// Derived from uBPF <https://github.com/iovisor/ubpf>
// Copyright 2015 Big Switch Networks, Inc
//      (uBPF: safety checks, originally in C)
// Copyright 2016 6WIND S.A. <[email protected]>
//      (Translation to Rust)
//
// 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 “verifier” performs simple checks when the eBPF program is loaded into the VM (before it is
//! interpreted or JIT-compiled). It has nothing to do with the much more elaborated verifier inside
//! Linux kernel. There is no verification regarding the program flow control (should be a Direct
//! Acyclic Graph) or the consistency for registers usage (the verifier of the kernel assigns types
//! to the registers and is much stricter).
//!
//! On the other hand, rbpf is not expected to run in kernel space.
//!
//! Improving the verifier would be nice, but this is not trivial (and Linux kernel is under GPL
//! license, so we cannot copy it).
//!
//! Contrary to the verifier of the Linux kernel, this one does not modify the bytecode at all.

use crate::{ebpf, error::UserDefinedError, user_error::UserError};

/// Error definitions
#[derive(Debug, thiserror::Error, PartialEq, Eq)]
pub enum VerifierError {
    /// ProgramLengthNotMultiple
    #[error("program length must be a multiple of {} octets", ebpf::INSN_SIZE)]
    ProgramLengthNotMultiple,
    /// ProgramTooLarge
    #[error("program too big, max {}, is {}", ebpf::PROG_MAX_INSNS, .0)]
    ProgramTooLarge(usize),
    /// NoProgram
    #[error("no program set, call prog_set() to load one")]
    NoProgram,
    ///InvalidLastInstruction
    #[error("program does not end with “EXIT” instruction")]
    InvalidLastInstruction,
    /// DivisionByZero
    #[error("division by 0 (insn #{0})")]
    DivisionByZero(usize),
    /// UnsupportedLeBeArgument
    #[error("unsupported argument for LE/BE (insn #{0})")]
    UnsupportedLeBeArgument(usize),
    /// LddwCannotBeLast
    #[error("LD_DW instruction cannot be last in program")]
    LddwCannotBeLast,
    /// IncompleteLddw
    #[error("incomplete LD_DW instruction (insn #{0})")]
    IncompleteLddw(usize),
    /// InfiniteLoop
    #[error("infinite loop (insn #{0})")]
    InfiniteLoop(usize),
    /// JumpOutOfCode
    #[error("jump out of code to #{0} (insn #{1})")]
    JumpOutOfCode(usize, usize),
    /// JumpToMiddleOfLddw
    #[error("jump to middle of LD_DW at #{0} (insn #{1})")]
    JumpToMiddleOfLddw(usize, usize),
    /// InvalidSourceRegister
    #[error("invalid source register (insn #{0})")]
    InvalidSourceRegister(usize),
    /// CannotWriteR10
    #[error("cannot write into register r10 (insn #{0})")]
    CannotWriteR10(usize),
    /// InvalidDestinationRegister
    #[error("invalid destination register (insn #{0})")]
    InvalidDestinationRegister(usize),
    /// UnknownOpCode
    #[error("unknown eBPF opcode {0:#2x} (insn #{1:?})")]
    UnknownOpCode(u8, usize),
    /// Shift with overflow
    #[error("Shift with overflow at instruction {0}")]
    ShiftWithOverflow(usize),
    /// Invalid register specified
    #[error("Invalid register specified at instruction {0}")]
    InvalidRegister(usize),
}
impl UserDefinedError for VerifierError {}
impl From<VerifierError> for UserError {
    fn from(error: VerifierError) -> Self {
        UserError::VerifierError(error)
    }
}

fn check_prog_len(prog: &[u8]) -> Result<(), VerifierError> {
    if prog.len() % ebpf::INSN_SIZE != 0 {
        return Err(VerifierError::ProgramLengthNotMultiple);
    }
    if prog.len() > ebpf::PROG_MAX_SIZE {
        return Err(VerifierError::ProgramTooLarge(prog.len() / ebpf::INSN_SIZE));
    }

    if prog.is_empty() {
        return Err(VerifierError::NoProgram);
    }
    let last_insn = ebpf::get_insn(prog, (prog.len() / ebpf::INSN_SIZE) - 1);
    if last_insn.opc != ebpf::EXIT {
        return Err(VerifierError::InvalidLastInstruction);
    }

    Ok(())
}

fn check_imm_nonzero(insn: &ebpf::Insn, insn_ptr: usize) -> Result<(), VerifierError> {
    if insn.imm == 0 {
        return Err(VerifierError::DivisionByZero(insn_ptr));
    }
    Ok(())
}

fn check_imm_endian(insn: &ebpf::Insn, insn_ptr: usize) -> Result<(), VerifierError> {
    match insn.imm {
        16 | 32 | 64 => Ok(()),
        _ => Err(VerifierError::UnsupportedLeBeArgument(insn_ptr)),
    }
}

fn check_load_dw(prog: &[u8], insn_ptr: usize) -> Result<(), VerifierError> {
    // We know we can reach next insn since we enforce an EXIT insn at the end of program, while
    // this function should be called only for LD_DW insn, that cannot be last in program.
    let next_insn = ebpf::get_insn(prog, insn_ptr + 1);
    if next_insn.opc != 0 {
        return Err(VerifierError::IncompleteLddw(insn_ptr));
    }
    Ok(())
}

fn check_jmp_offset(prog: &[u8], insn_ptr: usize) -> Result<(), VerifierError> {
    let insn = ebpf::get_insn(prog, insn_ptr);
    if insn.off == -1 {
        return Err(VerifierError::InfiniteLoop(insn_ptr));
    }

    let dst_insn_ptr = insn_ptr as isize + 1 + insn.off as isize;
    if dst_insn_ptr < 0 || dst_insn_ptr as usize >= (prog.len() / ebpf::INSN_SIZE) {
        return Err(VerifierError::JumpOutOfCode(
            dst_insn_ptr as usize,
            insn_ptr,
        ));
    }
    let dst_insn = ebpf::get_insn(prog, dst_insn_ptr as usize);
    if dst_insn.opc == 0 {
        return Err(VerifierError::JumpToMiddleOfLddw(
            dst_insn_ptr as usize,
            insn_ptr,
        ));
    }
    Ok(())
}

fn check_registers(insn: &ebpf::Insn, store: bool, insn_ptr: usize) -> Result<(), VerifierError> {
    if insn.src > 10 {
        return Err(VerifierError::InvalidSourceRegister(insn_ptr));
    }
    match (insn.dst, store) {
        (0..=9, _) | (10, true) => Ok(()),
        (10, false) => Err(VerifierError::CannotWriteR10(insn_ptr)),
        (_, _) => Err(VerifierError::InvalidDestinationRegister(insn_ptr)),
    }
}

/// Check that the imm is a valid shift operand
fn check_imm_shift(insn: &ebpf::Insn, insn_ptr: usize) -> Result<(), VerifierError> {
    if insn.imm < 0 || insn.imm as u64 >= 64 {
        return Err(VerifierError::ShiftWithOverflow(insn_ptr));
    }
    Ok(())
}

/// Check that the imm is a valid register number
fn check_imm_register(insn: &ebpf::Insn, insn_ptr: usize) -> Result<(), VerifierError> {
    if insn.imm < 0 || insn.imm > 10 {
        return Err(VerifierError::InvalidRegister(insn_ptr));
    }
    Ok(())
}

/// Default eBPF verifier
#[rustfmt::skip]
pub fn check(prog: &[u8]) -> Result<(), UserError> {
    check_prog_len(prog)?;

    let mut insn_ptr: usize = 0;
    while insn_ptr * ebpf::INSN_SIZE < prog.len() {
        let insn = ebpf::get_insn(prog, insn_ptr);
        let mut store = false;

        match insn.opc {

            // BPF_LD class
            ebpf::LD_ABS_B   => {},
            ebpf::LD_ABS_H   => {},
            ebpf::LD_ABS_W   => {},
            ebpf::LD_ABS_DW  => {},
            ebpf::LD_IND_B   => {},
            ebpf::LD_IND_H   => {},
            ebpf::LD_IND_W   => {},
            ebpf::LD_IND_DW  => {},

            ebpf::LD_DW_IMM  => {
                store = true;
                check_load_dw(prog, insn_ptr)?;
                insn_ptr += 1;
            },

            // BPF_LDX class
            ebpf::LD_B_REG   => {},
            ebpf::LD_H_REG   => {},
            ebpf::LD_W_REG   => {},
            ebpf::LD_DW_REG  => {},

            // BPF_ST class
            ebpf::ST_B_IMM   => store = true,
            ebpf::ST_H_IMM   => store = true,
            ebpf::ST_W_IMM   => store = true,
            ebpf::ST_DW_IMM  => store = true,

            // BPF_STX class
            ebpf::ST_B_REG   => store = true,
            ebpf::ST_H_REG   => store = true,
            ebpf::ST_W_REG   => store = true,
            ebpf::ST_DW_REG  => store = true,

            // BPF_ALU class
            ebpf::ADD32_IMM  => {},
            ebpf::ADD32_REG  => {},
            ebpf::SUB32_IMM  => {},
            ebpf::SUB32_REG  => {},
            ebpf::MUL32_IMM  => {},
            ebpf::MUL32_REG  => {},
            ebpf::DIV32_IMM  => { check_imm_nonzero(&insn, insn_ptr)?; },
            ebpf::DIV32_REG  => {},
            ebpf::OR32_IMM   => {},
            ebpf::OR32_REG   => {},
            ebpf::AND32_IMM  => {},
            ebpf::AND32_REG  => {},
            ebpf::LSH32_IMM  => { check_imm_shift(&insn, insn_ptr)?; },
            ebpf::LSH32_REG  => {},
            ebpf::RSH32_IMM  => { check_imm_shift(&insn, insn_ptr)?; },
            ebpf::RSH32_REG  => {},
            ebpf::NEG32      => {},
            ebpf::MOD32_IMM  => { check_imm_nonzero(&insn, insn_ptr)?; },
            ebpf::MOD32_REG  => {},
            ebpf::XOR32_IMM  => {},
            ebpf::XOR32_REG  => {},
            ebpf::MOV32_IMM  => {},
            ebpf::MOV32_REG  => {},
            ebpf::ARSH32_IMM => { check_imm_shift(&insn, insn_ptr)?; },
            ebpf::ARSH32_REG => {},
            ebpf::LE         => { check_imm_endian(&insn, insn_ptr)?; },
            ebpf::BE         => { check_imm_endian(&insn, insn_ptr)?; },

            // BPF_ALU64 class
            ebpf::ADD64_IMM  => {},
            ebpf::ADD64_REG  => {},
            ebpf::SUB64_IMM  => {},
            ebpf::SUB64_REG  => {},
            ebpf::MUL64_IMM  => {},
            ebpf::MUL64_REG  => {},
            ebpf::DIV64_IMM  => { check_imm_nonzero(&insn, insn_ptr)?; },
            ebpf::DIV64_REG  => {},
            ebpf::OR64_IMM   => {},
            ebpf::OR64_REG   => {},
            ebpf::AND64_IMM  => {},
            ebpf::AND64_REG  => {},
            ebpf::LSH64_IMM  => { check_imm_shift(&insn, insn_ptr)?; },
            ebpf::LSH64_REG  => {},
            ebpf::RSH64_IMM  => { check_imm_shift(&insn, insn_ptr)?; },
            ebpf::RSH64_REG  => {},
            ebpf::NEG64      => {},
            ebpf::MOD64_IMM  => { check_imm_nonzero(&insn, insn_ptr)?; },
            ebpf::MOD64_REG  => {},
            ebpf::XOR64_IMM  => {},
            ebpf::XOR64_REG  => {},
            ebpf::MOV64_IMM  => {},
            ebpf::MOV64_REG  => {},
            ebpf::ARSH64_IMM => { check_imm_shift(&insn, insn_ptr)?; },
            ebpf::ARSH64_REG => {},

            // BPF_JMP class
            ebpf::JA         => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JEQ_IMM    => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JEQ_REG    => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JGT_IMM    => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JGT_REG    => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JGE_IMM    => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JGE_REG    => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JLT_IMM    => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JLT_REG    => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JLE_IMM    => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JLE_REG    => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JSET_IMM   => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JSET_REG   => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JNE_IMM    => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JNE_REG    => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JSGT_IMM   => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JSGT_REG   => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JSGE_IMM   => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JSGE_REG   => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JSLT_IMM   => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JSLT_REG   => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JSLE_IMM   => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::JSLE_REG   => { check_jmp_offset(prog, insn_ptr)?; },
            ebpf::CALL_IMM   => {},
            ebpf::CALL_REG   => { check_imm_register(&insn, insn_ptr)?; },
            ebpf::EXIT       => {},

            _                => {
                return Err(VerifierError::UnknownOpCode(insn.opc, insn_ptr).into());
            }
        }

        check_registers(&insn, store, insn_ptr)?;

        insn_ptr += 1;
    }

    // insn_ptr should now be equal to number of instructions.
    if insn_ptr != prog.len() / ebpf::INSN_SIZE {
        return Err(VerifierError::JumpOutOfCode(insn_ptr, insn_ptr).into());
    }

    Ok(())
}