ISA: narrow the definition of legacy opcodes

[dthaler]

Currently for legacy packet access instructions, we deprecate an entire opcode. If some fields could instead be constrained to 0, it would leave more space un-reserved.

I think:

• ABS: dst_reg=0, src_reg=0, offset=0, imm=some offset
• IND: dst_reg=0 src_reg=register number, offset=0, imm=some offset

[dthaler]

Confirmed.

https://elixir.bootlin.com/linux/v6.8-rc1/source/kernel/bpf/verifier.c#L15101 has:

• SRC == any register
• IMM == 32-bit immediate

https://elixir.bootlin.com/linux/v6.8-rc1/source/kernel/bpf/verifier.c#L15127 checks that SRC = 0 for ABS.

[dthaler]

Proposed patch posted at https://mailarchive.ietf.org/arch/msg/bpf/1IajDsjWY2_43rX_qmloFkIADcw/

[dthaler]

Fixed in PR #99

[dthaler]

Fixed in draft -01