1 eBPF Instruction Set Specification, v1.0 ¶

This document specifies version 1.0 of the eBPF instruction set.

1.1 Documentation conventions ¶

For brevity, this document uses the type notion "u64", "u32", etc. to mean an unsigned integer whose width is the specified number of bits, and "s32", etc. to mean a signed integer of the specified number of bits.

1.2 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.

1.3 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 (i.e., constant) value after the basic instruction for a total of 128 bits.

The fields conforming an encoded basic instruction are stored in the following order:

opcode:8 src_reg:4 dst_reg:4 offset:16 imm:32 // In little-endian BPF.
opcode:8 dst_reg:4 src_reg:4 offset:16 imm:32 // In big-endian BPF.

imm: signed integer immediate value
offset: signed integer offset used with pointer arithmetic
src_reg: the source register number (0-10), except where otherwise specified (64-bit immediate instructions reuse this field for other purposes)
dst_reg: destination register number (0-10)
opcode: operation to perform

Note that the contents of multi-byte fields ('imm' and 'offset') are stored using big-endian byte ordering in big-endian BPF and little-endian byte ordering in little-endian BPF.

For example:

opcode                  offset imm          assembly
       src_reg dst_reg
07     0       1        00 00  44 33 22 11  r1 += 0x11223344 // little
       dst_reg src_reg
07     1       0        00 00  11 22 33 44  r1 += 0x11223344 // big

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

As discussed below in 64-bit immediate instructions, a 64-bit immediate instruction uses a 64-bit immediate value that is constructed as follows. The 64 bits following the basic instruction contain a pseudo instruction using the same format but with opcode, dst_reg, src_reg, and offset all set to zero, and imm containing the high 32 bits of the immediate value.

This is depicted in the following figure:

      basic_instruction
.-----------------------------.
|                             |
code:8 regs:8 offset:16 imm:32 unused:32 imm:32
                               |              |
                               '--------------'
                              pseudo instruction

Thus the 64-bit immediate value is constructed as follows:

imm64 = (next_imm << 32) | imm

where 'next_imm' refers to the imm value of the pseudo instruction following the basic instruction. The unused bytes in the pseudo instruction are reserved and shall be cleared to zero.

1.3.1 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

1.4 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)
code	source	instruction class

code

the operation code, whose meaning varies by instruction class

source

the source operand location, which unless otherwise specified is one of:

source	value	description
BPF_K	0x00	use 32-bit 'imm' value as source operand
BPF_X	0x08	use 'src_reg' register value as source operand

instruction class

the instruction class (see Instruction classes)

1.4.1 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, where 'src' and 'dst' refer to the values of the source and destination registers, respectively.

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 & mask)
BPF_RSH	0x70	dst >>= (src & mask)
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 dst >>= (src & mask)
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 = (u32) ((u32) dst + (u32) src)

where '(u32)' indicates that the upper 32 bits are zeroed.

BPF_ADD | BPF_X | BPF_ALU64 means:

dst = dst + src

BPF_XOR | BPF_K | BPF_ALU means:

dst = (u32) dst ^ (u32) imm32

BPF_XOR | BPF_K | BPF_ALU64 means:

dst = dst ^ 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.

Shift operations use a mask of 0x3F (63) for 64-bit operations and 0x1F (31) for 32-bit operations.

1.4.1.1 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 = htole16(dst)

BPF_ALU | BPF_TO_BE | BPF_END with imm = 64 means:

dst = htobe64(dst)

1.4.2 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	src	description	notes
BPF_JA	0x0	0x0	PC += offset	BPF_JMP only
BPF_JEQ	0x1	any	PC += offset if dst == src
BPF_JGT	0x2	any	PC += offset if dst > src	unsigned
BPF_JGE	0x3	any	PC += offset if dst >= src	unsigned
BPF_JSET	0x4	any	PC += offset if dst & src
BPF_JNE	0x5	any	PC += offset if dst != src
BPF_JSGT	0x6	any	PC += offset if dst > src	signed
BPF_JSGE	0x7	any	PC += offset if dst >= src	signed
BPF_CALL	0x8	0x0	call helper function by address	see Helper functions
BPF_CALL	0x8	0x1	call PC += offset	see Program-local functions
BPF_CALL	0x8	0x2	call helper function by BTF ID	see Helper functions
BPF_EXIT	0x9	0x0	return	BPF_JMP only
BPF_JLT	0xa	any	PC += offset if dst < src	unsigned
BPF_JLE	0xb	any	PC += offset if dst <= src	unsigned
BPF_JSLT	0xc	any	PC += offset if dst < src	signed
BPF_JSLE	0xd	any	PC += offset if dst <= src	signed

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

Example:

BPF_JSGE | BPF_X | BPF_JMP32 (0x7e) means:

if (s32)dst s>= (s32)src goto +offset

where 's>=' indicates a signed '>=' comparison.

1.4.2.1 Helper functions ¶

Helper functions are a concept whereby BPF programs can call into a set of function calls exposed by the underlying platform.

Historically, each helper function was identified by an address encoded in the imm field. The available helper functions may differ for each program type, but address values are unique across all program types.

Platforms that support the BPF Type Format (BTF) support identifying a helper function by a BTF ID encoded in the imm field, where the BTF ID identifies the helper name and type.

1.4.2.2 Program-local functions ¶

Program-local functions are functions exposed by the same BPF program as the caller, and are referenced by offset from the call instruction, similar to BPF_JA. A BPF_EXIT within the program-local function will return to the caller.

1.5 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)

1.5.1 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 + offset) = src

BPF_MEM | <size> | BPF_ST means:

*(size *) (dst + offset) = imm32

BPF_MEM | <size> | BPF_LDX means:

dst = *(size *) (src + offset)

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

1.5.2 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 + offset) += src

BPF_ATOMIC | BPF_DW | BPF_STX with 'imm' = BPF ADD means:

*(u64 *)(dst + offset) += src

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 with the value that was in memory before it was modified.

The BPF_XCHG operation atomically exchanges src with the value addressed by dst + offset.

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

1.5.3 64-bit immediate instructions ¶

Instructions with the BPF_IMM 'mode' modifier use the wide instruction encoding defined in Instruction encoding, and use the 'src' field of the basic instruction to hold an opcode subtype.

The following table defines a set of BPF_IMM | BPF_DW | BPF_LD instructions with opcode subtypes in the 'src' field, using new terms such as "map" defined further below:

opcode construction	opcode	src	pseudocode	imm type	dst type
BPF_IMM \| BPF_DW \| BPF_LD	0x18	0x0	dst = imm64	integer	integer
BPF_IMM \| BPF_DW \| BPF_LD	0x18	0x1	dst = map_by_fd(imm)	map fd	map
BPF_IMM \| BPF_DW \| BPF_LD	0x18	0x2	dst = map_val(map_by_fd(imm)) + next_imm	map fd	data pointer
BPF_IMM \| BPF_DW \| BPF_LD	0x18	0x3	dst = var_addr(imm)	variable id	data pointer
BPF_IMM \| BPF_DW \| BPF_LD	0x18	0x4	dst = code_addr(imm)	integer	code pointer
BPF_IMM \| BPF_DW \| BPF_LD	0x18	0x5	dst = map_by_idx(imm)	map index	map
BPF_IMM \| BPF_DW \| BPF_LD	0x18	0x6	dst = map_val(map_by_idx(imm)) + next_imm	map index	data pointer

where

map_by_fd(imm) means to convert a 32-bit file descriptor into an address of a map (see Maps)
map_by_idx(imm) means to convert a 32-bit index into an address of a map
map_val(map) gets the address of the first value in a given map
var_addr(imm) gets the address of a platform variable (see Platform Variables) with a given id
code_addr(imm) gets the address of the instruction at a specified relative offset in number of (64-bit) instructions
the 'imm type' can be used by disassemblers for display
the 'dst type' can be used for verification and JIT compilation purposes

1.5.3.1 Maps ¶

Maps are shared memory regions accessible by eBPF programs on some platforms. A map can have various semantics as defined in a separate document, and may or may not have a single contiguous memory region, but the 'map_val(map)' is currently only defined for maps that do have a single contiguous memory region.

Each map can have a file descriptor (fd) if supported by the platform, where 'map_by_fd(imm)' means to get the map with the specified file descriptor. Each BPF program can also be defined to use a set of maps associated with the program at load time, and 'map_by_idx(imm)' means to get the map with the given index in the set associated with the BPF program containing the instruction.

1.5.3.2 Platform Variables ¶

Platform variables are memory regions, identified by integer ids, exposed by the runtime and accessible by BPF programs on some platforms. The 'var_addr(imm)' operation means to get the address of the memory region identified by the given id.

1.5.4 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.