1 Linux implementation notes

This document provides more details specific to the Linux kernel implementation of the eBPF instruction set.

1.1 Byte swap instructions

BPF_FROM_LE and BPF_FROM_BE exist as aliases for BPF_TO_LE and BPF_TO_BE respectively.

1.2 Legacy BPF Packet access instructions

As mentioned in the ISA standard documentation, Linux has special eBPF instructions for access to packet data that have been carried over from classic BPF to retain the performance of legacy socket filters running in the eBPF interpreter.

The instructions come in two forms: BPF_ABS | <size> | BPF_LD and BPF_IND | <size> | BPF_LD.

These instructions are used to access packet data and can only be used when the program context is a pointer to a networking packet. BPF_ABS accesses packet data at an absolute offset specified by the immediate data and BPF_IND access packet data at an offset that includes the value of a register in addition to the immediate data.

These instructions have seven implicit operands:

  • Register R6 is an implicit input that must contain a pointer to a struct sk_buff.

  • Register R0 is an implicit output which contains the data fetched from the packet.

  • Registers R1-R5 are scratch registers that are clobbered by the instruction.

These instructions have an implicit program exit condition as well. If an eBPF program attempts access data beyond the packet boundary, the program execution will be aborted.

BPF_ABS | BPF_W | BPF_LD (0x20) means:

R0 = ntohl(*(u32 *) ((struct sk_buff *) R6->data + imm))

where ntohl() converts a 32-bit value from network byte order to host byte order.

BPF_IND | BPF_W | BPF_LD (0x40) means:

R0 = ntohl(*(u32 *) ((struct sk_buff *) R6->data + src + imm))