kernel / Documentation / bpf / instruction-set.rst

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========================================
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
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 = (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.

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.