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diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst new file mode 100644 index 000000000000..3704836fe6df --- /dev/null +++ b/Documentation/bpf/instruction-set.rst @@ -0,0 +1,279 @@ + +==================== +eBPF Instruction Set +==================== + +Registers and calling convention +================================ + +eBPF has 10 general purpose registers and a read-only frame pointer register, +all of which are 64-bits wide. + +The eBPF calling convention is defined as: + + * R0: return value from function calls, and exit value for eBPF programs + * R1 - R5: arguments for function calls + * R6 - R9: callee saved registers that function calls will preserve + * R10: read-only frame pointer to access stack + +R0 - R5 are scratch registers and eBPF programs needs to spill/fill them if +necessary across calls. + +Instruction encoding +==================== + +eBPF uses 64-bit instructions with the following encoding: + + ============= ======= =============== ==================== ============ + 32 bits (MSB) 16 bits 4 bits 4 bits 8 bits (LSB) + ============= ======= =============== ==================== ============ + immediate offset source register destination register opcode + ============= ======= =============== ==================== ============ + +Note that most instructions do not use all of the fields. +Unused fields shall be cleared to zero. + +Instruction classes +------------------- + +The three LSB bits of the 'opcode' field store the instruction class: + + ========= ===== =============================== + class value description + ========= ===== =============================== + BPF_LD 0x00 non-standard load operations + BPF_LDX 0x01 load into register operations + BPF_ST 0x02 store from immediate operations + BPF_STX 0x03 store from register operations + BPF_ALU 0x04 32-bit arithmetic operations + BPF_JMP 0x05 64-bit jump operations + BPF_JMP32 0x06 32-bit jump operations + BPF_ALU64 0x07 64-bit arithmetic operations + ========= ===== =============================== + +Arithmetic and jump instructions +================================ + +For arithmetic and jump instructions (BPF_ALU, BPF_ALU64, BPF_JMP and +BPF_JMP32), the 8-bit 'opcode' field is divided into three parts: + + ============== ====== ================= + 4 bits (MSB) 1 bit 3 bits (LSB) + ============== ====== ================= + operation code source instruction class + ============== ====== ================= + +The 4th bit encodes the source operand: + + ====== ===== ======================================== + source value description + ====== ===== ======================================== + BPF_K 0x00 use 32-bit immediate as source operand + BPF_X 0x08 use 'src_reg' register as source operand + ====== ===== ======================================== + +The four MSB bits store the operation code. + + +Arithmetic instructions +----------------------- + +BPF_ALU uses 32-bit wide operands while BPF_ALU64 uses 64-bit wide operands for +otherwise identical operations. +The code field encodes the operation as below: + + ======== ===== ========================== + code value description + ======== ===== ========================== + BPF_ADD 0x00 dst += src + BPF_SUB 0x10 dst -= src + BPF_MUL 0x20 dst \*= src + BPF_DIV 0x30 dst /= src + BPF_OR 0x40 dst \|= src + BPF_AND 0x50 dst &= src + BPF_LSH 0x60 dst <<= src + BPF_RSH 0x70 dst >>= src + BPF_NEG 0x80 dst = ~src + BPF_MOD 0x90 dst %= src + BPF_XOR 0xa0 dst ^= src + BPF_MOV 0xb0 dst = src + BPF_ARSH 0xc0 sign extending shift right + BPF_END 0xd0 endianness conversion + ======== ===== ========================== + +BPF_ADD | BPF_X | BPF_ALU means:: + + dst_reg = (u32) dst_reg + (u32) src_reg; + +BPF_ADD | BPF_X | BPF_ALU64 means:: + + dst_reg = dst_reg + src_reg + +BPF_XOR | BPF_K | BPF_ALU means:: + + src_reg = (u32) src_reg ^ (u32) imm32 + +BPF_XOR | BPF_K | BPF_ALU64 means:: + + src_reg = src_reg ^ imm32 + + +Jump instructions +----------------- + +BPF_JMP32 uses 32-bit wide operands while BPF_JMP uses 64-bit wide operands for +otherwise identical operations. +The code field encodes the operation as below: + + ======== ===== ========================= ============ + code value description notes + ======== ===== ========================= ============ + BPF_JA 0x00 PC += off BPF_JMP only + BPF_JEQ 0x10 PC += off if dst == src + BPF_JGT 0x20 PC += off if dst > src unsigned + BPF_JGE 0x30 PC += off if dst >= src unsigned + BPF_JSET 0x40 PC += off if dst & src + BPF_JNE 0x50 PC += off if dst != src + BPF_JSGT 0x60 PC += off if dst > src signed + BPF_JSGE 0x70 PC += off if dst >= src signed + BPF_CALL 0x80 function call + BPF_EXIT 0x90 function / program return BPF_JMP only + BPF_JLT 0xa0 PC += off if dst < src unsigned + BPF_JLE 0xb0 PC += off if dst <= src unsigned + BPF_JSLT 0xc0 PC += off if dst < src signed + BPF_JSLE 0xd0 PC += off if dst <= src signed + ======== ===== ========================= ============ + +The eBPF program needs to store the return value into register R0 before doing a +BPF_EXIT. + + +Load and store instructions +=========================== + +For load and store instructions (BPF_LD, BPF_LDX, BPF_ST and BPF_STX), the +8-bit 'opcode' field is divided as: + + ============ ====== ================= + 3 bits (MSB) 2 bits 3 bits (LSB) + ============ ====== ================= + mode size instruction class + ============ ====== ================= + +The size modifier is one of: + + ============= ===== ===================== + size modifier value description + ============= ===== ===================== + BPF_W 0x00 word (4 bytes) + BPF_H 0x08 half word (2 bytes) + BPF_B 0x10 byte + BPF_DW 0x18 double word (8 bytes) + ============= ===== ===================== + +The mode modifier is one of: + + ============= ===== ==================================== + mode modifier value description + ============= ===== ==================================== + BPF_IMM 0x00 used for 64-bit mov + BPF_ABS 0x20 legacy BPF packet access + BPF_IND 0x40 legacy BPF packet access + BPF_MEM 0x60 all normal load and store operations + BPF_ATOMIC 0xc0 atomic operations + ============= ===== ==================================== + +BPF_MEM | <size> | BPF_STX means:: + + *(size *) (dst_reg + off) = src_reg + +BPF_MEM | <size> | BPF_ST means:: + + *(size *) (dst_reg + off) = imm32 + +BPF_MEM | <size> | BPF_LDX means:: + + dst_reg = *(size *) (src_reg + off) + +Where size is one of: BPF_B or BPF_H or BPF_W or BPF_DW. + +Atomic operations +----------------- + +eBPF includes atomic operations, which use the immediate field for extra +encoding:: + + .imm = BPF_ADD, .code = BPF_ATOMIC | BPF_W | BPF_STX: lock xadd *(u32 *)(dst_reg + off16) += src_reg + .imm = BPF_ADD, .code = BPF_ATOMIC | BPF_DW | BPF_STX: lock xadd *(u64 *)(dst_reg + off16) += src_reg + +The basic atomic operations supported are:: + + BPF_ADD + BPF_AND + BPF_OR + BPF_XOR + +Each having equivalent semantics with the ``BPF_ADD`` example, that is: the +memory location addresed by ``dst_reg + off`` is atomically modified, with +``src_reg`` as the other operand. If the ``BPF_FETCH`` flag is set in the +immediate, then these operations also overwrite ``src_reg`` with the +value that was in memory before it was modified. + +The more special operations are:: + + BPF_XCHG + +This atomically exchanges ``src_reg`` with the value addressed by ``dst_reg + +off``. :: + + BPF_CMPXCHG + +This atomically compares the value addressed by ``dst_reg + off`` with +``R0``. If they match it is replaced with ``src_reg``. In either case, the +value that was there before is zero-extended and loaded back to ``R0``. + +Note that 1 and 2 byte atomic operations are not supported. + +Clang can generate atomic instructions by default when ``-mcpu=v3`` is +enabled. If a lower version for ``-mcpu`` is set, the only atomic instruction +Clang can generate is ``BPF_ADD`` *without* ``BPF_FETCH``. If you need to enable +the atomics features, while keeping a lower ``-mcpu`` version, you can use +``-Xclang -target-feature -Xclang +alu32``. + +You may encounter ``BPF_XADD`` - this is a legacy name for ``BPF_ATOMIC``, +referring to the exclusive-add operation encoded when the immediate field is +zero. + +16-byte instructions +-------------------- + +eBPF has one 16-byte instruction: ``BPF_LD | BPF_DW | BPF_IMM`` which consists +of two consecutive ``struct bpf_insn`` 8-byte blocks and interpreted as single +instruction that loads 64-bit immediate value into a dst_reg. + +Packet access instructions +-------------------------- + +eBPF has two non-generic instructions: (BPF_ABS | <size> | BPF_LD) and +(BPF_IND | <size> | BPF_LD) which are used to access packet data. + +They had to be carried over from classic BPF to have strong performance of +socket filters running in eBPF interpreter. These instructions can only +be used when interpreter context is a pointer to ``struct sk_buff`` and +have seven implicit operands. Register R6 is an implicit input that must +contain pointer to sk_buff. Register R0 is an implicit output which contains +the data fetched from the packet. Registers R1-R5 are scratch registers +and must not be used to store the data across BPF_ABS | BPF_LD or +BPF_IND | BPF_LD instructions. + +These instructions have implicit program exit condition as well. When +eBPF program is trying to access the data beyond the packet boundary, +the interpreter will abort the execution of the program. JIT compilers +therefore must preserve this property. src_reg and imm32 fields are +explicit inputs to these instructions. + +For example, BPF_IND | BPF_W | BPF_LD means:: + + R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32)) + +and R1 - R5 are clobbered. |