fork-like clone test

src/test/clone_no_new_stack.zig

@@ -0,0 +1,61 @@
+const std = @import("std");
+const linux = std.os.linux;
+const clone = linux.CLONE;
+
+pub fn main() !void {
+    // SIGCHLD: Send signal to parent on exit (required for waitpid)
+    const flags = clone.FILES | clone.FS | linux.SIG.CHLD;
+
+    const msg = "Child: Hello\n";
+    const msg_len = msg.len;
+
+    // We use inline assembly to perform the clone syscall and handle the child path completely to
+    // avoid the compiler generating code that relies on the parent's stack frame in the child
+    // process (where the stack is empty).
+    const ret = asm volatile (
+        \\ syscall
+        \\ test %%rax, %%rax
+        \\ jnz 1f
+        \\
+        \\ # Child Path
+        \\ # Write to stdout
+        \\ mov $1, %%rdi        # fd = 1 (stdout)
+        \\ mov %[msg], %%rsi    # buffer
+        \\ mov %[len], %%rdx    # length
+        \\ mov $1, %%rax        # SYS_write
+        \\ syscall
+        \\
+        \\ # Exit
+        \\ mov $0, %%rdi        # code = 0
+        \\ mov $60, %%rax       # SYS_exit
+        \\ syscall
+        \\
+        \\ # Should not be reached
+        \\ ud2
+        \\
+        \\ 1:
+        \\ # Parent Path continues
+        : [ret] "={rax}" (-> usize),
+        : [number] "{rax}" (@intFromEnum(linux.syscalls.X64.clone)),
+          [arg1] "{rdi}" (flags),
+          [arg2] "{rsi}" (0),
+          [arg3] "{rdx}" (0),
+          [arg4] "{r10}" (0),
+          [arg5] "{r8}" (0),
+          [msg] "r" (msg.ptr),
+          [len] "r" (msg_len),
+        : .{ .rcx = true, .r11 = true, .memory = true });
+
+    // Parent Process
+    const child_pid: i32 = @intCast(ret);
+    if (child_pid < 0) {
+        _ = linux.syscall3(.write, 1, @intFromPtr("Parent: Clone failed\n"), 21);
+        return;
+    }
+
+    var status: u32 = 0;
+    // wait4 for the child to exit
+    _ = linux.syscall4(.wait4, @as(usize, @intCast(child_pid)), @intFromPtr(&status), 0, 0);
+
+    _ = linux.syscall3(.write, 1, @intFromPtr("Parent: Goodbye\n"), 16);
+}