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bpf, docs: Define signed modulo as using truncated division

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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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Dave Thaler 2023-10-17 20:30:20 +00:00 committed by Daniel Borkmann
parent bb6a88885f
commit 0e133a1337
1 changed files with 8 additions and 0 deletions
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Documentation/bpf/standardization/instruction-set.rst
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@ -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.