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const std = @import("std");
const builtin = @import("builtin");
const x86 = @import("x86.zig");
const utils = @import("utils.zig");
const loadValue = @import("mem.zig").loadValue;
const Hook = @This();
const HookType = enum {
vmt,
detour,
};
const Rel32Patch = struct {
offset: u32, // offset into the trampoline
dest: u32, // absolute address rel32 points to
orig: u32, // original data in the instruction
};
// Windows doesn't use PIC
const PICPatch = switch (builtin.os.tag) {
.linux => struct {
offset: u32, // offset into the original function
orig: u32, // original data in the instruction
},
.windows => void,
else => @compileError("Unsupported OS"),
};
const HookData = union(HookType) {
const HookVMTResult = struct {
vt: [*]*const anyopaque,
index: u32,
};
const HookDetourResult = struct {
func: [*]u8,
trampoline: []u8,
rel32_patch: ?Rel32Patch = null,
pic_patch: ?PICPatch = null,
};
vmt: HookVMTResult,
detour: HookDetourResult,
};
orig: ?*const anyopaque,
data: HookData,
pub fn hookVMT(vt: [*]*const anyopaque, index: usize, target: *const anyopaque) !Hook {
const orig: *const anyopaque = vt[index];
const entry_ptr: [*]u8 = @ptrCast(vt + index);
const bytes = std.mem.toBytes(target);
try utils.patchCode(entry_ptr, &bytes, 0b001); // restore to read-only
return Hook{
.orig = orig,
.data = .{
.vmt = .{
.vt = vt,
.index = index,
},
},
};
}
// Trampoline memory must have rwx permissions
pub fn hookDetour(func: *anyopaque, target: *const anyopaque, trampoline: []u8) !Hook {
var mem: [*]u8 = @ptrCast(func);
// Hook the underlying thing if the function jmp immediately.
while (mem[0] == x86.Opcode.Op1IW.jmpiw) {
const offset = loadValue(u32, mem + 1);
mem = @ptrFromInt(@intFromPtr(mem + 5) +% offset);
}
var rel32_patch: ?Rel32Patch = null;
var pic_patch: ?PICPatch = null;
var len: usize = 0;
while (true) : (len += try x86.x86_len(mem + len)) {
if (len >= 5) break;
// No checks for rel16 at all. I don't think we will encounter them.
const op0 = mem[len];
switch (op0) {
x86.Opcode.Op1I8.jmpi8,
x86.Opcode.Op1I8.jcxz,
x86.Opcode.Op1I8.jo,
x86.Opcode.Op1I8.jno,
x86.Opcode.Op1I8.jb,
x86.Opcode.Op1I8.jnb,
x86.Opcode.Op1I8.jz,
x86.Opcode.Op1I8.jnz,
x86.Opcode.Op1I8.jna,
x86.Opcode.Op1I8.ja,
x86.Opcode.Op1I8.js,
x86.Opcode.Op1I8.jns,
x86.Opcode.Op1I8.jp,
x86.Opcode.Op1I8.jnp,
x86.Opcode.Op1I8.jl,
x86.Opcode.Op1I8.jnl,
x86.Opcode.Op1I8.jng,
x86.Opcode.Op1I8.jg,
=> {
// TODO: Make it rel32 jump in the trampoline
return error.BadInstruction;
},
x86.Opcode.Op1IW.jmpiw,
x86.Opcode.Op1IW.call,
=> {
const offset = loadValue(u32, mem + len + 1);
rel32_patch = .{
.offset = len + 1,
.dest = @intFromPtr(mem + len + 5) +% offset,
.orig = offset,
};
if (op0 == x86.Opcode.Op1IW.call and builtin.os.tag == .linux) {
// Look for PIC pattern:
// call __i686.get_pc_thunk.reg
// add reg, imm32
if (utils.matchPIC(mem + len)) |off| {
const imm32 = loadValue(u32, mem + len + off);
pic_patch = .{
.offset = len + off,
.orig = imm32,
};
}
}
},
x86.Opcode.op2_byte => {
const op1 = mem[len + 1];
switch (op1) {
x86.Opcode.Op2IW.joii,
x86.Opcode.Op2IW.jnoii,
x86.Opcode.Op2IW.jbii,
x86.Opcode.Op2IW.jnbii,
x86.Opcode.Op2IW.jzii,
x86.Opcode.Op2IW.jnzii,
x86.Opcode.Op2IW.jnaii,
x86.Opcode.Op2IW.jaii,
x86.Opcode.Op2IW.jsii,
x86.Opcode.Op2IW.jnsii,
x86.Opcode.Op2IW.jpii,
x86.Opcode.Op2IW.jnpii,
x86.Opcode.Op2IW.jlii,
x86.Opcode.Op2IW.jnlii,
x86.Opcode.Op2IW.jngii,
x86.Opcode.Op2IW.jgii,
=> {
const offset = loadValue(u32, mem + len + 2);
rel32_patch = .{
.offset = len + 2,
.dest = @intFromPtr(mem + len + 6) +% offset,
.orig = offset,
};
},
else => {},
}
},
else => {},
}
}
const trampoline_size = len + 5;
if (trampoline.len < trampoline_size) {
return error.OutOfTrampoline;
}
@memcpy(trampoline[0..len], mem);
trampoline[len] = x86.Opcode.Op1IW.jmpiw;
const jmp1_offset: *align(1) u32 = @ptrCast(trampoline.ptr + len + 1);
jmp1_offset.* = @intFromPtr(mem + len) -% @intFromPtr(trampoline.ptr + len + 5);
if (rel32_patch) |r| {
const rel_patch: *align(1) u32 = @ptrCast(trampoline.ptr + r.offset);
rel_patch.* = r.dest -% (@intFromPtr(trampoline.ptr + r.offset + 4));
}
var detour: [5]u8 = undefined;
detour[0] = x86.Opcode.Op1IW.jmpiw;
const jmp2_offset: *align(1) u32 = @ptrCast(&detour[1]);
jmp2_offset.* = @intFromPtr(target) -% @intFromPtr(mem + 5);
try utils.patchCode(mem, detour[0..], 0b101);
if (builtin.os.tag == .linux) {
if (pic_patch) |p| {
const delta: u32 = @intFromPtr(trampoline.ptr) -% @intFromPtr(mem);
const new_value: u32 = p.orig -% delta;
const bytes = std.mem.toBytes(new_value);
try utils.patchCode(mem + p.offset, &bytes, 0b101);
}
}
return Hook{
.orig = trampoline.ptr,
.data = .{ .detour = .{
.func = mem,
.trampoline = trampoline[0..trampoline_size],
.rel32_patch = rel32_patch,
.pic_patch = pic_patch,
} },
};
}
pub fn unhook(self: *Hook) !void {
const orig = self.orig orelse return;
switch (self.data) {
.vmt => |v| {
const entry_ptr: [*]u8 = @ptrCast(v.vt + v.index);
const bytes = std.mem.toBytes(orig);
try utils.patchCode(entry_ptr, &bytes, 0b001); // restore to read-only
},
.detour => |v| {
if (v.rel32_patch) |r| {
const orig_patch: *align(1) u32 = @ptrCast(v.trampoline.ptr + r.offset);
orig_patch.* = r.orig;
}
try utils.patchCode(v.func, v.trampoline[0 .. v.trampoline.len - 5], 0b101);
if (builtin.os.tag == .linux) {
if (v.pic_patch) |p| {
const bytes = std.mem.toBytes(p.orig);
try utils.patchCode(v.func + p.offset, &bytes, 0b101);
}
}
},
}
self.orig = null;
}
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