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![Jose E. Marchesi](/assets/img/avatar_default.png)
[Changes from V4: - s/regs:16/regs:8 in figure.] [Changes from V3: - Back to src_reg and dst_reg, since they denote register numbers as opposed to the values stored in these registers.] [Changes from V2: - Use src and dst consistently in the document. - Use a more graphical depiction of the 128-bit instruction. - Remove `Where:' fragment. - Clarify that unused bits are reserved and shall be zeroed.] [Changes from V1: - Use rst literal blocks for figures. - Avoid using | in the basic instruction/pseudo instruction figure. - Rebased to today's bpf-next master branch.] This patch modifies instruction-set.rst so it documents the encoding of BPF instructions in terms of how the bytes are stored (be it in an ELF file or as bytes in a memory buffer to be loaded into the kernel or some other BPF consumer) as opposed to how the instruction looks like once loaded. This is hopefully easier to understand by implementors looking to generate and/or consume bytes conforming BPF instructions. The patch also clarifies that the unused bytes in a pseudo-instruction shall be cleared with zeros. Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com> Acked-by: Yonghong Song <yhs@fb.com> Acked-by: David Vernet <void@manifault.com> Link: https://lore.kernel.org/r/87h6v6i0da.fsf_-_@oracle.com Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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.. contents::
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.. sectnum::
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========================================
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eBPF Instruction Set Specification, v1.0
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========================================
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This document specifies version 1.0 of the eBPF instruction set.
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Documentation conventions
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=========================
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For brevity, this document uses the type notion "u64", "u32", etc.
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to mean an unsigned integer whose width is the specified number of bits.
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Registers and calling convention
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================================
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eBPF has 10 general purpose registers and a read-only frame pointer register,
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all of which are 64-bits wide.
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The eBPF calling convention is defined as:
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* R0: return value from function calls, and exit value for eBPF programs
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* R1 - R5: arguments for function calls
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* R6 - R9: callee saved registers that function calls will preserve
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* R10: read-only frame pointer to access stack
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R0 - R5 are scratch registers and eBPF programs needs to spill/fill them if
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necessary across calls.
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Instruction encoding
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====================
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eBPF has two instruction encodings:
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* the basic instruction encoding, which uses 64 bits to encode an instruction
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* the wide instruction encoding, which appends a second 64-bit immediate (i.e.,
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constant) value after the basic instruction for a total of 128 bits.
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The fields conforming an encoded basic instruction are stored in the
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following order::
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opcode:8 src_reg:4 dst_reg:4 offset:16 imm:32 // In little-endian BPF.
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opcode:8 dst_reg:4 src_reg:4 offset:16 imm:32 // In big-endian BPF.
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**imm**
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signed integer immediate value
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**offset**
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signed integer offset used with pointer arithmetic
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**src_reg**
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the source register number (0-10), except where otherwise specified
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(`64-bit immediate instructions`_ reuse this field for other purposes)
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**dst_reg**
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destination register number (0-10)
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**opcode**
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operation to perform
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Note that the contents of multi-byte fields ('imm' and 'offset') are
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stored using big-endian byte ordering in big-endian BPF and
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little-endian byte ordering in little-endian BPF.
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For example::
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opcode offset imm assembly
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src_reg dst_reg
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07 0 1 00 00 44 33 22 11 r1 += 0x11223344 // little
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dst_reg src_reg
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07 1 0 00 00 11 22 33 44 r1 += 0x11223344 // big
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Note that most instructions do not use all of the fields.
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Unused fields shall be cleared to zero.
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As discussed below in `64-bit immediate instructions`_, a 64-bit immediate
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instruction uses a 64-bit immediate value that is constructed as follows.
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The 64 bits following the basic instruction contain a pseudo instruction
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using the same format but with opcode, dst_reg, src_reg, and offset all set to zero,
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and imm containing the high 32 bits of the immediate value.
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This is depicted in the following figure::
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basic_instruction
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.-----------------------------.
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code:8 regs:8 offset:16 imm:32 unused:32 imm:32
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'--------------'
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pseudo instruction
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Thus the 64-bit immediate value is constructed as follows:
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imm64 = (next_imm << 32) | imm
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where 'next_imm' refers to the imm value of the pseudo instruction
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following the basic instruction. The unused bytes in the pseudo
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instruction are reserved and shall be cleared to zero.
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Instruction classes
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-------------------
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The three LSB bits of the 'opcode' field store the instruction class:
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========= ===== =============================== ===================================
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class value description reference
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========= ===== =============================== ===================================
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BPF_LD 0x00 non-standard load operations `Load and store instructions`_
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BPF_LDX 0x01 load into register operations `Load and store instructions`_
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BPF_ST 0x02 store from immediate operations `Load and store instructions`_
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BPF_STX 0x03 store from register operations `Load and store instructions`_
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BPF_ALU 0x04 32-bit arithmetic operations `Arithmetic and jump instructions`_
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BPF_JMP 0x05 64-bit jump operations `Arithmetic and jump instructions`_
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BPF_JMP32 0x06 32-bit jump operations `Arithmetic and jump instructions`_
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BPF_ALU64 0x07 64-bit arithmetic operations `Arithmetic and jump instructions`_
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========= ===== =============================== ===================================
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Arithmetic and jump instructions
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================================
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For arithmetic and jump instructions (``BPF_ALU``, ``BPF_ALU64``, ``BPF_JMP`` and
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``BPF_JMP32``), the 8-bit 'opcode' field is divided into three parts:
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============== ====== =================
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4 bits (MSB) 1 bit 3 bits (LSB)
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============== ====== =================
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code source instruction class
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============== ====== =================
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**code**
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the operation code, whose meaning varies by instruction class
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**source**
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the source operand location, which unless otherwise specified is one of:
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====== ===== ==============================================
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source value description
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====== ===== ==============================================
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BPF_K 0x00 use 32-bit 'imm' value as source operand
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BPF_X 0x08 use 'src_reg' register value as source operand
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====== ===== ==============================================
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**instruction class**
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the instruction class (see `Instruction classes`_)
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Arithmetic instructions
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-----------------------
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``BPF_ALU`` uses 32-bit wide operands while ``BPF_ALU64`` uses 64-bit wide operands for
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otherwise identical operations.
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The 'code' field encodes the operation as below, where 'src' and 'dst' refer
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to the values of the source and destination registers, respectively.
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======== ===== ==========================================================
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code value description
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======== ===== ==========================================================
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BPF_ADD 0x00 dst += src
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BPF_SUB 0x10 dst -= src
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BPF_MUL 0x20 dst \*= src
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BPF_DIV 0x30 dst = (src != 0) ? (dst / src) : 0
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BPF_OR 0x40 dst \|= src
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BPF_AND 0x50 dst &= src
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BPF_LSH 0x60 dst <<= src
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BPF_RSH 0x70 dst >>= src
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BPF_NEG 0x80 dst = ~src
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BPF_MOD 0x90 dst = (src != 0) ? (dst % src) : dst
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BPF_XOR 0xa0 dst ^= src
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BPF_MOV 0xb0 dst = src
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BPF_ARSH 0xc0 sign extending shift right
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BPF_END 0xd0 byte swap operations (see `Byte swap instructions`_ below)
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======== ===== ==========================================================
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Underflow and overflow are allowed during arithmetic operations, meaning
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the 64-bit or 32-bit value will wrap. If eBPF program execution would
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result in division by zero, the destination register is instead set to zero.
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If execution would result in modulo by zero, for ``BPF_ALU64`` the value of
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the destination register is unchanged whereas for ``BPF_ALU`` the upper
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32 bits of the destination register are zeroed.
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``BPF_ADD | BPF_X | BPF_ALU`` means::
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dst = (u32) ((u32) dst + (u32) src)
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where '(u32)' indicates that the upper 32 bits are zeroed.
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``BPF_ADD | BPF_X | BPF_ALU64`` means::
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dst = dst + src
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``BPF_XOR | BPF_K | BPF_ALU`` means::
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dst = (u32) dst ^ (u32) imm32
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``BPF_XOR | BPF_K | BPF_ALU64`` means::
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dst = dst ^ imm32
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Also note that the division and modulo operations are unsigned. Thus, for
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``BPF_ALU``, 'imm' is first interpreted as an unsigned 32-bit value, whereas
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for ``BPF_ALU64``, 'imm' is first sign extended to 64 bits and the result
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interpreted as an unsigned 64-bit value. There are no instructions for
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signed division or modulo.
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Byte swap instructions
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~~~~~~~~~~~~~~~~~~~~~~
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The byte swap instructions use an instruction class of ``BPF_ALU`` and a 4-bit
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'code' field of ``BPF_END``.
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The byte swap instructions operate on the destination register
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only and do not use a separate source register or immediate value.
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The 1-bit source operand field in the opcode is used to select what byte
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order the operation convert from or to:
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========= ===== =================================================
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source value description
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========= ===== =================================================
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BPF_TO_LE 0x00 convert between host byte order and little endian
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BPF_TO_BE 0x08 convert between host byte order and big endian
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========= ===== =================================================
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The 'imm' field encodes the width of the swap operations. The following widths
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are supported: 16, 32 and 64.
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Examples:
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``BPF_ALU | BPF_TO_LE | BPF_END`` with imm = 16 means::
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dst = htole16(dst)
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``BPF_ALU | BPF_TO_BE | BPF_END`` with imm = 64 means::
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dst = htobe64(dst)
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Jump instructions
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-----------------
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``BPF_JMP32`` uses 32-bit wide operands while ``BPF_JMP`` uses 64-bit wide operands for
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otherwise identical operations.
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The 'code' field encodes the operation as below:
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======== ===== ========================= ============
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code value description notes
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======== ===== ========================= ============
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BPF_JA 0x00 PC += off BPF_JMP only
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BPF_JEQ 0x10 PC += off if dst == src
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BPF_JGT 0x20 PC += off if dst > src unsigned
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BPF_JGE 0x30 PC += off if dst >= src unsigned
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BPF_JSET 0x40 PC += off if dst & src
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BPF_JNE 0x50 PC += off if dst != src
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BPF_JSGT 0x60 PC += off if dst > src signed
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BPF_JSGE 0x70 PC += off if dst >= src signed
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BPF_CALL 0x80 function call
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BPF_EXIT 0x90 function / program return BPF_JMP only
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BPF_JLT 0xa0 PC += off if dst < src unsigned
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BPF_JLE 0xb0 PC += off if dst <= src unsigned
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BPF_JSLT 0xc0 PC += off if dst < src signed
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BPF_JSLE 0xd0 PC += off if dst <= src signed
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======== ===== ========================= ============
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The eBPF program needs to store the return value into register R0 before doing a
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BPF_EXIT.
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Load and store instructions
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===========================
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For load and store instructions (``BPF_LD``, ``BPF_LDX``, ``BPF_ST``, and ``BPF_STX``), the
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8-bit 'opcode' field is divided as:
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============ ====== =================
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3 bits (MSB) 2 bits 3 bits (LSB)
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============ ====== =================
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mode size instruction class
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============ ====== =================
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The mode modifier is one of:
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============= ===== ==================================== =============
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mode modifier value description reference
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============= ===== ==================================== =============
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BPF_IMM 0x00 64-bit immediate instructions `64-bit immediate instructions`_
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BPF_ABS 0x20 legacy BPF packet access (absolute) `Legacy BPF Packet access instructions`_
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BPF_IND 0x40 legacy BPF packet access (indirect) `Legacy BPF Packet access instructions`_
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BPF_MEM 0x60 regular load and store operations `Regular load and store operations`_
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BPF_ATOMIC 0xc0 atomic operations `Atomic operations`_
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============= ===== ==================================== =============
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The size modifier is one of:
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============= ===== =====================
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size modifier value description
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============= ===== =====================
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BPF_W 0x00 word (4 bytes)
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BPF_H 0x08 half word (2 bytes)
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BPF_B 0x10 byte
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BPF_DW 0x18 double word (8 bytes)
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============= ===== =====================
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Regular load and store operations
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---------------------------------
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The ``BPF_MEM`` mode modifier is used to encode regular load and store
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instructions that transfer data between a register and memory.
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``BPF_MEM | <size> | BPF_STX`` means::
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*(size *) (dst + offset) = src
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``BPF_MEM | <size> | BPF_ST`` means::
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*(size *) (dst + offset) = imm32
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``BPF_MEM | <size> | BPF_LDX`` means::
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dst = *(size *) (src + offset)
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Where size is one of: ``BPF_B``, ``BPF_H``, ``BPF_W``, or ``BPF_DW``.
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Atomic operations
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-----------------
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Atomic operations are operations that operate on memory and can not be
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interrupted or corrupted by other access to the same memory region
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by other eBPF programs or means outside of this specification.
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All atomic operations supported by eBPF are encoded as store operations
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that use the ``BPF_ATOMIC`` mode modifier as follows:
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* ``BPF_ATOMIC | BPF_W | BPF_STX`` for 32-bit operations
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* ``BPF_ATOMIC | BPF_DW | BPF_STX`` for 64-bit operations
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* 8-bit and 16-bit wide atomic operations are not supported.
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The 'imm' field is used to encode the actual atomic operation.
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Simple atomic operation use a subset of the values defined to encode
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arithmetic operations in the 'imm' field to encode the atomic operation:
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======== ===== ===========
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imm value description
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======== ===== ===========
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BPF_ADD 0x00 atomic add
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BPF_OR 0x40 atomic or
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BPF_AND 0x50 atomic and
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BPF_XOR 0xa0 atomic xor
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======== ===== ===========
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``BPF_ATOMIC | BPF_W | BPF_STX`` with 'imm' = BPF_ADD means::
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*(u32 *)(dst + offset) += src
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``BPF_ATOMIC | BPF_DW | BPF_STX`` with 'imm' = BPF ADD means::
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*(u64 *)(dst + offset) += src
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In addition to the simple atomic operations, there also is a modifier and
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two complex atomic operations:
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=========== ================ ===========================
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imm value description
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=========== ================ ===========================
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BPF_FETCH 0x01 modifier: return old value
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BPF_XCHG 0xe0 | BPF_FETCH atomic exchange
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BPF_CMPXCHG 0xf0 | BPF_FETCH atomic compare and exchange
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=========== ================ ===========================
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The ``BPF_FETCH`` modifier is optional for simple atomic operations, and
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always set for the complex atomic operations. If the ``BPF_FETCH`` flag
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is set, then the operation also overwrites ``src`` with the value that
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was in memory before it was modified.
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The ``BPF_XCHG`` operation atomically exchanges ``src`` with the value
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addressed by ``dst + offset``.
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The ``BPF_CMPXCHG`` operation atomically compares the value addressed by
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``dst + offset`` with ``R0``. If they match, the value addressed by
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``dst + offset`` is replaced with ``src``. In either case, the
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value that was at ``dst + offset`` before the operation is zero-extended
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and loaded back to ``R0``.
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64-bit immediate instructions
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-----------------------------
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Instructions with the ``BPF_IMM`` 'mode' modifier use the wide instruction
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encoding for an extra imm64 value.
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There is currently only one such instruction.
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``BPF_LD | BPF_DW | BPF_IMM`` means::
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dst = imm64
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Legacy BPF Packet access instructions
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-------------------------------------
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eBPF previously introduced special instructions for access to packet data that were
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carried over from classic BPF. However, these instructions are
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deprecated and should no longer be used.
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