1 eBPF Instruction Set Specification, v1.0 ¶

This document specifies version 1.0 of the eBPF instruction set.

1.1 Registers and calling convention ¶

eBPF has 10 general purpose registers and a read-only frame pointer register, all of which are 64-bits wide.

The eBPF calling convention is defined as:

R0: return value from function calls, and exit value for eBPF programs
R1 - R5: arguments for function calls
R6 - R9: callee saved registers that function calls will preserve
R10: read-only frame pointer to access stack

R0 - R5 are scratch registers and eBPF programs needs to spill/fill them if necessary across calls.

1.2 Instruction encoding ¶

eBPF has two instruction encodings:

the basic instruction encoding, which uses 64 bits to encode an instruction
the wide instruction encoding, which appends a second 64-bit immediate value (imm64) after the basic instruction for a total of 128 bits.

The basic instruction encoding looks as follows:

32 bits (MSB)	16 bits	4 bits	4 bits	8 bits (LSB)
immediate	offset	source register	destination register	opcode

Note that most instructions do not use all of the fields. Unused fields shall be cleared to zero.

1.2.1 Instruction classes ¶

The three LSB bits of the ‘opcode’ field store the instruction class:

class	value	description	reference
BPF_LD	0x00	non-standard load operations	Load and store instructions
BPF_LDX	0x01	load into register operations	Load and store instructions
BPF_ST	0x02	store from immediate operations	Load and store instructions
BPF_STX	0x03	store from register operations	Load and store instructions
BPF_ALU	0x04	32-bit arithmetic operations	Arithmetic and jump instructions
BPF_JMP	0x05	64-bit jump operations	Arithmetic and jump instructions
BPF_JMP32	0x06	32-bit jump operations	Arithmetic and jump instructions
BPF_ALU64	0x07	64-bit arithmetic operations	Arithmetic and jump instructions

1.3 Arithmetic and jump instructions ¶

For arithmetic and jump instructions (BPF_ALU, BPF_ALU64, BPF_JMP and BPF_JMP32), the 8-bit ‘opcode’ field is divided into three parts:

4 bits (MSB)	1 bit	3 bits (LSB)
operation code	source	instruction class

The 4th bit encodes the source operand:

source

value

description

BPF_K

0x00

use 32-bit immediate as source operand

BPF_X

0x08

use ‘src_reg’ register as source operand

The four MSB bits store the operation code.

1.3.1 Arithmetic instructions ¶

BPF_ALU uses 32-bit wide operands while BPF_ALU64 uses 64-bit wide operands for otherwise identical operations. The ‘code’ field encodes the operation as below:

code	value	description
BPF_ADD	0x00	dst += src
BPF_SUB	0x10	dst -= src
BPF_MUL	0x20	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 != 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, for BPF_ALU64 the value of the destination register is unchanged whereas for BPF_ALU the upper 32 bits of the destination register are zeroed.

BPF_ADD | BPF_X | BPF_ALU means:

dst_reg = (u32) dst_reg + (u32) src_reg;

BPF_ADD | BPF_X | BPF_ALU64 means:

dst_reg = dst_reg + src_reg

BPF_XOR | BPF_K | BPF_ALU means:

src_reg = (u32) src_reg ^ (u32) imm32

BPF_XOR | BPF_K | BPF_ALU64 means:

src_reg = src_reg ^ imm32

Also note that the division and modulo operations are unsigned. Thus, for BPF_ALU, ‘imm’ is first interpreted as an unsigned 32-bit value, whereas for BPF_ALU64, ‘imm’ is first sign extended to 64 bits and the result interpreted as an unsigned 64-bit value. There are no instructions for signed division or modulo.

1.3.1.1 Byte swap instructions ¶

The byte swap instructions use an instruction class of BPF_ALU and a 4-bit ‘code’ field of BPF_END.

The byte swap instructions operate on the destination register only and do not use a separate source register or immediate value.

The 1-bit source operand field in the opcode is used to select what byte order the operation convert from or to:

source	value	description
BPF_TO_LE	0x00	convert between host byte order and little endian
BPF_TO_BE	0x08	convert between host byte order and big endian

The ‘imm’ field encodes the width of the swap operations. The following widths are supported: 16, 32 and 64.

Examples:

BPF_ALU | BPF_TO_LE | BPF_END with imm = 16 means:

dst_reg = htole16(dst_reg)

BPF_ALU | BPF_TO_BE | BPF_END with imm = 64 means:

dst_reg = htobe64(dst_reg)

1.3.2 Jump instructions ¶

BPF_JMP32 uses 32-bit wide operands while BPF_JMP uses 64-bit wide operands for otherwise identical operations. The ‘code’ field encodes the operation as below:

code	value	description	notes
BPF_JA	0x00	PC += off	BPF_JMP only
BPF_JEQ	0x10	PC += off if dst == src
BPF_JGT	0x20	PC += off if dst > src	unsigned
BPF_JGE	0x30	PC += off if dst >= src	unsigned
BPF_JSET	0x40	PC += off if dst & src
BPF_JNE	0x50	PC += off if dst != src
BPF_JSGT	0x60	PC += off if dst > src	signed
BPF_JSGE	0x70	PC += off if dst >= src	signed
BPF_CALL	0x80	function call
BPF_EXIT	0x90	function / program return	BPF_JMP only
BPF_JLT	0xa0	PC += off if dst < src	unsigned
BPF_JLE	0xb0	PC += off if dst <= src	unsigned
BPF_JSLT	0xc0	PC += off if dst < src	signed
BPF_JSLE	0xd0	PC += off if dst <= src	signed

The eBPF program needs to store the return value into register R0 before doing a BPF_EXIT.

1.4 Load and store instructions ¶

For load and store instructions (BPF_LD, BPF_LDX, BPF_ST, and BPF_STX), the 8-bit ‘opcode’ field is divided as:

3 bits (MSB)	2 bits	3 bits (LSB)
mode	size	instruction class

The mode modifier is one of:

mode modifier

value

description

reference

BPF_IMM

0x00

64-bit immediate instructions

64-bit immediate instructions

BPF_ABS

0x20

legacy BPF packet access (absolute)

Legacy BPF Packet access instructions

BPF_IND

0x40

legacy BPF packet access (indirect)

Legacy BPF Packet access instructions

BPF_MEM

0x60

regular load and store operations

Regular load and store operations

BPF_ATOMIC

0xc0

atomic operations

Atomic operations

The size modifier is one of:

size modifier

value

description

BPF_W

0x00

word (4 bytes)

BPF_H

0x08

half word (2 bytes)

BPF_B

0x10

byte

BPF_DW

0x18

double word (8 bytes)

1.4.1 Regular load and store operations ¶

The BPF_MEM mode modifier is used to encode regular load and store instructions that transfer data between a register and memory.

BPF_MEM | <size> | BPF_STX means:

*(size *) (dst_reg + off) = src_reg

BPF_MEM | <size> | BPF_ST means:

*(size *) (dst_reg + off) = imm32

BPF_MEM | <size> | BPF_LDX means:

dst_reg = *(size *) (src_reg + off)

Where size is one of: BPF_B, BPF_H, BPF_W, or BPF_DW.

1.4.2 Atomic operations ¶

Atomic operations are operations that operate on memory and can not be interrupted or corrupted by other access to the same memory region by other eBPF programs or means outside of this specification.

All atomic operations supported by eBPF are encoded as store operations that use the BPF_ATOMIC mode modifier as follows:

BPF_ATOMIC | BPF_W | BPF_STX for 32-bit operations
BPF_ATOMIC | BPF_DW | BPF_STX for 64-bit operations
8-bit and 16-bit wide atomic operations are not supported.

The ‘imm’ field is used to encode the actual atomic operation. Simple atomic operation use a subset of the values defined to encode arithmetic operations in the ‘imm’ field to encode the atomic operation:

imm	value	description
BPF_ADD	0x00	atomic add
BPF_OR	0x40	atomic or
BPF_AND	0x50	atomic and
BPF_XOR	0xa0	atomic xor

BPF_ATOMIC | BPF_W | BPF_STX with ‘imm’ = BPF_ADD means:

*(u32 *)(dst_reg + off16) += src_reg

BPF_ATOMIC | BPF_DW | BPF_STX with ‘imm’ = BPF ADD means:

*(u64 *)(dst_reg + off16) += src_reg

In addition to the simple atomic operations, there also is a modifier and two complex atomic operations:

imm	value	description
BPF_FETCH	0x01	modifier: return old value
BPF_XCHG	0xe0 \| BPF_FETCH	atomic exchange
BPF_CMPXCHG	0xf0 \| BPF_FETCH	atomic compare and exchange

The BPF_FETCH modifier is optional for simple atomic operations, and always set for the complex atomic operations. If the BPF_FETCH flag is set, then the operation also overwrites src_reg with the value that was in memory before it was modified.

The BPF_XCHG operation atomically exchanges src_reg with the value addressed by dst_reg + off.

The BPF_CMPXCHG operation atomically compares the value addressed by dst_reg + off with R0. If they match, the value addressed by dst_reg + off is replaced with src_reg. In either case, the value that was at dst_reg + off before the operation is zero-extended and loaded back to R0.

1.4.3 64-bit immediate instructions ¶

Instructions with the BPF_IMM ‘mode’ modifier use the wide instruction encoding for an extra imm64 value.

There is currently only one such instruction.

BPF_LD | BPF_DW | BPF_IMM means:

dst_reg = imm64

1.4.4 Legacy BPF Packet access instructions ¶

eBPF previously introduced special instructions for access to packet data that were carried over from classic BPF. However, these instructions are deprecated and should no longer be used.