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) + Byte swap instructions ~~~~~~~~~~~~~~~~~~~~~~ -- 2.33.4
