1.. contents:: 2.. sectnum:: 3 4========================== 5Linux implementation notes 6========================== 7 8This document provides more details specific to the Linux kernel implementation of the eBPF instruction set. 9 10Byte swap instructions 11====================== 12 13``BPF_FROM_LE`` and ``BPF_FROM_BE`` exist as aliases for ``BPF_TO_LE`` and ``BPF_TO_BE`` respectively. 14 15Jump instructions 16================= 17 18``BPF_CALL | BPF_X | BPF_JMP`` (0x8d), where the helper function 19integer would be read from a specified register, is not currently supported 20by the verifier. Any programs with this instruction will fail to load 21until such support is added. 22 23Maps 24==== 25 26Linux only supports the 'map_val(map)' operation on array maps with a single element. 27 28Linux uses an fd_array to store maps associated with a BPF program. Thus, 29map_by_idx(imm) uses the fd at that index in the array. 30 31Variables 32========= 33 34The following 64-bit immediate instruction specifies that a variable address, 35which corresponds to some integer stored in the 'imm' field, should be loaded: 36 37========================= ====== === ========================================= =========== ============== 38opcode construction opcode src pseudocode imm type dst type 39========================= ====== === ========================================= =========== ============== 40BPF_IMM | BPF_DW | BPF_LD 0x18 0x3 dst = var_addr(imm) variable id data pointer 41========================= ====== === ========================================= =========== ============== 42 43On Linux, this integer is a BTF ID. 44 45Legacy BPF Packet access instructions 46===================================== 47 48As mentioned in the `ISA standard documentation <instruction-set.rst#legacy-bpf-packet-access-instructions>`_, 49Linux has special eBPF instructions for access to packet data that have been 50carried over from classic BPF to retain the performance of legacy socket 51filters running in the eBPF interpreter. 52 53The instructions come in two forms: ``BPF_ABS | <size> | BPF_LD`` and 54``BPF_IND | <size> | BPF_LD``. 55 56These instructions are used to access packet data and can only be used when 57the program context is a pointer to a networking packet. ``BPF_ABS`` 58accesses packet data at an absolute offset specified by the immediate data 59and ``BPF_IND`` access packet data at an offset that includes the value of 60a register in addition to the immediate data. 61 62These instructions have seven implicit operands: 63 64* Register R6 is an implicit input that must contain a pointer to a 65 struct sk_buff. 66* Register R0 is an implicit output which contains the data fetched from 67 the packet. 68* Registers R1-R5 are scratch registers that are clobbered by the 69 instruction. 70 71These instructions have an implicit program exit condition as well. If an 72eBPF program attempts access data beyond the packet boundary, the 73program execution will be aborted. 74 75``BPF_ABS | BPF_W | BPF_LD`` (0x20) means:: 76 77 R0 = ntohl(*(u32 *) ((struct sk_buff *) R6->data + imm)) 78 79where ``ntohl()`` converts a 32-bit value from network byte order to host byte order. 80 81``BPF_IND | BPF_W | BPF_LD`` (0x40) means:: 82 83 R0 = ntohl(*(u32 *) ((struct sk_buff *) R6->data + src + imm)) 84