From: Dave Thaler <dthaler@xxxxxxxxxxxxx> Improve English readability Signed-off-by: Dave Thaler <dthaler@xxxxxxxxxxxxx> --- Documentation/bpf/instruction-set.rst | 110 +++++++++++++++++--------- Documentation/bpf/linux-notes.rst | 5 ++ 2 files changed, 77 insertions(+), 38 deletions(-) diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst index 7c1e245df..4d5a506be 100644 --- a/Documentation/bpf/instruction-set.rst +++ b/Documentation/bpf/instruction-set.rst @@ -7,6 +7,9 @@ eBPF Instruction Set Specification, v1.0 This document specifies version 1.0 of the eBPF instruction set. +The eBPF instruction set consists of eleven 64 bit registers, a program counter, +and an implementation-specific amount (e.g., 512 bytes) of stack space. + Documentation conventions ========================= @@ -27,12 +30,24 @@ The eBPF calling convention is defined as: * 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. +Registers R0 - R5 are caller-saved registers, meaning the BPF program needs to either +spill them to the BPF stack or move them to callee saved registers if these +arguments are to be reused across multiple function calls. Spilling means +that the value in the register is moved to the BPF stack. The reverse operation +of moving the variable from the BPF stack to the register is called filling. +The reason for spilling/filling is due to the limited number of registers. + +Upon entering execution of an eBPF program, registers R1 - R5 initially can contain +the input arguments for the program (similar to the argc/argv pair for a typical C program). +The actual number of registers used, and their meaning, is defined by the program type; +for example, a networking program might have an argument that includes network packet data +and/or metadata. Instruction encoding ==================== +An eBPF program is a sequence of instructions. + eBPF has two instruction encodings: * the basic instruction encoding, which uses 64 bits to encode an instruction @@ -65,7 +80,7 @@ opcode operation to perform Note that most instructions do not use all of the fields. -Unused fields shall be cleared to zero. +Unused fields must be set 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. @@ -92,7 +107,9 @@ and destination registers, respectively, rather than the register number. Instruction classes ------------------- -The three LSB bits of the 'opcode' field store the instruction class: +The encoding of the 'opcode' field varies and can be determined from +the three least significant bits (LSB) of the 'opcode' field which holds +the "instruction class", as follows: ========= ===== =============================== =================================== class value description reference @@ -138,9 +155,11 @@ instruction class Arithmetic instructions ----------------------- -``BPF_ALU`` uses 32-bit wide operands while ``BPF_ALU64`` uses 64-bit wide operands for +Instruction class ``BPF_ALU`` uses 32-bit wide operands (zeroing the upper 32 bits +of the destination register) while ``BPF_ALU64`` uses 64-bit wide operands for otherwise identical operations. -The 'code' field encodes the operation as below: + +The 4-bit 'code' field encodes the operation as follows: ======== ===== ========================================================== code value description @@ -170,21 +189,23 @@ the destination register is instead set to zero. If execution would result in modulo by zero, the destination register is instead left unchanged. -``BPF_ADD | BPF_X | BPF_ALU`` means:: +Examples: + +``BPF_ADD | BPF_X | BPF_ALU`` (0x0c) means:: dst = (u32) (dst + src) where '(u32)' indicates truncation to 32 bits. -``BPF_ADD | BPF_X | BPF_ALU64`` means:: +``BPF_ADD | BPF_X | BPF_ALU64`` (0x0f) means:: dst = dst + src -``BPF_XOR | BPF_K | BPF_ALU`` means:: +``BPF_XOR | BPF_K | BPF_ALU`` (0xa4) means:: src = (u32) src ^ (u32) imm -``BPF_XOR | BPF_K | BPF_ALU64`` means:: +``BPF_XOR | BPF_K | BPF_ALU64`` (0xa7) means:: src = src ^ imm @@ -202,8 +223,9 @@ The byte swap instructions use an instruction class of ``BPF_ALU`` and a 4-bit 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: +Byte swap instructions use the 1-bit 'source' field in the 'opcode' field +as follows. Instead of indicating the source operator, it is instead +used to select what byte order the operation converts from or to: ========= ===== ================================================= source value description @@ -213,24 +235,33 @@ 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) +are supported: 16, 32 and 64. The following table summarizes the resulting +possibilities: + +============================= ========= === ======== ================== +opcode construction opcode imm mnemonic pseudocode +============================= ========= === ======== ================== +BPF_END | BPF_TO_LE | BPF_ALU 0xd4 16 le16 dst dst = htole16(dst) +BPF_END | BPF_TO_LE | BPF_ALU 0xd4 32 le32 dst dst = htole32(dst) +BPF_END | BPF_TO_LE | BPF_ALU 0xd4 64 le64 dst dst = htole64(dst) +BPF_END | BPF_TO_BE | BPF_ALU 0xdc 16 be16 dst dst = htobe16(dst) +BPF_END | BPF_TO_BE | BPF_ALU 0xdc 32 be32 dst dst = htobe32(dst) +BPF_END | BPF_TO_BE | BPF_ALU 0xdc 64 be64 dst dst = htobe64(dst) +============================= ========= === ======== ================== + +where + +* mnenomic indicates a short form that might be displayed by some tools such as disassemblers +* 'htoleNN()' indicates converting a NN-bit value from host byte order to little-endian byte order +* 'htobeNN()' indicates converting a NN-bit value from host byte order to big-endian byte order Jump instructions ----------------- -``BPF_JMP32`` uses 32-bit wide operands while ``BPF_JMP`` uses 64-bit wide operands for +Instruction class ``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: + +The 4-bit 'code' field encodes the operation as below, where PC is the program counter: ======== ===== ========================= ============ code value description notes @@ -251,9 +282,6 @@ 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. - Helper functions ~~~~~~~~~~~~~~~~ Helper functions are a concept whereby BPF programs can call into a @@ -283,7 +311,8 @@ For load and store instructions (``BPF_LD``, ``BPF_LDX``, ``BPF_ST``, and ``BPF_ mode size instruction class ============ ====== ================= -The mode modifier is one of: +mode + one of: ============= ===== ==================================== ============= mode modifier value description reference @@ -295,7 +324,8 @@ The mode modifier is one of: BPF_ATOMIC 0xc0 atomic operations `Atomic operations`_ ============= ===== ==================================== ============= -The size modifier is one of: +size + one of: ============= ===== ===================== size modifier value description @@ -306,6 +336,9 @@ The size modifier is one of: BPF_DW 0x18 double word (8 bytes) ============= ===== ===================== +instruction class + the instruction class (see `Instruction classes`_) + Regular load and store operations --------------------------------- @@ -324,7 +357,7 @@ instructions that transfer data between a register and memory. dst = *(size *) (src + offset) -Where size is one of: ``BPF_B``, ``BPF_H``, ``BPF_W``, or ``BPF_DW``. +where size is one of: ``BPF_B``, ``BPF_H``, ``BPF_W``, or ``BPF_DW``. Atomic operations ----------------- @@ -336,9 +369,11 @@ 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. +* ``BPF_ATOMIC | BPF_W | BPF_STX`` (0xc3) for 32-bit operations +* ``BPF_ATOMIC | BPF_DW | BPF_STX`` (0xdb) for 64-bit operations + +Note that 8-bit (``BPF_B``) and 16-bit (``BPF_H``) wide atomic operations are not supported, +nor is ``BPF_ATOMIC | <size> | BPF_ST``. The 'imm' field is used to encode the actual atomic operation. Simple atomic operation use a subset of the values defined to encode @@ -353,16 +388,15 @@ BPF_AND 0x50 atomic and BPF_XOR 0xa0 atomic xor ======== ===== =========== - -``BPF_ATOMIC | BPF_W | BPF_STX`` with 'imm' = BPF_ADD means:: +``BPF_ATOMIC | BPF_W | BPF_STX`` (0xc3) with 'imm' = BPF_ADD means:: *(u32 *)(dst + offset) += src -``BPF_ATOMIC | BPF_DW | BPF_STX`` with 'imm' = BPF ADD means:: +``BPF_ATOMIC | BPF_DW | BPF_STX`` (0xdb) with 'imm' = BPF ADD means:: *(u64 *)(dst + offset) += src -In addition to the simple atomic operations, there also is a modifier and +In addition to the simple atomic operations above, there also is a modifier and two complex atomic operations: =========== ================ =========================== diff --git a/Documentation/bpf/linux-notes.rst b/Documentation/bpf/linux-notes.rst index 956b0c866..5860b73ea 100644 --- a/Documentation/bpf/linux-notes.rst +++ b/Documentation/bpf/linux-notes.rst @@ -7,6 +7,11 @@ Linux implementation notes This document provides more details specific to the Linux kernel implementation of the eBPF instruction set. +Stack space +====================== + +Linux currently supports 512 bytes of stack space. + Byte swap instructions ====================== -- 2.33.4
