bpf

const (
	// MaxInstructions is the maximum number of instructions in a BPF program,
	// and is equal to Linux's BPF_MAXINSNS.
	MaxInstructions = 4096

	// ScratchMemRegisters is the number of M registers in a BPF virtual machine,
	// and is equal to Linux's BPF_MEMWORDS.
	ScratchMemRegisters = 16
)

const (
	// Instruction class, stored in bits 0-2.
	Ld   = 0x00 // load into A
	Ldx  = 0x01 // load into X
	St   = 0x02 // store from A
	Stx  = 0x03 // store from X
	Alu  = 0x04 // arithmetic
	Jmp  = 0x05 // jump
	Ret  = 0x06 // return
	Misc = 0x07

	// Size of a load, stored in bits 3-4.
	W = 0x00 // 32 bits
	H = 0x08 // 16 bits
	B = 0x10 // 8 bits

	// Source operand for a load, stored in bits 5-7.
	// Address mode numbers in the comments come from Linux's
	// Documentation/networking/filter.txt.
	Imm = 0x00 // immediate value K (mode 4)
	Abs = 0x20 // data in input at byte offset K (mode 1)
	Ind = 0x40 // data in input at byte offset X+K (mode 2)
	Mem = 0x60 // M[K] (mode 3)
	Len = 0x80 // length of the input in bytes ("BPF extension len")
	Msh = 0xa0 // 4 * lower nibble of input at byte offset K (mode 5)

	// Source operands for arithmetic, jump, and return instructions.
	// Arithmetic and jump instructions can use K or X as source operands.
	// Return instructions can use K or A as source operands.
	K = 0x00 // still mode 4
	X = 0x08 // mode 0
	A = 0x10 // mode 9

	// Arithmetic instructions, stored in bits 4-7.
	Add = 0x00
	Sub = 0x10 // A - src
	Mul = 0x20
	Div = 0x30 // A / src
	Or  = 0x40
	And = 0x50
	Lsh = 0x60 // A << src
	Rsh = 0x70 // A >> src
	Neg = 0x80 // -A (src ignored)
	Mod = 0x90 // A % src
	Xor = 0xa0

	// Jump instructions, stored in bits 4-7.
	Ja   = 0x00 // unconditional (uses K for jump offset)
	Jeq  = 0x10 // if A == src
	Jgt  = 0x20 // if A > src
	Jge  = 0x30 // if A >= src
	Jset = 0x40 // if (A & src) != 0

	// Miscellaneous instructions, stored in bits 3-7.
	Tax = 0x00 // A = X
	Txa = 0x80 // X = A

)

const (
	// DivisionByZero indicates that a program contains, or executed, a
	// division or modulo by zero.
	DivisionByZero = iota

	// InvalidEndOfProgram indicates that the last instruction of a program is
	// not a return.
	InvalidEndOfProgram

	// InvalidInstructionCount indicates that a program has zero instructions
	// or more than MaxInstructions instructions.
	InvalidInstructionCount

	// InvalidJumpTarget indicates that a program contains a jump whose target
	// is outside of the program's bounds.
	InvalidJumpTarget

	// InvalidLoad indicates that a program executed an invalid load of input
	// data.
	InvalidLoad

	// InvalidOpcode indicates that a program contains an instruction with an
	// invalid opcode.
	InvalidOpcode

	// InvalidRegister indicates that a program contains a load from, or store
	// to, a non-existent M register (index >= ScratchMemRegisters).
	InvalidRegister
)