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bpf, docs: Define signed modulo as using truncated division
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> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Acked-by: David Vernet <void@manifault.com> Acked-by: Eduard Zingerman <eddyz87@gmail.com> Link: https://lore.kernel.org/bpf/20231017203020.1500-1-dthaler1968@googlemail.com
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@ -283,6 +283,14 @@ For signed operations (``BPF_SDIV`` and ``BPF_SMOD``), for ``BPF_ALU``,
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is first :term:`sign extended<Sign Extend>` from 32 to 64 bits, and then
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interpreted as a 64-bit signed value.
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Note that there are varying definitions of the signed modulo operation
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when the dividend or divisor are negative, where implementations often
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vary by language such that Python, Ruby, etc. differ from C, Go, Java,
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etc. This specification requires that signed modulo use truncated division
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(where -13 % 3 == -1) as implemented in C, Go, etc.:
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a % n = a - n * trunc(a / n)
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The ``BPF_MOVSX`` instruction does a move operation with sign extension.
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``BPF_ALU | BPF_MOVSX`` :term:`sign extends<Sign Extend>` 8-bit and 16-bit operands into 32
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bit operands, and zeroes the remaining upper 32 bits.
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