Re: [PATCH v3 2/3] overflow: Introduce add_wrap(), sub_wrap(), and mul_wrap()

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 2/5/24 07:31, Marco Elver wrote:
> On Mon, 5 Feb 2024 at 10:12, Kees Cook <keescook@chromium.org> wrote:
>>
>> Provide helpers that will perform wrapping addition, subtraction, or
>> multiplication without tripping the arithmetic wrap-around sanitizers. The
>> first argument is the type under which the wrap-around should happen
>> with. In other words, these two calls will get very different results:
>>
>>          mul_wrap(int, 50, 50) == 2500
>>          mul_wrap(u8,  50, 50) ==  196
>>
>> Add to the selftests to validate behavior and lack of side-effects.
>>
>> Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: linux-hardening@vger.kernel.org
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>>   include/linux/overflow.h | 54 ++++++++++++++++++++++++++++++++++++++++
>>   lib/overflow_kunit.c     | 23 ++++++++++++++---
>>   2 files changed, 73 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
>> index 4e741ebb8005..9b8c05bdb788 100644
>> --- a/include/linux/overflow.h
>> +++ b/include/linux/overflow.h
>> @@ -64,6 +64,24 @@ static inline bool __must_check __must_check_overflow(bool overflow)
>>   #define check_add_overflow(a, b, d)    \
>>          __must_check_overflow(__builtin_add_overflow(a, b, d))
>>
>> +/**
>> + * add_wrap() - Intentionally perform a wrapping addition
>> + * @type: type for result of calculation
>> + * @a: first addend
>> + * @b: second addend
>> + *
>> + * Return the potentially wrapped-around addition without
>> + * tripping any wrap-around sanitizers that may be enabled.
>> + */
>> +#define add_wrap(type, a, b)                           \
>> +       ({                                              \
>> +               type __val;                             \
>> +               if (check_add_overflow(a, b, &__val)) { \
>> +                       /* do nothing */                \
> 
> The whole reason check_*_overflow() exists is to wrap the builtin in a
> function with __must_check. Here the result is explicitly ignored, so
> do we have to go through the check_add_overflow indirection? Why not
> just use the builtin directly? It might make sense to make the
> compiler's job a little easier, because I predict that
> __must_check_overflow will be outlined with enough instrumentation
> (maybe it should have been __always_inline).

Yeah; I think that directly calling __builtin_*_overflow() is a bit
cleaner.

I wonder if there is any particular reason for not doing that.

In any case, this version of the add_wrap() helper with the `type` as
parameter looks much better than the v1 that relied on `typeof(a)`. :)

So,

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Thanks!
--
Gustavo