RE: [PATCH 11/16] byteorder: provide a linux/byteorder.h with {be, le}_to_cpu() and cpu_to_{be, le}() macros

[David Laight]

From: Joe Perches
> 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@xxxxxxxxxx> wrote:
> > > On Wed, May 28, 2014 at 3:45 AM, David Laight <David.Laight@xxxxxxxxxx> 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

And you'd have to be very, very careful with constants:
#define SOME_CONST xxxxx
	cp_to_be(SOME_CONST)
isn't likely to be the right size.

	David



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