Enable CLOEXEC by default and allow it to be changed after creation.

src/memfd.rs

@@ -90,7 +90,7 @@ impl Default for MemfdOptions {
     fn default() -> Self {
         Self {
             allow_sealing: false,
-            cloexec: false,
+            cloexec: true,
             hugetlb: None,
         }
     }

tests/memfd.rs

@@ -9,9 +9,19 @@ fn test_memfd_default() {
     let meta0 = m0.as_file().metadata().unwrap();
     assert_eq!(meta0.len(), 0);
     assert_eq!(meta0.is_file(), true);
+    assert_eq!(get_close_on_exec(&m0).unwrap(), true);
     drop(m0)
 }
 
+#[test]
+fn test_memfd_no_cloexec() {
+    let memfd = memfd::MemfdOptions::default()
+        .close_on_exec(false)
+        .create("no-cloexec")
+        .unwrap();
+    assert_eq!(get_close_on_exec(&memfd).unwrap(), false);
+}
+
 #[test]
 fn test_memfd_multi() {
     let opts = memfd::MemfdOptions::default();
@@ -40,3 +50,16 @@ fn test_memfd_from_into() {
         .right()
         .expect("unexpected conversion from a non-memfd file");
 }
+
+/// Check if the close-on-exec flag is set for the memfd.
+pub fn get_close_on_exec(memfd: &memfd::Memfd) -> std::io::Result<bool> {
+    // SAFETY: The syscall called has no soundness implications (i.e. does not mess with
+    // process memory in weird ways, checks its arguments for correctness, etc.). Furthermore
+    // due to invariants of `Memfd` this syscall is provided a valid file descriptor.
+    let flags = unsafe { libc::fcntl(memfd.as_file().as_raw_fd(), libc::F_GETFD, 0) };
+    if flags == -1 {
+        Err(std::io::Error::last_os_error())
+    } else {
+        Ok(flags & libc::FD_CLOEXEC != 0)
+    }
+}