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main.zig
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main.zig
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const std = @import("std");
const c = @cImport({
@cInclude("sys/ptrace.h");
@cInclude("sys/user.h");
@cInclude("sys/wait.h");
@cInclude("errno.h");
});
const cNullPtr: ?*anyopaque = null;
fn execChild(
arena: *std.heap.ArenaAllocator,
args: []const []const u8,
childPid: std.os.pid_t,
) std.process.ExecvError {
_ = c.ptrace(c.PTRACE_TRACEME, childPid, cNullPtr, cNullPtr);
return std.process.execv(
arena.allocator(),
args[1..],
);
}
const ABIArguments = struct {
regs: c.user_regs_struct,
fn nth(aa: ABIArguments, i: u8) c_ulonglong {
std.debug.assert(i < 4);
return switch (i) {
0 => aa.regs.rdi,
1 => aa.regs.rsi,
2 => aa.regs.rdx,
else => unreachable,
};
}
fn setNth(aa: *ABIArguments, i: u8, value: c_ulonglong) void {
std.debug.assert(i < 4);
switch (i) {
0 => {
aa.regs.rdi = value;
},
1 => {
aa.regs.rsi = value;
},
2 => {
aa.regs.rdx = value;
},
else => unreachable,
}
}
fn result(aa: ABIArguments) c_ulonglong {
return aa.regs.rax;
}
fn setResult(aa: *ABIArguments, value: c_ulonglong) void {
aa.regs.rax = value;
}
fn function(aa: ABIArguments) c_ulonglong {
return aa.regs.orig_rax;
}
};
const ChildManager = struct {
arena: *std.heap.ArenaAllocator,
childPid: std.os.pid_t,
fn getABIArguments(cm: ChildManager) ABIArguments {
var args = ABIArguments{ .regs = undefined };
_ = c.ptrace(c.PTRACE_GETREGS, cm.childPid, cNullPtr, &args.regs);
return args;
}
fn setABIArguments(cm: ChildManager, args: *ABIArguments) void {
_ = c.ptrace(c.PTRACE_SETREGS, cm.childPid, cNullPtr, &args.regs);
}
fn childReadData(
cm: ChildManager,
address: c_ulonglong,
length: c_ulonglong,
) !std.ArrayList(u8) {
var data = std.ArrayList(u8).init(cm.arena.allocator());
while (data.items.len < length) {
var word = c.ptrace(
c.PTRACE_PEEKDATA,
cm.childPid,
address + data.items.len,
cNullPtr,
);
for (std.mem.asBytes(&word)) |byte| {
if (data.items.len == length) {
break;
}
try data.append(byte);
}
}
return data;
}
fn childWaitForSyscall(cm: ChildManager) i32 {
var status: i32 = 0;
_ = c.ptrace(c.PTRACE_SYSCALL, cm.childPid, cNullPtr, cNullPtr);
_ = c.waitpid(cm.childPid, &status, 0);
return status;
}
const hooks = &[_]struct {
syscall: c_ulonglong,
hook: *const fn (ChildManager, *ABIArguments) anyerror!void,
}{.{
.syscall = @intFromEnum(std.os.linux.syscalls.X64.write),
.hook = writeHandler,
}};
fn childInterceptSyscalls(
cm: *ChildManager,
) !void {
while (true) {
// Handle syscall entrance
const status = cm.childWaitForSyscall();
if (status == 0) {
break;
}
var args: ABIArguments = cm.getABIArguments();
const address = args.function();
for (hooks) |hook| {
if (address == hook.syscall) {
try hook.hook(cm.*, &args);
}
}
}
}
fn writeHandler(cm: ChildManager, entryArgs: *ABIArguments) anyerror!void {
const fd = entryArgs.nth(0);
const dataAddress = entryArgs.nth(1);
var dataLength = entryArgs.nth(2);
// Truncate some bytes
if (dataLength > 2) {
dataLength -= 2;
entryArgs.setNth(2, dataLength);
cm.setABIArguments(entryArgs);
}
const data = try cm.childReadData(dataAddress, dataLength);
defer data.deinit();
std.debug.print("Got a write on {}: {s}\n", .{ fd, data.items });
// Handle syscall exit
_ = cm.childWaitForSyscall();
var exitArgs = cm.getABIArguments();
dataLength = exitArgs.nth(2);
if (dataLength > 2) {
// Force the writes to stop after the first one by returning EIO.
var result: c_ulonglong = 0;
result = result -% c.EIO;
exitArgs.setResult(result);
cm.setABIArguments(&exitArgs);
}
}
};
pub fn main() !void {
var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
defer arena.deinit();
var args = try std.process.argsAlloc(arena.allocator());
std.debug.assert(args.len >= 2);
const pid = try std.os.fork();
if (pid < 0) {
std.debug.print("Fork failed!\n", .{});
return;
} else if (pid == 0) {
return execChild(&arena, args, pid);
} else {
var cm = ChildManager{ .arena = &arena, .childPid = pid };
try cm.childInterceptSyscalls();
}
}