fix(vmm/persist): do not close uffd socket

We prevent Rust from closing the socket file descriptor to avoid a
potential race condition between the mappings message and the connection
shutdown.  If the latter arrives at the UFFD handler first, the handler
never sees the mappings.

Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>

Author: kalyazin

CHANGELOG.md

@@ -44,6 +44,10 @@ and this project adheres to
   the `SendCtrlAltDel` command not working for ACPI-enabled guest kernels, by
   dropping the i8042.nopnp argument from the default kernel command line
   Firecracker constructs.
+- [#5122](https://github.com/firecracker-microvm/firecracker/pull/5122): Keep
+  the UFFD Unix domain socket open to prevent the race condition between the
+  guest memory mappings message and the shutdown event that was sometimes
+  causing arrival of an empty message on the UFFD handler side.
 
 ## [1.11.0]
 

src/vmm/src/persist.rs

@@ -6,6 +6,7 @@
 use std::fmt::Debug;
 use std::fs::{File, OpenOptions};
 use std::io::{self, Write};
+use std::mem::forget;
 use std::os::unix::io::AsRawFd;
 use std::os::unix::net::UnixStream;
 use std::path::Path;
@@ -657,6 +658,11 @@ fn send_uffd_handshake(
         uffd.as_raw_fd(),
     )?;
 
+    // We prevent Rust from closing the socket file descriptor to avoid a potential race condition
+    // between the mappings message and the connection shutdown. If the latter arrives at the UFFD
+    // handler first, the handler never sees the mappings.
+    forget(socket);
+
     Ok(())
 }