debug: add FK:ENTER and FK:INNER markers to trace fork EOPNOTSUPP

redox-rt/src/proc.rs

@@ -962,7 +962,9 @@ pub fn create_set_addr_space_buf_for_fork(
 /// descriptors from other schemes are reobtained with `dup`, and grants referencing such file
 /// descriptors are reobtained through `fmap`. Other mappings are kept but duplicated using CoW.
 pub fn fork_impl(args: &ForkArgs<'_>) -> Result<usize> {
+    let _ = syscall::write(1, b"FK:ENTER\n");
     let old_mask = crate::signal::get_sigmask()?;
+    let _ = syscall::write(1, b"FK:SIG OK\n");
     let pid = unsafe {
         Error::demux(__relibc_internal_fork_wrapper(
             core::ptr::from_ref::<ForkArgs>(args) as usize,
@@ -984,6 +986,7 @@ pub enum ForkArgs<'a> {
 }
 
 pub fn fork_inner(initial_rsp: *mut usize, args: &ForkArgs) -> Result<usize> {
+    let _ = syscall::write(1, b"FK:INNER\n");
     let (cur_filetable_fd, new_filetable_fd, new_proc_fd, new_thr_fd, new_pid);
 
     {