// SPDX-License-Identifier: (Apache-2.0 OR MIT)
// Derived from uBPF <https://github.com/iovisor/ubpf>
// Copyright 2015 Big Switch Networks, Inc
//      (uBPF: VM architecture, parts of the interpreter, originally in C)
// Copyright 2016 6WIND S.A. <quentin.monnet@6wind.com>
//      (Translation to Rust, MetaBuff/multiple classes addition, hashmaps for helpers)

use crate::ebpf;
use crate::ebpf::MAX_CALL_DEPTH;
use crate::lib::*;
use crate::stack::{StackFrame, StackUsage};

#[allow(clippy::too_many_arguments)]
fn check_mem(
    addr: u64,
    len: usize,
    access_type: &str,
    insn_ptr: usize,
    mbuff: &[u8],
    mem: &[u8],
    stack: &[u8],
    allowed_memory: &HashSet<u64>,
) -> Result<(), Error> {
    if let Some(addr_end) = addr.checked_add(len as u64) {
        if mbuff.as_ptr() as u64 <= addr && addr_end <= mbuff.as_ptr() as u64 + mbuff.len() as u64 {
            return Ok(());
        }
        if mem.as_ptr() as u64 <= addr && addr_end <= mem.as_ptr() as u64 + mem.len() as u64 {
            return Ok(());
        }
        if stack.as_ptr() as u64 <= addr && addr_end <= stack.as_ptr() as u64 + stack.len() as u64 {
            return Ok(());
        }
        if allowed_memory.contains(&addr) {
            return Ok(());
        }
    }

    Err(Error::new(ErrorKind::Other, format!(
        "Error: out of bounds memory {} (insn #{:?}), addr {:#x}, size {:?}\nmbuff: {:#x}/{:#x}, mem: {:#x}/{:#x}, stack: {:#x}/{:#x}",
        access_type, insn_ptr, addr, len,
        mbuff.as_ptr() as u64, mbuff.len(),
        mem.as_ptr() as u64, mem.len(),
        stack.as_ptr() as u64, stack.len()
    )))
}

pub fn execute_program(
    prog_: Option<&[u8]>,
    stack_usage: Option<&StackUsage>,
    mem: &[u8],
    mbuff: &[u8],
    helpers: &HashMap<u32, ebpf::Helper>,
    allowed_memory: &HashSet<u64>,
) -> Result<u64, Error> {
    const U32MAX: u64 = u32::MAX as u64;
    const SHIFT_MASK_64: u64 = 0x3f;

    let (prog, stack_usage) = match prog_ {
        Some(prog) => (prog, stack_usage.unwrap()),
        None => Err(Error::new(
            ErrorKind::Other,
            "Error: No program set, call prog_set() to load one",
        ))?,
    };
    let stack = vec![0u8; ebpf::STACK_SIZE];
    let mut stacks = [StackFrame::new(); MAX_CALL_DEPTH];
    let mut stack_frame_idx = 0;

    // R1 points to beginning of memory area, R10 to stack
    let mut reg: [u64; 11] = [
        0,
        0,
        0,
        0,
        0,
        0,
        0,
        0,
        0,
        0,
        stack.as_ptr() as u64 + stack.len() as u64,
    ];
    if !mbuff.is_empty() {
        reg[1] = mbuff.as_ptr() as u64;
    } else if !mem.is_empty() {
        reg[1] = mem.as_ptr() as u64;
    }

    let check_mem_load = |addr: u64, len: usize, insn_ptr: usize| {
        check_mem(
            addr,
            len,
            "load",
            insn_ptr,
            mbuff,
            mem,
            &stack,
            allowed_memory,
        )
    };
    let check_mem_store = |addr: u64, len: usize, insn_ptr: usize| {
        check_mem(
            addr,
            len,
            "store",
            insn_ptr,
            mbuff,
            mem,
            &stack,
            allowed_memory,
        )
    };

    // Loop on instructions
    let mut insn_ptr: usize = 0;
    while insn_ptr * ebpf::INSN_SIZE < prog.len() {
        let insn = ebpf::get_insn(prog, insn_ptr);
        if stack_frame_idx < MAX_CALL_DEPTH {
            if let Some(usage) = stack_usage.stack_usage_for_local_func(insn_ptr) {
                stacks[stack_frame_idx].set_stack_usage(usage);
            }
        }
        insn_ptr += 1;
        let _dst = insn.dst as usize;
        let _src = insn.src as usize;

        let mut do_jump = || {
            insn_ptr = (insn_ptr as i16 + insn.off) as usize;
        };

        macro_rules! unsigned_u64 {
            ($imm:expr) => {
                ($imm as u32) as u64
            };
        }

        #[rustfmt::skip]
        #[allow(clippy::let_unit_value)] // assign, to avoid #[rustfmt::skip] on an expression
        let _ = match insn.opc {

            // BPF_LD class
            // LD_ABS_* and LD_IND_* are supposed to load pointer to data from metadata buffer.
            // Since this pointer is constant, and since we already know it (mem), do not
            // bother re-fetching it, just use mem already.
            ebpf::LD_ABS_B   => reg[0] = unsafe {
                let x = (mem.as_ptr() as u64 + (insn.imm as u32) as u64) as *const u8;
                check_mem_load(x as u64, 8, insn_ptr)?;
                x.read_unaligned() as u64
            },
            ebpf::LD_ABS_H   => reg[0] = unsafe {
                let x = (mem.as_ptr() as u64 + (insn.imm as u32) as u64) as *const u16;
                check_mem_load(x as u64, 8, insn_ptr)?;
                x.read_unaligned() as u64
            },
            ebpf::LD_ABS_W   => reg[0] = unsafe {
                let x = (mem.as_ptr() as u64 + (insn.imm as u32) as u64) as *const u32;
                check_mem_load(x as u64, 8, insn_ptr)?;
                x.read_unaligned() as u64
            },
            ebpf::LD_ABS_DW  => reg[0] = unsafe {
                let x = (mem.as_ptr() as u64 + (insn.imm as u32) as u64) as *const u64;
                check_mem_load(x as u64, 8, insn_ptr)?;
                x.read_unaligned()
            },
            ebpf::LD_IND_B   => reg[0] = unsafe {
                let x = (mem.as_ptr() as u64 + reg[_src] + (insn.imm as u32) as u64) as *const u8;
                check_mem_load(x as u64, 8, insn_ptr)?;
                x.read_unaligned() as u64
            },
            ebpf::LD_IND_H   => reg[0] = unsafe {
                let x = (mem.as_ptr() as u64 + reg[_src] + (insn.imm as u32) as u64) as *const u16;
                check_mem_load(x as u64, 8, insn_ptr)?;
                x.read_unaligned() as u64
            },
            ebpf::LD_IND_W   => reg[0] = unsafe {
                let x = (mem.as_ptr() as u64 + reg[_src] + (insn.imm as u32) as u64) as *const u32;
                check_mem_load(x as u64, 8, insn_ptr)?;
                x.read_unaligned() as u64
            },
            ebpf::LD_IND_DW  => reg[0] = unsafe {
                let x = (mem.as_ptr() as u64 + reg[_src] + (insn.imm as u32) as u64) as *const u64;
                check_mem_load(x as u64, 8, insn_ptr)?;
                x.read_unaligned()
            },

            ebpf::LD_DW_IMM  => {
                let next_insn = ebpf::get_insn(prog, insn_ptr);
                insn_ptr += 1;
                reg[_dst] = ((insn.imm as u32) as u64) + ((next_insn.imm as u64) << 32);
            },

            // BPF_LDX class
            ebpf::LD_B_REG   => reg[_dst] = unsafe {
                let x = (reg[_src] as *const u8).wrapping_offset(insn.off as isize);
                check_mem_load(x as u64, 1, insn_ptr)?;
                x.read_unaligned() as u64
            },
            ebpf::LD_H_REG   => reg[_dst] = unsafe {
                let x = (reg[_src] as *const u8).wrapping_offset(insn.off as isize) as *const u16;
                check_mem_load(x as u64, 2, insn_ptr)?;
                x.read_unaligned() as u64
            },
            ebpf::LD_W_REG   => reg[_dst] = unsafe {
                let x = (reg[_src] as *const u8).wrapping_offset(insn.off as isize) as *const u32;
                check_mem_load(x as u64, 4, insn_ptr)?;
                x.read_unaligned() as u64
            },
            ebpf::LD_DW_REG  => reg[_dst] = unsafe {
                let x = (reg[_src] as *const u8).wrapping_offset(insn.off as isize) as *const u64;
                check_mem_load(x as u64, 8, insn_ptr)?;
                x.read_unaligned()
            },

            // BPF_ST class
            ebpf::ST_B_IMM   => unsafe {
                let x = (reg[_dst] as *const u8).wrapping_offset(insn.off as isize) as *mut u8;
                check_mem_store(x as u64, 1, insn_ptr)?;
                x.write_unaligned(insn.imm as u8);
            },
            ebpf::ST_H_IMM   => unsafe {
                let x = (reg[_dst] as *const u8).wrapping_offset(insn.off as isize) as *mut u16;
                check_mem_store(x as u64, 2, insn_ptr)?;
                x.write_unaligned(insn.imm as u16);
            },
            ebpf::ST_W_IMM   => unsafe {
                let x = (reg[_dst] as *const u8).wrapping_offset(insn.off as isize) as *mut u32;
                check_mem_store(x as u64, 4, insn_ptr)?;
                x.write_unaligned(insn.imm as u32);
            },
            ebpf::ST_DW_IMM  => unsafe {
                let x = (reg[_dst] as *const u8).wrapping_offset(insn.off as isize) as *mut u64;
                check_mem_store(x as u64, 8, insn_ptr)?;
                x.write_unaligned(insn.imm as u64);
            },

            // BPF_STX class
            ebpf::ST_B_REG   => unsafe {
                let x = (reg[_dst] as *const u8).wrapping_offset(insn.off as isize) as *mut u8;
                check_mem_store(x as u64, 1, insn_ptr)?;
                x.write_unaligned(reg[_src] as u8);
            },
            ebpf::ST_H_REG   => unsafe {
                let x = (reg[_dst] as *const u8).wrapping_offset(insn.off as isize) as *mut u16;
                check_mem_store(x as u64, 2, insn_ptr)?;
                x.write_unaligned(reg[_src] as u16);
            },
            ebpf::ST_W_REG   => unsafe {
                let x = (reg[_dst] as *const u8).wrapping_offset(insn.off as isize) as *mut u32;
                check_mem_store(x as u64, 4, insn_ptr)?;
                x.write_unaligned(reg[_src] as u32);
            },
            ebpf::ST_DW_REG  => unsafe {
                let x = (reg[_dst] as *const u8).wrapping_offset(insn.off as isize) as *mut u64;
                check_mem_store(x as u64, 8, insn_ptr)?;
                x.write_unaligned(reg[_src]);
            },
            ebpf::ST_W_XADD  => unimplemented!(),
            ebpf::ST_DW_XADD => unimplemented!(),

            // BPF_ALU class
            // TODO Check how overflow works in kernel. Should we &= U32MAX all src register value
            // before we do the operation?
            // Cf ((0x11 << 32) - (0x1 << 32)) as u32 VS ((0x11 << 32) as u32 - (0x1 << 32) as u32
            ebpf::ADD32_IMM  => reg[_dst] = (reg[_dst] as i32).wrapping_add(insn.imm)         as u64, //((reg[_dst] & U32MAX) + insn.imm  as u64)     & U32MAX,
            ebpf::ADD32_REG  => reg[_dst] = (reg[_dst] as i32).wrapping_add(reg[_src] as i32) as u64, //((reg[_dst] & U32MAX) + (reg[_src] & U32MAX)) & U32MAX,
            ebpf::SUB32_IMM  => reg[_dst] = (reg[_dst] as i32).wrapping_sub(insn.imm)         as u64,
            ebpf::SUB32_REG  => reg[_dst] = (reg[_dst] as i32).wrapping_sub(reg[_src] as i32) as u64,
            ebpf::MUL32_IMM  => reg[_dst] = (reg[_dst] as i32).wrapping_mul(insn.imm)         as u64,
            ebpf::MUL32_REG  => reg[_dst] = (reg[_dst] as i32).wrapping_mul(reg[_src] as i32) as u64,
            ebpf::DIV32_IMM if insn.imm as u32 == 0 => reg[_dst] = 0,
            ebpf::DIV32_IMM  => reg[_dst] = (reg[_dst] as u32 / insn.imm              as u32) as u64,
            ebpf::DIV32_REG if reg[_src] as u32 == 0 => reg[_dst] = 0,
            ebpf::DIV32_REG  => reg[_dst] = (reg[_dst] as u32 / reg[_src]             as u32) as u64,
            ebpf::OR32_IMM   =>   reg[_dst] = (reg[_dst] as u32             | insn.imm  as u32) as u64,
            ebpf::OR32_REG   =>   reg[_dst] = (reg[_dst] as u32             | reg[_src] as u32) as u64,
            ebpf::AND32_IMM  =>   reg[_dst] = (reg[_dst] as u32             & insn.imm  as u32) as u64,
            ebpf::AND32_REG  =>   reg[_dst] = (reg[_dst] as u32             & reg[_src] as u32) as u64,
            // As for the 64-bit version, we should mask the number of bits to shift with
            // 0x1f, but .wrappping_shr() already takes care of it for us.
            ebpf::LSH32_IMM  =>   reg[_dst] = (reg[_dst] as u32).wrapping_shl(insn.imm  as u32) as u64,
            ebpf::LSH32_REG  =>   reg[_dst] = (reg[_dst] as u32).wrapping_shl(reg[_src] as u32) as u64,
            ebpf::RSH32_IMM  =>   reg[_dst] = (reg[_dst] as u32).wrapping_shr(insn.imm  as u32) as u64,
            ebpf::RSH32_REG  =>   reg[_dst] = (reg[_dst] as u32).wrapping_shr(reg[_src] as u32) as u64,
            ebpf::NEG32      => { reg[_dst] = (reg[_dst] as i32).wrapping_neg()                 as u64; reg[_dst] &= U32MAX; },
            ebpf::MOD32_IMM if insn.imm as u32 == 0 => (),
            ebpf::MOD32_IMM  =>   reg[_dst] = (reg[_dst] as u32             % insn.imm  as u32) as u64,
            ebpf::MOD32_REG if reg[_src] as u32 == 0 => (),
            ebpf::MOD32_REG  =>   reg[_dst] = (reg[_dst] as u32 % reg[_src]             as u32) as u64,
            ebpf::XOR32_IMM  =>   reg[_dst] = (reg[_dst] as u32             ^ insn.imm  as u32) as u64,
            ebpf::XOR32_REG  =>   reg[_dst] = (reg[_dst] as u32             ^ reg[_src] as u32) as u64,
            ebpf::MOV32_IMM  =>   reg[_dst] = insn.imm   as u32                                 as u64,
            ebpf::MOV32_REG  =>   reg[_dst] = (reg[_src] as u32)                                as u64,
            // As for the 64-bit version, we should mask the number of bits to shift with
            // 0x1f, but .wrappping_shr() already takes care of it for us.
            ebpf::ARSH32_IMM => { reg[_dst] = (reg[_dst] as i32).wrapping_shr(insn.imm  as u32) as u64; reg[_dst] &= U32MAX; },
            ebpf::ARSH32_REG => { reg[_dst] = (reg[_dst] as i32).wrapping_shr(reg[_src] as u32) as u64; reg[_dst] &= U32MAX; },
            ebpf::LE         => {
                reg[_dst] = match insn.imm {
                    16 => (reg[_dst] as u16).to_le() as u64,
                    32 => (reg[_dst] as u32).to_le() as u64,
                    64 =>  reg[_dst].to_le(),
                    _  => unreachable!(),
                };
            },
            ebpf::BE         => {
                reg[_dst] = match insn.imm {
                    16 => (reg[_dst] as u16).to_be() as u64,
                    32 => (reg[_dst] as u32).to_be() as u64,
                    64 =>  reg[_dst].to_be(),
                    _  => unreachable!(),
                };
            },

            // BPF_ALU64 class
            ebpf::ADD64_IMM  => reg[_dst] = reg[_dst].wrapping_add(insn.imm as u64),
            ebpf::ADD64_REG  => reg[_dst] = reg[_dst].wrapping_add(reg[_src]),
            ebpf::SUB64_IMM  => reg[_dst] = reg[_dst].wrapping_sub(insn.imm as u64),
            ebpf::SUB64_REG  => reg[_dst] = reg[_dst].wrapping_sub(reg[_src]),
            ebpf::MUL64_IMM  => reg[_dst] = reg[_dst].wrapping_mul(insn.imm as u64),
            ebpf::MUL64_REG  => reg[_dst] = reg[_dst].wrapping_mul(reg[_src]),
            ebpf::DIV64_IMM if insn.imm == 0 => reg[_dst] = 0,
            ebpf::DIV64_IMM  => reg[_dst]                       /= insn.imm as u64,
            ebpf::DIV64_REG if reg[_src] == 0 => reg[_dst] = 0,
            ebpf::DIV64_REG  => reg[_dst] /= reg[_src],
            ebpf::OR64_IMM   => reg[_dst] |=  insn.imm as u64,
            ebpf::OR64_REG   => reg[_dst] |=  reg[_src],
            ebpf::AND64_IMM  => reg[_dst] &=  insn.imm as u64,
            ebpf::AND64_REG  => reg[_dst] &=  reg[_src],
            ebpf::LSH64_IMM  => reg[_dst] <<= insn.imm as u64 & SHIFT_MASK_64,
            ebpf::LSH64_REG  => reg[_dst] <<= reg[_src] & SHIFT_MASK_64,
            ebpf::RSH64_IMM  => reg[_dst] >>= insn.imm as u64 & SHIFT_MASK_64,
            ebpf::RSH64_REG  => reg[_dst] >>= reg[_src] & SHIFT_MASK_64,
            ebpf::NEG64      => reg[_dst] = -(reg[_dst] as i64) as u64,
            ebpf::MOD64_IMM if insn.imm == 0 => (),
            ebpf::MOD64_IMM  => reg[_dst] %=  insn.imm as u64,
            ebpf::MOD64_REG if reg[_src] == 0 => (),
            ebpf::MOD64_REG  => reg[_dst] %= reg[_src],
            ebpf::XOR64_IMM  => reg[_dst] ^= insn.imm  as u64,
            ebpf::XOR64_REG  => reg[_dst] ^= reg[_src],
            ebpf::MOV64_IMM  => reg[_dst] =  insn.imm  as u64,
            ebpf::MOV64_REG  => reg[_dst] =  reg[_src],
            ebpf::ARSH64_IMM => reg[_dst] = (reg[_dst] as i64 >> (insn.imm as u64 & SHIFT_MASK_64))  as u64,
            ebpf::ARSH64_REG => reg[_dst] = (reg[_dst] as i64 >> (reg[_src] as u64 & SHIFT_MASK_64)) as u64,

            // BPF_JMP class
            // TODO: check this actually works as expected for signed / unsigned ops
            // J-EQ, J-NE, J-GT, J-GE, J-LT, J-LE: unsigned
            // JS-GT, JS-GE, JS-LT, JS-LE: signed
            ebpf::JA         =>                                             do_jump(),
            ebpf::JEQ_IMM    => if  reg[_dst] == unsigned_u64!(insn.imm)  { do_jump(); },
            ebpf::JEQ_REG    => if  reg[_dst] == reg[_src]                { do_jump(); },
            ebpf::JGT_IMM    => if  reg[_dst] >  unsigned_u64!(insn.imm)  { do_jump(); },
            ebpf::JGT_REG    => if  reg[_dst] >  reg[_src]                { do_jump(); },
            ebpf::JGE_IMM    => if  reg[_dst] >= unsigned_u64!(insn.imm)  { do_jump(); },
            ebpf::JGE_REG    => if  reg[_dst] >= reg[_src]                { do_jump(); },
            ebpf::JLT_IMM    => if  reg[_dst] <  unsigned_u64!(insn.imm)  { do_jump(); },
            ebpf::JLT_REG    => if  reg[_dst] <  reg[_src]                { do_jump(); },
            ebpf::JLE_IMM    => if  reg[_dst] <= unsigned_u64!(insn.imm)  { do_jump(); },
            ebpf::JLE_REG    => if  reg[_dst] <= reg[_src]                { do_jump(); },
            ebpf::JSET_IMM   => if  reg[_dst] &  insn.imm as u64 != 0     { do_jump(); },
            ebpf::JSET_REG   => if  reg[_dst] &  reg[_src]       != 0     { do_jump(); },
            ebpf::JNE_IMM    => if  reg[_dst] != unsigned_u64!(insn.imm)  { do_jump(); },
            ebpf::JNE_REG    => if  reg[_dst] != reg[_src]                { do_jump(); },
            ebpf::JSGT_IMM   => if  reg[_dst] as i64  >  insn.imm  as i64 { do_jump(); },
            ebpf::JSGT_REG   => if  reg[_dst] as i64  >  reg[_src] as i64 { do_jump(); },
            ebpf::JSGE_IMM   => if  reg[_dst] as i64  >= insn.imm  as i64 { do_jump(); },
            ebpf::JSGE_REG   => if  reg[_dst] as i64  >= reg[_src] as i64 { do_jump(); },
            ebpf::JSLT_IMM   => if (reg[_dst] as i64) <  insn.imm  as i64 { do_jump(); },
            ebpf::JSLT_REG   => if (reg[_dst] as i64) <  reg[_src] as i64 { do_jump(); },
            ebpf::JSLE_IMM   => if  reg[_dst] as i64  <= insn.imm  as i64 { do_jump(); },
            ebpf::JSLE_REG   => if  reg[_dst] as i64  <= reg[_src] as i64 { do_jump(); },

            // BPF_JMP32 class
            ebpf::JEQ_IMM32  => if  reg[_dst] as u32  == insn.imm  as u32      { do_jump(); },
            ebpf::JEQ_REG32  => if  reg[_dst] as u32  == reg[_src] as u32      { do_jump(); },
            ebpf::JGT_IMM32  => if  reg[_dst] as u32  >  insn.imm  as u32      { do_jump(); },
            ebpf::JGT_REG32  => if  reg[_dst] as u32  >  reg[_src] as u32      { do_jump(); },
            ebpf::JGE_IMM32  => if  reg[_dst] as u32  >= insn.imm  as u32      { do_jump(); },
            ebpf::JGE_REG32  => if  reg[_dst] as u32  >= reg[_src] as u32      { do_jump(); },
            ebpf::JLT_IMM32  => if (reg[_dst] as u32) <  insn.imm  as u32      { do_jump(); },
            ebpf::JLT_REG32  => if (reg[_dst] as u32) <  reg[_src] as u32      { do_jump(); },
            ebpf::JLE_IMM32  => if  reg[_dst] as u32  <= insn.imm  as u32      { do_jump(); },
            ebpf::JLE_REG32  => if  reg[_dst] as u32  <= reg[_src] as u32      { do_jump(); },
            ebpf::JSET_IMM32 => if  reg[_dst] as u32  &  insn.imm  as u32 != 0 { do_jump(); },
            ebpf::JSET_REG32 => if  reg[_dst] as u32  &  reg[_src] as u32 != 0 { do_jump(); },
            ebpf::JNE_IMM32  => if  reg[_dst] as u32  != insn.imm  as u32      { do_jump(); },
            ebpf::JNE_REG32  => if  reg[_dst] as u32  != reg[_src] as u32      { do_jump(); },
            ebpf::JSGT_IMM32 => if  reg[_dst] as i32  >  insn.imm              { do_jump(); },
            ebpf::JSGT_REG32 => if  reg[_dst] as i32  >  reg[_src] as i32      { do_jump(); },
            ebpf::JSGE_IMM32 => if  reg[_dst] as i32  >= insn.imm              { do_jump(); },
            ebpf::JSGE_REG32 => if  reg[_dst] as i32  >= reg[_src] as i32      { do_jump(); },
            ebpf::JSLT_IMM32 => if (reg[_dst] as i32) <  insn.imm              { do_jump(); },
            ebpf::JSLT_REG32 => if (reg[_dst] as i32) <  reg[_src] as i32      { do_jump(); },
            ebpf::JSLE_IMM32 => if  reg[_dst] as i32  <= insn.imm              { do_jump(); },
            ebpf::JSLE_REG32 => if  reg[_dst] as i32  <= reg[_src] as i32      { do_jump(); },

            // Do not delegate the check to the verifier, since registered functions can be
            // changed after the program has been verified.
            ebpf::CALL       => {
                match _src {
                    // Call helper function
                    0 => {
                        if let Some(function) = helpers.get(&(insn.imm as u32)) {
                            reg[0] = function(reg[1], reg[2], reg[3], reg[4], reg[5]);
                        } else {
                            Err(Error::new(
                                ErrorKind::Other,
                                format!(
                                    "Error: unknown helper function (id: {:#x})",
                                    insn.imm as u32
                                )
                            ))?;
                        }
                    }
                    // eBPF-to-eBPF call
                    1 => {
                        if stack_frame_idx >= MAX_CALL_DEPTH {
                            Err(Error::new(
                                ErrorKind::Other,
                                format!(
                                    "Error: too many nested calls (max: {MAX_CALL_DEPTH})"
                                )
                            ))?;
                        }
                        stacks[stack_frame_idx].save_registers(&reg[6..=9]);
                        stacks[stack_frame_idx].save_return_address(insn_ptr);
                        // Why we don't need to check the stack usage here?
                        // When the stack is exhausted, if there are instructions in the new function
                        // that read or write to the stack, check_mem_load or check_mem_store will return an error.
                        reg[10] -= stacks[stack_frame_idx].get_stack_usage().stack_usage() as u64;
                        stack_frame_idx += 1;
                        insn_ptr += insn.imm as usize;
                    }
                    _ => {
                        Err(Error::new(
                            ErrorKind::Other,
                            format!("Error: unsupported call type #{} (insn #{})",
                                _src,
                                insn_ptr-1
                            )
                        ))?;
                    }
                }
            }
            ebpf::TAIL_CALL  => unimplemented!(),
            ebpf::EXIT       => {
                if stack_frame_idx > 0 {
                    stack_frame_idx -= 1;
                    reg[6..=9].copy_from_slice(&stacks[stack_frame_idx].get_registers());
                    insn_ptr = stacks[stack_frame_idx].get_return_address();
                    reg[10] += stacks[stack_frame_idx].get_stack_usage().stack_usage() as u64;
                } else {
                    return Ok(reg[0]);
                }
            }

            _                => unreachable!()
        };
    }

    unreachable!()
}