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

[Michal Wajdeczko]

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:

#define make_u16_from_u8(hi, lo) \
	((u16)(FIELD_PREP_CONST(GENMASK(15, 8), (hi)) | \
	       FIELD_PREP_CONST(GENMASK(7, 0), (lo))))

#define make_u32_from_u16(hi, lo) \
	((u32)(FIELD_PREP_CONST(GENMASK(31, 16), (hi)) | \
	       FIELD_PREP_CONST(GENMASK(15, 0), (lo))))

#define make_u64_from_u32(hi, lo) \
	((u64)(FIELD_PREP_CONST(GENMASK_ULL(63, 32), (hi)) | \
	       FIELD_PREP_CONST(GENMASK_ULL(31, 0), (lo))))

but then it will not match simplicity of the lower|upper_XX_bits macros