On Tue, Oct 17, 2023 at 08:30:20PM +0000, Dave Thaler wrote: > 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> Acked-by: David Vernet <void@xxxxxxxxxxxxx> +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 > >
