Re: [PATCH bpf-next] bpf, docs: Define signed modulo as using truncated division

[David Vernet <void@manifault.com>]

On Tue, Oct 17, 2023 at 08:30:20PM +0000, Dave Thaler wrote:
> From: Dave Thaler <dthaler@microsoft.com>
> 
> There's different mathematical definitions (truncated, floored,
> rounded, etc.) and different languages have chosen different
> definitions [0][1].  E.g., languages/libraries that follow Knuth
> use a different mathematical definition than C uses.  This
> patch specifies which definition BPF uses, as verified by
> Eduard [2] and others.
> 
> [0]: https://en.wikipedia.org/wiki/Modulo#Variants_of_the_definition
> [1]: https://torstencurdt.com/tech/posts/modulo-of-negative-numbers/
> [2]: https://lore.kernel.org/bpf/57e6fefadaf3b2995bb259fa8e711c7220ce5290.camel@gmail.com/
> 
> Signed-off-by: Dave Thaler <dthaler@microsoft.com>

Acked-by: David Vernet <void@manifault.com>

+cc Eduard as well in case he wants to take a look.

> ---
>  Documentation/bpf/standardization/instruction-set.rst | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/Documentation/bpf/standardization/instruction-set.rst b/Documentation/bpf/standardization/instruction-set.rst
> index c5d53a6e8c7..245b6defc29 100644
> --- a/Documentation/bpf/standardization/instruction-set.rst
> +++ b/Documentation/bpf/standardization/instruction-set.rst
> @@ -283,6 +283,14 @@ For signed operations (``BPF_SDIV`` and ``BPF_SMOD``), for ``BPF_ALU``,
>  is first :term:`sign extended<Sign Extend>` from 32 to 64 bits, and then
>  interpreted as a 64-bit signed value.
>  
> +Note that there are varying definitions of the signed modulo operation
> +when the dividend or divisor are negative, where implementations often
> +vary by language such that Python, Ruby, etc.  differ from C, Go, Java,
> +etc. This specification requires that signed modulo use truncated division
> +(where -13 % 3 == -1) as implemented in C, Go, etc.:
> +
> +   a % n = a - n * trunc(a / n)
> +
>  The ``BPF_MOVSX`` instruction does a move operation with sign extension.
>  ``BPF_ALU | BPF_MOVSX`` :term:`sign extends<Sign Extend>` 8-bit and 16-bit operands into 32
>  bit operands, and zeroes the remaining upper 32 bits.
> -- 
> 2.33.4
> 
>