diff --git a/src/libos/src/fs/events.rs b/src/libos/src/fs/events.rs
index 55ce6e3a..c2c2ce6a 100644
--- a/src/libos/src/fs/events.rs
+++ b/src/libos/src/fs/events.rs
@@ -2,13 +2,37 @@ use crate::events::{Event, EventFilter, Notifier, Observer};
 use crate::prelude::*;
 
 bitflags! {
+    /// I/O Events
+    #[rustfmt::skip]
     pub struct IoEvents: u32 {
-        const IN = 0x001; // = POLLIN
-        const OUT = 0x004; // = POLLOUT
-        const PRI = 0x002; // = POLLPRI
-        const ERR = 0x008; // = POLLERR
+        const IN    = 0x0001; // = POLLIN
+        const PRI   = 0x0002; // = POLLPRI
+        const OUT   = 0x0004; // = POLLOUT
+        const ERR   = 0x0008; // = POLLERR
+        const HUP   = 0x0010; // = POLLHUP
+        const NVAL  = 0x0020; // = POLLNVAL
         const RDHUP = 0x2000; // = POLLRDHUP
-        const HUP = 0x010; // = POLLHUP
+    }
+}
+
+impl IoEvents {
+    pub fn from_raw(raw: u32) -> Self {
+        if Self::contains_unrecognizable_bits(raw) {
+            warn!("contain unknow flags: {:#x}", raw);
+        }
+        Self::from_bits_truncate(raw)
+    }
+
+    fn contains_unrecognizable_bits(raw: u32) -> bool {
+        // Help to detect four valid but mostly useless flags that we do not
+        // handle, yet: POLLRDNORM, POLLRDBAND, POLLWRNORM, annd POLLWRBAND.
+
+        let all_raw = Self::all().to_raw();
+        (raw & !all_raw) != 0
+    }
+
+    pub fn to_raw(&self) -> u32 {
+        self.bits()
     }
 }
 
diff --git a/src/libos/src/net/io_multiplexing/epoll/host_file_epoller.rs b/src/libos/src/net/io_multiplexing/epoll/host_file_epoller.rs
index ce65d0fc..8980681b 100644
--- a/src/libos/src/net/io_multiplexing/epoll/host_file_epoller.rs
+++ b/src/libos/src/net/io_multiplexing/epoll/host_file_epoller.rs
@@ -156,7 +156,7 @@ impl HostFileEpoller {
         let mut host_files = self.host_files.lock().unwrap();
         for raw_event in &raw_events[..count] {
             let raw_event = unsafe { raw_event.assume_init() };
-            let io_events = IoEvents::from_bits_truncate(raw_event.events as u32);
+            let io_events = IoEvents::from_raw(raw_event.events as u32);
             let host_fd = raw_event.u64 as u32;
 
             let host_file = match host_files.get(&host_fd) {
diff --git a/src/libos/src/net/io_multiplexing/epoll/mod.rs b/src/libos/src/net/io_multiplexing/epoll/mod.rs
index f1f8a6d8..8e8f0a23 100644
--- a/src/libos/src/net/io_multiplexing/epoll/mod.rs
+++ b/src/libos/src/net/io_multiplexing/epoll/mod.rs
@@ -55,7 +55,7 @@ impl EpollEvent {
     }
 
     pub fn from_c(c_event: &libc::epoll_event) -> Self {
-        let mask = IoEvents::from_bits_truncate(c_event.events as u32);
+        let mask = IoEvents::from_raw(c_event.events as u32);
         let user_data = c_event.u64;
         Self { mask, user_data }
     }