Re: [PATCH 1/2] wordpart.h: Helpers for making u16/u32/u64 values

[Michal Wajdeczko]

On 15.02.2024 23:47, Kees Cook wrote:
> On Thu, Feb 15, 2024 at 09:40:40PM +0100, Michal Wajdeczko wrote:
>> On 14.02.2024 23:09, Kees Cook wrote:
>>> On Wed, Feb 14, 2024 at 10:46:53PM +0100, Michal Wajdeczko wrote:
>>>> It is quite common practice to make u16, u32 or u64 values from
>>>> smaller words.  Add simple helpers for that.
>>>>
>>>> Signed-off-by: Michal Wajdeczko <michal.wajdeczko@xxxxxxxxx>
>>>> ---
>>>> v2: new macro names due to conflict with crypto/aria.h
>>>>     explicit cast and truncation everywhere (Alexey)
>>>>     moved to wordpart.h (Andy)
>>>> ---
>>>> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>>>> Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
>>>> Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
>>>> Cc: Jani Nikula <jani.nikula@xxxxxxxxx>
>>>> ---
>>>>  include/linux/wordpart.h | 32 ++++++++++++++++++++++++++++++++
>>>>  1 file changed, 32 insertions(+)
>>>>
>>>> diff --git a/include/linux/wordpart.h b/include/linux/wordpart.h
>>>> index f6f8f83b15b0..8c75a5355112 100644
>>>> --- a/include/linux/wordpart.h
>>>> +++ b/include/linux/wordpart.h
>>>> @@ -31,6 +31,38 @@
>>>>   */
>>>>  #define lower_16_bits(n) ((u16)((n) & 0xffff))
>>>>  
>>>> +/**
>>>> + * make_u16_from_u8 - make u16 value from two u8 values
>>>> + * @hi: value representing upper 8 bits
>>>> + * @lo: value representing lower 8 bits
>>>> + */
>>>> +#define make_u16_from_u8(hi, lo) ((u16)((u16)(u8)(hi) << 8 | (u8)(lo)))
>>>
>>> Do we want to actually do type validation here? Right now it's just
>>> cast/truncating, which based on the version log is by design. Is silent
>>> truncation the right thing to do?
>>
>> note that even FIELD_PREP() is doing silent truncation and these macros
>> here could be treated as specialized/simplified variants of FIELD_PREP()
>> as alternate implementation can look like:
> 
> Also, isn't all of this endian-specific?

endianness shouldn't matter here

so I guess the only question now is whether we want to have simple
direct macros like lower|upper_bits() or go with macros build on top of
the FIELD_PREP_CONST() or drop the idea completely and force the drivers
to invent the wheel on its own