Documentation/bpf/instruction-set.rst - linux - Git at Google

 .. contents::
 .. sectnum::

 ========================================
 eBPF Instruction Set Specification, v1.0
 ========================================

 This document specifies version 1.0 of the eBPF instruction set.

 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.

 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 (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.

 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`_
 =========  =====  ===============================  ===================================

 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`_)

 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.

 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)

 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.

 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.

 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.

 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)
   =============  =====  =====================

 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``.

 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``.

 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

 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.

 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.

 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.
	.. contents::
	.. sectnum::

	========================================
	eBPF Instruction Set Specification, v1.0
	========================================

	This document specifies version 1.0 of the eBPF instruction set.

	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.

	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 (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.

	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`_
	========= ===== =============================== ===================================

	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`_)

	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.

	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)

	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.

	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.

	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.

	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)
	============= ===== =====================

	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``.

	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``.

	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

	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.

	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.

	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.