rbpf/src/ebpf.rs

// Copyright 2016 6WIND S.A. <quentin.monnet@6wind.com>
//
// Licensed under the Apache License, Version 2.0 <http://www.apache.org/licenses/LICENSE-2.0> or
// the MIT license <http://opensource.org/licenses/MIT>, at your option. This file may not be
// copied, modified, or distributed except according to those terms.


//! This module contains all the definitions related to eBPF.
//!
//! The number of bytes in an instruction, the maximum number of instructions in a program, and
//! also all operation codes are defined here as constants.
//!
//! The structure for an instruction used by this crate, as well as the function to extract it from
//! a program, is also defined in the module.
//!
//! To learn more about these instructions, see the Linux kernel documentation:
//! <https://www.kernel.org/doc/Documentation/networking/filter.txt>, or for a shorter version of
//! the list of the operation codes: <https://github.com/iovisor/bpf-docs/blob/master/eBPF.md>

/// Maximum number of instructions in an eBPF program.
pub const PROG_MAX_INSNS: usize = 4096;
/// Size of an eBPF instructions, in bytes.
pub const INSN_SIZE: usize = 8;
/// Maximum size of an eBPF program, in bytes.
pub const PROG_MAX_SIZE: usize = PROG_MAX_INSNS * INSN_SIZE;
/// Stack for the eBPF stack, in bytes.
pub const STACK_SIZE: usize = 512;

// eBPF op codes.
// See also https://www.kernel.org/doc/Documentation/networking/filter.txt

// Three least significant bits are operation class:
/// BPF operation class: load from immediate.
pub const BPF_LD    : u8 = 0x00;
/// BPF operation class: load from register.
pub const BPF_LDX   : u8 = 0x01;
/// BPF operation class: store immediate.
pub const BPF_ST    : u8 = 0x02;
/// BPF operation class: store value from register.
pub const BPF_STX   : u8 = 0x03;
/// BPF operation class: 32 bits arithmetic operation.
pub const BPF_ALU   : u8 = 0x04;
/// BPF operation class: jump.
pub const BPF_JMP   : u8 = 0x05;
// [ class 6 unused, reserved for future use ]
/// BPF operation class: 64 bits arithmetic operation.
pub const BPF_ALU64 : u8 = 0x07;

// For load and store instructions:
// +------------+--------+------------+
// |   3 bits   | 2 bits |   3 bits   |
// |    mode    |  size  | insn class |
// +------------+--------+------------+
// (MSB)                          (LSB)

// Size modifiers:
/// BPF size modifier: word (4 bytes).
pub const BPF_W     : u8 = 0x00;
/// BPF size modifier: half-word (2 bytes).
pub const BPF_H     : u8 = 0x08;
/// BPF size modifier: byte (1 byte).
pub const BPF_B     : u8 = 0x10;
/// BPF size modifier: double word (8 bytes).
pub const BPF_DW    : u8 = 0x18;

// Mode modifiers:
/// BPF mode modifier: immediate value.
pub const BPF_IMM   : u8 = 0x00;
/// BPF mode modifier: absolute load.
pub const BPF_ABS   : u8 = 0x20;
/// BPF mode modifier: indirect load.
pub const BPF_IND   : u8 = 0x40;
/// BPF mode modifier: load from / store to memory.
pub const BPF_MEM   : u8 = 0x60;
// [ 0x80 reserved ]
// [ 0xa0 reserved ]
/// BPF mode modifier: exclusive add.
pub const BPF_XADD  : u8 = 0xc0;

// For arithmetic (BPF_ALU/BPF_ALU64) and jump (BPF_JMP) instructions:
// +----------------+--------+--------+
// |     4 bits     |1 b.|   3 bits   |
// | operation code | src| insn class |
// +----------------+----+------------+
// (MSB)                          (LSB)

// Source modifiers:
/// BPF source operand modifier: 32-bit immediate value.
pub const BPF_K     : u8 = 0x00;
/// BPF source operand modifier: `src` register.
pub const BPF_X     : u8 = 0x08;

// Operation codes -- BPF_ALU or BPF_ALU64 classes:
/// BPF ALU/ALU64 operation code: addition.
pub const BPF_ADD   : u8 = 0x00;
/// BPF ALU/ALU64 operation code: subtraction.
pub const BPF_SUB   : u8 = 0x10;
/// BPF ALU/ALU64 operation code: multiplication.
pub const BPF_MUL   : u8 = 0x20;
/// BPF ALU/ALU64 operation code: division.
pub const BPF_DIV   : u8 = 0x30;
/// BPF ALU/ALU64 operation code: or.
pub const BPF_OR    : u8 = 0x40;
/// BPF ALU/ALU64 operation code: and.
pub const BPF_AND   : u8 = 0x50;
/// BPF ALU/ALU64 operation code: left shift.
pub const BPF_LSH   : u8 = 0x60;
/// BPF ALU/ALU64 operation code: right shift.
pub const BPF_RSH   : u8 = 0x70;
/// BPF ALU/ALU64 operation code: negation.
pub const BPF_NEG   : u8 = 0x80;
/// BPF ALU/ALU64 operation code: modulus.
pub const BPF_MOD   : u8 = 0x90;
/// BPF ALU/ALU64 operation code: exclusive or.
pub const BPF_XOR   : u8 = 0xa0;
/// BPF ALU/ALU64 operation code: move.
pub const BPF_MOV   : u8 = 0xb0;
/// BPF ALU/ALU64 operation code: sign extending right shift.
pub const BPF_ARSH  : u8 = 0xc0;
/// BPF ALU/ALU64 operation code: endianness conversion.
pub const BPF_END   : u8 = 0xd0;

// Operation codes -- BPF_JMP class:
/// BPF JMP operation code: jump.
pub const BPF_JA    : u8 = 0x00;
/// BPF JMP operation code: jump if equal.
pub const BPF_JEQ   : u8 = 0x10;
/// BPF JMP operation code: jump if greater than.
pub const BPF_JGT   : u8 = 0x20;
/// BPF JMP operation code: jump if greater or equal.
pub const BPF_JGE   : u8 = 0x30;
/// BPF JMP operation code: jump if `src` & `reg`.
pub const BPF_JSET  : u8 = 0x40;
/// BPF JMP operation code: jump if not equal.
pub const BPF_JNE   : u8 = 0x50;
/// BPF JMP operation code: jump if greater than (signed).
pub const BPF_JSGT  : u8 = 0x60;
/// BPF JMP operation code: jump if greater or equal (signed).
pub const BPF_JSGE  : u8 = 0x70;
/// BPF JMP operation code: helper function call.
pub const BPF_CALL  : u8 = 0x80;
/// BPF JMP operation code: return from program.
pub const BPF_EXIT  : u8 = 0x90;

// Op codes
// (Following operation names are not “official”, but may be proper to rbpf; Linux kernel only
// combines above flags and does not attribute a name per operation.)

/// BPF opcode: `ldabsb src, dst, imm`.
pub const LD_ABS_B   : u8 = BPF_LD    | BPF_ABS | BPF_B;
/// BPF opcode: `ldabsh src, dst, imm`.
pub const LD_ABS_H   : u8 = BPF_LD    | BPF_ABS | BPF_H;
/// BPF opcode: `ldabsw src, dst, imm`.
pub const LD_ABS_W   : u8 = BPF_LD    | BPF_ABS | BPF_W;
/// BPF opcode: `ldabsdw src, dst, imm`.
pub const LD_ABS_DW  : u8 = BPF_LD    | BPF_ABS | BPF_DW;
/// BPF opcode: `ldindb src, dst, imm`.
pub const LD_IND_B   : u8 = BPF_LD    | BPF_IND | BPF_B;
/// BPF opcode: `ldindh src, dst, imm`.
pub const LD_IND_H   : u8 = BPF_LD    | BPF_IND | BPF_H;
/// BPF opcode: `ldindw src, dst, imm`.
pub const LD_IND_W   : u8 = BPF_LD    | BPF_IND | BPF_W;
/// BPF opcode: `ldinddw src, dst, imm`.
pub const LD_IND_DW  : u8 = BPF_LD    | BPF_IND | BPF_DW;

/// BPF opcode: `lddw dst, imm` /// `dst = imm`.
pub const LD_DW_IMM  : u8 = BPF_LD    | BPF_IMM | BPF_DW;
/// BPF opcode: `ldxb dst, [src + off]` /// `dst = (src + off) as u8`.
pub const LD_B_REG   : u8 = BPF_LDX   | BPF_MEM | BPF_B;
/// BPF opcode: `ldxh dst, [src + off]` /// `dst = (src + off) as u16`.
pub const LD_H_REG   : u8 = BPF_LDX   | BPF_MEM | BPF_H;
/// BPF opcode: `ldxw dst, [src + off]` /// `dst = (src + off) as u32`.
pub const LD_W_REG   : u8 = BPF_LDX   | BPF_MEM | BPF_W;
/// BPF opcode: `ldxdw dst, [src + off]` /// `dst = (src + off) as u64`.
pub const LD_DW_REG  : u8 = BPF_LDX   | BPF_MEM | BPF_DW;
/// BPF opcode: `stb [dst + off], imm` /// `(dst + offset) as u8 = imm`.
pub const ST_B_IMM   : u8 = BPF_ST    | BPF_MEM | BPF_B;
/// BPF opcode: `sth [dst + off], imm` /// `(dst + offset) as u16 = imm`.
pub const ST_H_IMM   : u8 = BPF_ST    | BPF_MEM | BPF_H;
/// BPF opcode: `stw [dst + off], imm` /// `(dst + offset) as u32 = imm`.
pub const ST_W_IMM   : u8 = BPF_ST    | BPF_MEM | BPF_W;
/// BPF opcode: `stdw [dst + off], imm` /// `(dst + offset) as u64 = imm`.
pub const ST_DW_IMM  : u8 = BPF_ST    | BPF_MEM | BPF_DW;
/// BPF opcode: `stxb [dst + off], src` /// `(dst + offset) as u8 = src`.
pub const ST_B_REG   : u8 = BPF_STX   | BPF_MEM | BPF_B;
/// BPF opcode: `stxh [dst + off], src` /// `(dst + offset) as u16 = src`.
pub const ST_H_REG   : u8 = BPF_STX   | BPF_MEM | BPF_H;
/// BPF opcode: `stxw [dst + off], src` /// `(dst + offset) as u32 = src`.
pub const ST_W_REG   : u8 = BPF_STX   | BPF_MEM | BPF_W;
/// BPF opcode: `stxdw [dst + off], src` /// `(dst + offset) as u64 = src`.
pub const ST_DW_REG  : u8 = BPF_STX   | BPF_MEM | BPF_DW;

/// BPF opcode: `stxxaddw [dst + off], src`.
pub const ST_W_XADD  : u8 = BPF_STX   | BPF_XADD | BPF_W;
/// BPF opcode: `stxxadddw [dst + off], src`.
pub const ST_DW_XADD : u8 = BPF_STX   | BPF_XADD | BPF_DW;

/// BPF opcode: `add32 dst, imm` /// `dst += imm`.
pub const ADD32_IMM  : u8 = BPF_ALU   | BPF_K   | BPF_ADD;
/// BPF opcode: `add32 dst, src` /// `dst += src`.
pub const ADD32_REG  : u8 = BPF_ALU   | BPF_X   | BPF_ADD;
/// BPF opcode: `sub32 dst, imm` /// `dst -= imm`.
pub const SUB32_IMM  : u8 = BPF_ALU   | BPF_K   | BPF_SUB;
/// BPF opcode: `sub32 dst, src` /// `dst -= src`.
pub const SUB32_REG  : u8 = BPF_ALU   | BPF_X   | BPF_SUB;
/// BPF opcode: `mul32 dst, imm` /// `dst *= imm`.
pub const MUL32_IMM  : u8 = BPF_ALU   | BPF_K   | BPF_MUL;
/// BPF opcode: `mul32 dst, src` /// `dst *= src`.
pub const MUL32_REG  : u8 = BPF_ALU   | BPF_X   | BPF_MUL;
/// BPF opcode: `div32 dst, imm` /// `dst /= imm`.
pub const DIV32_IMM  : u8 = BPF_ALU   | BPF_K   | BPF_DIV;
/// BPF opcode: `div32 dst, src` /// `dst /= src`.
pub const DIV32_REG  : u8 = BPF_ALU   | BPF_X   | BPF_DIV;
/// BPF opcode: `or32 dst, imm` /// `dst |= imm`.
pub const OR32_IMM   : u8 = BPF_ALU   | BPF_K   | BPF_OR;
/// BPF opcode: `or32 dst, src` /// `dst |= src`.
pub const OR32_REG   : u8 = BPF_ALU   | BPF_X   | BPF_OR;
/// BPF opcode: `and32 dst, imm` /// `dst &= imm`.
pub const AND32_IMM  : u8 = BPF_ALU   | BPF_K   | BPF_AND;
/// BPF opcode: `and32 dst, src` /// `dst &= src`.
pub const AND32_REG  : u8 = BPF_ALU   | BPF_X   | BPF_AND;
/// BPF opcode: `lsh32 dst, imm` /// `dst <<= imm`.
pub const LSH32_IMM  : u8 = BPF_ALU   | BPF_K   | BPF_LSH;
/// BPF opcode: `lsh32 dst, src` /// `dst <<= src`.
pub const LSH32_REG  : u8 = BPF_ALU   | BPF_X   | BPF_LSH;
/// BPF opcode: `rsh32 dst, imm` /// `dst >>= imm`.
pub const RSH32_IMM  : u8 = BPF_ALU   | BPF_K   | BPF_RSH;
/// BPF opcode: `rsh32 dst, src` /// `dst >>= src`.
pub const RSH32_REG  : u8 = BPF_ALU   | BPF_X   | BPF_RSH;
/// BPF opcode: `neg32 dst` /// `dst = -dst`.
pub const NEG32      : u8 = BPF_ALU   | BPF_NEG;
/// BPF opcode: `mod32 dst, imm` /// `dst %= imm`.
pub const MOD32_IMM  : u8 = BPF_ALU   | BPF_K   | BPF_MOD;
/// BPF opcode: `mod32 dst, src` /// `dst %= src`.
pub const MOD32_REG  : u8 = BPF_ALU   | BPF_X   | BPF_MOD;
/// BPF opcode: `xor32 dst, imm` /// `dst ^= imm`.
pub const XOR32_IMM  : u8 = BPF_ALU   | BPF_K   | BPF_XOR;
/// BPF opcode: `xor32 dst, src` /// `dst ^= src`.
pub const XOR32_REG  : u8 = BPF_ALU   | BPF_X   | BPF_XOR;
/// BPF opcode: `mov32 dst, imm` /// `dst = imm`.
pub const MOV32_IMM  : u8 = BPF_ALU   | BPF_K   | BPF_MOV;
/// BPF opcode: `mov32 dst, src` /// `dst = src`.
pub const MOV32_REG  : u8 = BPF_ALU   | BPF_X   | BPF_MOV;
/// BPF opcode: `arsh32 dst, imm` /// `dst >>= imm (arithmetic)`.
///
/// <https://en.wikipedia.org/wiki/Arithmetic_shift>
pub const ARSH32_IMM : u8 = BPF_ALU   | BPF_K   | BPF_ARSH;
/// BPF opcode: `arsh32 dst, src` /// `dst >>= src (arithmetic)`.
///
/// <https://en.wikipedia.org/wiki/Arithmetic_shift>
pub const ARSH32_REG : u8 = BPF_ALU   | BPF_X   | BPF_ARSH;

/// BPF opcode: `le dst` /// `dst = htole<imm>(dst), with imm in {16, 32, 64}`.
pub const LE         : u8 = BPF_ALU   | BPF_K   | BPF_END;
/// BPF opcode: `be dst` /// `dst = htobe<imm>(dst), with imm in {16, 32, 64}`.
pub const BE         : u8 = BPF_ALU   | BPF_X   | BPF_END;

/// BPF opcode: `add64 dst, imm` /// `dst += imm`.
pub const ADD64_IMM  : u8 = BPF_ALU64 | BPF_K   | BPF_ADD;
/// BPF opcode: `add64 dst, src` /// `dst += src`.
pub const ADD64_REG  : u8 = BPF_ALU64 | BPF_X   | BPF_ADD;
/// BPF opcode: `sub64 dst, imm` /// `dst -= imm`.
pub const SUB64_IMM  : u8 = BPF_ALU64 | BPF_K   | BPF_SUB;
/// BPF opcode: `sub64 dst, src` /// `dst -= src`.
pub const SUB64_REG  : u8 = BPF_ALU64 | BPF_X   | BPF_SUB;
/// BPF opcode: `div64 dst, imm` /// `dst /= imm`.
pub const MUL64_IMM  : u8 = BPF_ALU64 | BPF_K   | BPF_MUL;
/// BPF opcode: `div64 dst, src` /// `dst /= src`.
pub const MUL64_REG  : u8 = BPF_ALU64 | BPF_X   | BPF_MUL;
/// BPF opcode: `div64 dst, imm` /// `dst /= imm`.
pub const DIV64_IMM  : u8 = BPF_ALU64 | BPF_K   | BPF_DIV;
/// BPF opcode: `div64 dst, src` /// `dst /= src`.
pub const DIV64_REG  : u8 = BPF_ALU64 | BPF_X   | BPF_DIV;
/// BPF opcode: `or64 dst, imm` /// `dst |= imm`.
pub const OR64_IMM   : u8 = BPF_ALU64 | BPF_K   | BPF_OR;
/// BPF opcode: `or64 dst, src` /// `dst |= src`.
pub const OR64_REG   : u8 = BPF_ALU64 | BPF_X   | BPF_OR;
/// BPF opcode: `and64 dst, imm` /// `dst &= imm`.
pub const AND64_IMM  : u8 = BPF_ALU64 | BPF_K   | BPF_AND;
/// BPF opcode: `and64 dst, src` /// `dst &= src`.
pub const AND64_REG  : u8 = BPF_ALU64 | BPF_X   | BPF_AND;
/// BPF opcode: `lsh64 dst, imm` /// `dst <<= imm`.
pub const LSH64_IMM  : u8 = BPF_ALU64 | BPF_K   | BPF_LSH;
/// BPF opcode: `lsh64 dst, src` /// `dst <<= src`.
pub const LSH64_REG  : u8 = BPF_ALU64 | BPF_X   | BPF_LSH;
/// BPF opcode: `rsh64 dst, imm` /// `dst >>= imm`.
pub const RSH64_IMM  : u8 = BPF_ALU64 | BPF_K   | BPF_RSH;
/// BPF opcode: `rsh64 dst, src` /// `dst >>= src`.
pub const RSH64_REG  : u8 = BPF_ALU64 | BPF_X   | BPF_RSH;
/// BPF opcode: `neg64 dst, imm` /// `dst = -dst`.
pub const NEG64      : u8 = BPF_ALU64 | BPF_NEG;
/// BPF opcode: `mod64 dst, imm` /// `dst %= imm`.
pub const MOD64_IMM  : u8 = BPF_ALU64 | BPF_K   | BPF_MOD;
/// BPF opcode: `mod64 dst, src` /// `dst %= src`.
pub const MOD64_REG  : u8 = BPF_ALU64 | BPF_X   | BPF_MOD;
/// BPF opcode: `xor64 dst, imm` /// `dst ^= imm`.
pub const XOR64_IMM  : u8 = BPF_ALU64 | BPF_K   | BPF_XOR;
/// BPF opcode: `xor64 dst, src` /// `dst ^= src`.
pub const XOR64_REG  : u8 = BPF_ALU64 | BPF_X   | BPF_XOR;
/// BPF opcode: `mov64 dst, imm` /// `dst = imm`.
pub const MOV64_IMM  : u8 = BPF_ALU64 | BPF_K   | BPF_MOV;
/// BPF opcode: `mov64 dst, src` /// `dst = src`.
pub const MOV64_REG  : u8 = BPF_ALU64 | BPF_X   | BPF_MOV;
/// BPF opcode: `arsh64 dst, imm` /// `dst >>= imm (arithmetic)`.
///
/// <https://en.wikipedia.org/wiki/Arithmetic_shift>
pub const ARSH64_IMM : u8 = BPF_ALU64 | BPF_K   | BPF_ARSH;
/// BPF opcode: `arsh64 dst, src` /// `dst >>= src (arithmetic)`.
///
/// <https://en.wikipedia.org/wiki/Arithmetic_shift>
pub const ARSH64_REG : u8 = BPF_ALU64 | BPF_X   | BPF_ARSH;

/// BPF opcode: `ja +off` /// `PC += off`.
pub const JA         : u8 = BPF_JMP   | BPF_JA;
/// BPF opcode: `jeq dst, imm, +off` /// `PC += off if dst == imm`.
pub const JEQ_IMM    : u8 = BPF_JMP   | BPF_K   | BPF_JEQ;
/// BPF opcode: `jeq dst, src, +off` /// `PC += off if dst == src`.
pub const JEQ_REG    : u8 = BPF_JMP   | BPF_X   | BPF_JEQ;
/// BPF opcode: `jgt dst, imm, +off` /// `PC += off if dst > imm`.
pub const JGT_IMM    : u8 = BPF_JMP   | BPF_K   | BPF_JGT;
/// BPF opcode: `jgt dst, src, +off` /// `PC += off if dst > src`.
pub const JGT_REG    : u8 = BPF_JMP   | BPF_X   | BPF_JGT;
/// BPF opcode: `jge dst, imm, +off` /// `PC += off if dst >= imm`.
pub const JGE_IMM    : u8 = BPF_JMP   | BPF_K   | BPF_JGE;
/// BPF opcode: `jge dst, src, +off` /// `PC += off if dst >= src`.
pub const JGE_REG    : u8 = BPF_JMP   | BPF_X   | BPF_JGE;
/// BPF opcode: `jset dst, imm, +off` /// `PC += off if dst & imm`.
pub const JSET_IMM   : u8 = BPF_JMP   | BPF_K   | BPF_JSET;
/// BPF opcode: `jset dst, src, +off` /// `PC += off if dst & src`.
pub const JSET_REG   : u8 = BPF_JMP   | BPF_X   | BPF_JSET;
/// BPF opcode: `jne dst, imm, +off` /// `PC += off if dst != imm`.
pub const JNE_IMM    : u8 = BPF_JMP   | BPF_K   | BPF_JNE;
/// BPF opcode: `jne dst, src, +off` /// `PC += off if dst != src`.
pub const JNE_REG    : u8 = BPF_JMP   | BPF_X   | BPF_JNE;
/// BPF opcode: `jsgt dst, imm, +off` /// `PC += off if dst > imm (signed)`.
pub const JSGT_IMM   : u8 = BPF_JMP   | BPF_K   | BPF_JSGT;
/// BPF opcode: `jsgt dst, src, +off` /// `PC += off if dst > src (signed)`.
pub const JSGT_REG   : u8 = BPF_JMP   | BPF_X   | BPF_JSGT;
/// BPF opcode: `jsge dst, imm, +off` /// `PC += off if dst >= imm (signed)`.
pub const JSGE_IMM   : u8 = BPF_JMP   | BPF_K   | BPF_JSGE;
/// BPF opcode: `jsge dst, src, +off` /// `PC += off if dst >= src (signed)`.
pub const JSGE_REG   : u8 = BPF_JMP   | BPF_X   | BPF_JSGE;

/// BPF opcode: `call imm` /// helper function call to helper with key `imm`.
pub const CALL       : u8 = BPF_JMP   | BPF_CALL;
/// BPF opcode: tail call.
pub const TAIL_CALL  : u8 = BPF_JMP   | BPF_X | BPF_CALL;
/// BPF opcode: `exit` /// `return r0`.
pub const EXIT       : u8 = BPF_JMP   | BPF_EXIT;

// Used in JIT
/// Mask to extract the operation class from an operation code.
pub const BPF_CLS_MASK    : u8 = 0x07;
/// Mask to extract the arithmetic operation code from an instruction operation code.
pub const BPF_ALU_OP_MASK : u8 = 0xf0;

/// Prototype of an eBPF helper function.
pub type Helper = fn (u64, u64, u64, u64, u64) -> u64;

/// An eBPF instruction.
///
/// See <https://www.kernel.org/doc/Documentation/networking/filter.txt> for the Linux kernel
/// documentation about eBPF, or <https://github.com/iovisor/bpf-docs/blob/master/eBPF.md> for a
/// more concise version.
#[derive(Debug, PartialEq)]
pub struct Insn {
    /// Operation code.
    pub opc: u8,
    /// Destination register operand.
    pub dst: u8,
    /// Source register operand.
    pub src: u8,
    /// Offset operand.
    pub off: i16,
    /// Immediate value operand.
    pub imm: i32,
}

/// Get the instruction at `idx` of an eBPF program. `idx` is the index (number) of the
/// instruction (not a byte offset). The first instruction has index 0.
///
/// # Panics
///
/// Panics if it is not possible to get the instruction (if idx is too high, or last instruction is
/// incomplete).
///
/// # Examples
///
/// ```
/// use rbpf::ebpf;
///
/// let prog = &[
///     0xb7, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
///     0x95, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
///     ];
/// let insn = ebpf::get_insn(prog, 1);
/// assert_eq!(insn.opc, 0x95);
/// ```
///
/// The example below will panic, since the last instruction is not complete and cannot be loaded.
///
/// ```rust,should_panic
/// use rbpf::ebpf;
///
/// let prog = &[
///     0xb7, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
///     0x95, 0x00, 0x00, 0x00, 0x00, 0x00              // two bytes missing
///     ];
/// let insn = ebpf::get_insn(prog, 1);
/// ```
pub fn get_insn(prog: &[u8], idx: usize) -> Insn {
    // This guard should not be needed in most cases, since the verifier already checks the program
    // size, and indexes should be fine in the interpreter/JIT. But this function is publicly
    // available and user can call it with any `idx`, so we have to check anyway.
    if (idx + 1) * INSN_SIZE > prog.len() {
        panic!("Error: cannot reach instruction at index {:?} in program containing {:?} bytes",
               idx, prog.len());
    }
    Insn {
        opc:  prog[INSN_SIZE * idx],
        dst:  prog[INSN_SIZE * idx + 1] & 0x0f,
        src: (prog[INSN_SIZE * idx + 1] & 0xf0) >> 4,
        off: unsafe {
            let x = prog.as_ptr().offset((INSN_SIZE * idx + 2) as isize) as *const i16; *x
        },
        imm: unsafe {
            let x = prog.as_ptr().offset((INSN_SIZE * idx + 4) as isize) as *const i32; *x
        },
    }
}

/// Return a vector of `struct Insn` built from a program.
///
/// This is provided as a convenience for users wishing to manipulate a vector of instructions, for
/// example for dumping the program instruction after instruction with a custom format.
///
/// Note that the two parts of `LD_DW_IMM` instructions (spanning on 64 bits) are considered as two
/// distinct instructions.
///
/// # Examples
///
/// ```
/// use rbpf::ebpf;
///
/// let prog = &[
///     0x18, 0x00, 0x00, 0x00, 0x88, 0x77, 0x66, 0x55,
///     0x00, 0x00, 0x00, 0x00, 0x44, 0x33, 0x22, 0x11,
///     0x95, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
/// ];
///
/// let v = ebpf::to_insn_vec(prog);
/// assert_eq!(v, vec![
///     ebpf::Insn {
///         opc: 0x18,
///         dst: 0,
///         src: 0,
///         off: 0,
///         imm: 0x55667788
///     },
///     ebpf::Insn {
///         opc: 0,
///         dst: 0,
///         src: 0,
///         off: 0,
///         imm: 0x11223344
///     },
///     ebpf::Insn {
///         opc: 0x95,
///         dst: 0,
///         src: 0,
///         off: 0,
///         imm: 0
///     },
/// ]);
/// ```
pub fn to_insn_vec(prog: &[u8]) -> Vec<Insn> {
    if prog.len() % INSN_SIZE != 0 {
        panic!("Error: eBPF program length must be a multiple of {:?} octets",
               INSN_SIZE);
    }

    let mut res = vec![];
    let mut insn_ptr:usize = 0;

    while insn_ptr * INSN_SIZE < prog.len() {
        let insn = get_insn(prog, insn_ptr);
        res.push(insn);
        insn_ptr += 1;
    };
    res
}