redox-rt: refresh inherited filetable before rebuilding fd table

Backport of upstream relibc 580eab8b ('Refresh filetable data before
filetable creation'). FdTbl::from_binary_fd reconstructs an exec'd
process's fd table from the inherited filetable fd. A plain lseek(0) did
not force the backing scheme to re-materialize the current descriptor
set, so an exec'd child (e.g. a login shell) could read a STALE snapshot
and end up with a desynchronized fd -- observed as an interactive shell
whose pty-slave stdin (fd 0) returns nothing while writes to fd 1/2 work.
Issue a SYS_DUP2 'refresh' on the filetable fd before reading it.

redox-rt/src/sys.rs

@@ -965,7 +965,25 @@ impl FdTbl {
     pub fn from_binary_fd(filetable_fd: FdGuardUpper) -> Result<Self> {
         let mut fdtbl = Self::new();
         let files_reader_fd = filetable_fd.as_raw_fd();
-        let _ = filetable_fd.lseek(0, syscall::flag::SEEK_SET);
+        // Refresh the inherited filetable's data before reading it, so an
+        // exec'd process reconstructs its fd table from the LATEST parent state
+        // rather than a stale snapshot. Without this, some inherited fds (e.g.
+        // an interactive shell's pty-slave stdin) are left desynchronized and
+        // reads return nothing even though writes to sibling fds work.
+        // Upstream relibc 580eab8b ("Refresh filetable data before filetable
+        // creation"). A plain lseek did not force the underlying scheme to
+        // re-materialize the current descriptor set.
+        let buf = b"refresh";
+        unsafe {
+            syscall::syscall4(
+                syscall::SYS_DUP2,
+                files_reader_fd,
+                files_reader_fd,
+                buf.as_ptr() as usize,
+                buf.len(),
+            )
+        }?;
+
         fdtbl.resize(Self::DEFAULT_CAPACITY);
 
         let mut reader = crate::proc::FileBufReader::from_fd(files_reader_fd);