On Tue, Sep 27, 2022 at 06:59:50PM +0000, dthaler1968@xxxxxxxxxxxxxx wrote: > From: Dave Thaler <dthaler@xxxxxxxxxxxxx> > > Signed-off-by: Dave Thaler <dthaler@xxxxxxxxxxxxx> > --- > Documentation/bpf/instruction-set.rst | 19 +++++++++++++++++-- > 1 file changed, 17 insertions(+), 2 deletions(-) > > diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst > index a24bc5d53..3c5a63612 100644 > --- a/Documentation/bpf/instruction-set.rst > +++ b/Documentation/bpf/instruction-set.rst > @@ -103,19 +103,26 @@ code value description > BPF_ADD 0x00 dst += src > BPF_SUB 0x10 dst -= src > BPF_MUL 0x20 dst \*= src > -BPF_DIV 0x30 dst /= src > +BPF_DIV 0x30 dst = (src != 0) ? (dst / src) : 0 > BPF_OR 0x40 dst \|= src > BPF_AND 0x50 dst &= src > BPF_LSH 0x60 dst <<= src > BPF_RSH 0x70 dst >>= src > BPF_NEG 0x80 dst = ~src > -BPF_MOD 0x90 dst %= src > +BPF_MOD 0x90 dst = (src != 0) ? (dst % src) : dst > BPF_XOR 0xa0 dst ^= src > BPF_MOV 0xb0 dst = src > BPF_ARSH 0xc0 sign extending shift right > BPF_END 0xd0 byte swap operations (see `Byte swap instructions`_ below) > ======== ===== ========================================================== > > +Underflow and overflow are allowed during arithmetic operations, > +meaning the 64-bit or 32-bit value will wrap. If > +eBPF program execution would result in division by zero, > +the destination register is instead set to zero. > +If execution would result in modulo by zero, > +the destination register is instead left unchanged. > + > ``BPF_ADD | BPF_X | BPF_ALU`` means:: > > dst_reg = (uint32_t) dst_reg + (uint32_t) src_reg; > @@ -135,6 +142,14 @@ where '(uint32_t)' indicates truncation to 32 bits. > src_reg = src_reg ^ imm32 > > > +Also note that the modulo operation often varies by language > +when the dividend or divisor are negative, where Python, Ruby, etc. > +differ from C, Go, Java, etc. This specification requires that > +modulo use truncated division (where -13 % 3 == -1) as implemented > +in C, Go, etc.: > + > + a % n = a - n * trunc(a / n) > + Interesting bit of info, but I'm not sure how it relates to the ISA doc. I think it's more important to say that BPF ISA only supports unsigned div/mod. There are no instructions for signed div/mod. > Byte swap instructions > ~~~~~~~~~~~~~~~~~~~~~~ > > -- > 2.33.4 >
