userspace: implement OSS ksuinit

userspace/ksuinit/src/main.rs

@@ -0,0 +1,19 @@
+#![no_main]
+
+mod init;
+mod loader;
+
+use rustix::{cstr, runtime::execve};
+/// # Safety
+/// This is the entry point of the program
+/// We cannot use the main because rust will abort if we don't have std{in/out/err}
+/// https://github.com/rust-lang/rust/blob/3071aefdb2821439e2e6f592f41a4d28e40c1e79/library/std/src/sys/unix/mod.rs#L80
+/// So we use the C main function and call rust code from there
+#[no_mangle]
+pub unsafe extern "C" fn main(_argc: i32, argv: *const *const u8, envp: *const *const u8) -> i32 {
+    let _ = init::init();
+    unsafe {
+        execve(cstr!("/init"), argv, envp);
+    }
+    0
+}