From: Dave Thaler <dthaler@xxxxxxxxxxxxx> 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@xxxxxxxxx/ Signed-off-by: Dave Thaler <dthaler@xxxxxxxxxxxxx> --- 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
