Re: [Bpf] Signed modulo operations

[Eduard Zingerman <eddyz87@gmail.com>]

On Sat, 2023-09-30 at 07:48 -0700, Alexei Starovoitov wrote:
> On Fri, Sep 29, 2023 at 2:03 PM Dave Thaler
> <dthaler=40microsoft.com@dmarc.ietf.org> wrote:
> > 
> > Perhaps text like the proposed snippet quoted in the exchange above should be
> > added around the new text that now appears in the doc, i.e. the ambiguous text
> > is currently:
> > > For signed operations (``BPF_SDIV`` and ``BPF_SMOD``), for ``BPF_ALU``,
> > > 'imm' is interpreted as a 32-bit signed value. For ``BPF_ALU64``, 'imm'
> > > is first :term:`sign extended<Sign Extend>` from 32 to 64 bits, and then
> > > interpreted as a 64-bit signed value.
> 
> That's what we have in the doc and it's a correct description.
> Which part is ambiguous?
> 

As far as I understand Dave suggests to add exact specification for
the SMOD instruction as "signed modulo" might have different definitions [1].

I double checked and current clang implementation generates SMOD for
LLVM's 'srem' operation [2], which follows C semantics and defines
remainder as:

  remainder = a - n * trunc(divident / divisor)

> This instruction returns the remainder of a division (where the
> result is either zero or has the same sign as the dividend, op1)

And this is consistent with interpreter logic in core.c, e.g.:

	ALU64_MOD_K:
		switch (OFF) {
		case 0: ... break;
		case 1:
			AX = div64_s64(DST, IMM); /* implemented as '/' */
			DST = DST - AX * IMM;
			break;
		}
		CONT;

[1] https://en.wikipedia.org/wiki/Modulo
[2] https://llvm.org/docs/LangRef.html#srem-instruction