wrote:
>> On Wed, May 28, 2014 at 5:05 PM, Cody P Schafer <dev@...yps.com> wrote:
>> > On Wed, May 28, 2014 at 3:45 AM, David Laight <David.Laight@...lab.com> wrote:
>> >> From: Cody P Schafer
>> >>> Rather manually specifying the size of the integer to be converted, key
>> >>> off of the type size. Reduces duplicate size info and the occurance of
>> >>> certain types of bugs (using the wrong sized conversion).
>> >> ...
>> >>> +#define be_to_cpu(v) \
>> >>> +     __builtin_choose_expr(sizeof(v) == sizeof(uint8_t) , v, \
>> >>> +     __builtin_choose_expr(sizeof(v) == sizeof(uint16_t), be16_to_cpu(v), \
>> >>> +     __builtin_choose_expr(sizeof(v) == sizeof(uint32_t), be32_to_cpu(v), \
>> >>> +     __builtin_choose_expr(sizeof(v) == sizeof(uint64_t), be64_to_cpu(v), \
>> >>> +             (void)0))))
>> >> ...
>> >>
>> >> I'm not at all sure that using the 'size' of the constant will reduce
>> >> the number of bugs - it just introduces a whole new category of bugs.
>> >
>> > Certainly, if you mis-size the argument (and thus have missized one of
>> > the variables containing the be value, probably a bug anyhow), there
>> > will be problems.
>> >
>> > I put this interface together because of an actual bug I wrote into
>> > the initial code of the hv_24x7 driver (resized a struct member
>> > without adjusting the be*_to_cpu() sizing).
>> > Having this "auto sizing" macro means I can avoid encoding the size of
>> > a struct field in multiple places.
>>
>> To clarify, the point I'm making here is that this simply cuts out 1
>> more place we can screw up endianness conversion sizing.
>
> It does screw up other types when you do things like:
>
>         u8 foo = some_function();
>
>         cpu_to_be(foo + 1);
>
> the return value is sizeof(int) not u8

Yep, that is a very good argument against the cpu_to_{be,le}()
variants. It might make sense to remove them and just have the
{be,le}_to_cpu() ones.
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