eBPF Instruction Set¶
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.
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.
Instruction classes¶
The three LSB bits of the ‘opcode’ field store the instruction class:
class value description BPF_LD 0x00 non-standard load operations BPF_LDX 0x01 load into register operations BPF_ST 0x02 store from immediate operations BPF_STX 0x03 store from register operations BPF_ALU 0x04 32-bit arithmetic operations BPF_JMP 0x05 64-bit jump operations BPF_JMP32 0x06 32-bit jump operations BPF_ALU64 0x07 64-bit arithmetic operations
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.
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 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 BPF_XOR 0xa0 dst ^= src BPF_MOV 0xb0 dst = src BPF_ARSH 0xc0 sign extending shift right BPF_END 0xd0 byte swap operations (see separate section below)
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
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 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)
BPF_FROM_LE
and BPF_FROM_BE
exist as aliases for BPF_TO_LE
and
BPF_TO_BE
respectively.
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.
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 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)
The mode modifier is one of:
mode modifier value description BPF_IMM 0x00 64-bit immediate instructions BPF_ABS 0x20 legacy BPF packet access (absolute) BPF_IND 0x40 legacy BPF packet access (indirect) BPF_MEM 0x60 regular load and store operations BPF_ATOMIC 0xc0 atomic operations
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
.
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 operationsBPF_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
BPF_XADD
is a deprecated name for BPF_ATOMIC | BPF_ADD
.
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
.
Clang can generate atomic instructions by default when -mcpu=v3
is
enabled. If a lower version for -mcpu
is set, the only atomic instruction
Clang can generate is BPF_ADD
without BPF_FETCH
. If you need to enable
the atomics features, while keeping a lower -mcpu
version, you can use
-Xclang -target-feature -Xclang +alu32
.
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
Legacy BPF Packet access instructions¶
eBPF has special 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 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 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 after a call to
BPF_ABS | BPF_LD
orBPF_IND | BPF_LD
instructions.
These instructions have an implicit program exit condition as well. When an eBPF program is trying to access the data beyond the packet boundary, the program execution will be aborted.
BPF_ABS | BPF_W | BPF_LD
means:
R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + imm32))
BPF_IND | BPF_W | BPF_LD
means:
R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32))