fix: Properly handle status message on older kernels

The status message has evolved, with fields being added in subsequent
kernel versions. The parsing of this message excepts a recent version of
it, making it fail to parse on older kernels.

This is fixed by putting the newer fields behind an option. On parsing,
those are set only if the received status payload is big enough.

src/status.rs

@@ -17,9 +17,10 @@ const RATE_LIMITING: Field = 16..20;
 const BACKLOG_LIMIT: Field = 20..24;
 const LOST: Field = 24..28;
 const BACKLOG: Field = 28..32;
+pub const MINIMAL_STATUS_MESSAGE_LEN: usize = BACKLOG.end;
 const FEATURE_BITMAP: Field = 32..36;
 const BACKLOG_WAIT_TIME: Field = 36..40;
-pub const STATUS_MESSAGE_LEN: usize = BACKLOG_WAIT_TIME.end;
+pub const MAXIMAL_STATUS_MESSAGE_LEN: usize = BACKLOG_WAIT_TIME.end;
 
 #[derive(Debug, PartialEq, Eq, Clone, Default)]
 #[non_exhaustive]
@@ -39,10 +40,13 @@ pub struct StatusMessage {
     pub lost: u32,
     /// Messages waiting in queue
     pub backlog: u32,
+
+    // Starting here, those fields may not be present in older kernels, hence
+    // the use of Option.
     /// bitmap of kernel audit features
-    pub feature_bitmap: u32,
+    pub feature_bitmap: Option<u32>,
     /// Message queue wait timeout
-    pub backlog_wait_time: u32,
+    pub backlog_wait_time: Option<u32>,
 }
 
 impl StatusMessage {
@@ -72,10 +76,10 @@ impl<T: AsRef<[u8]>> StatusMessageBuffer<T> {
 
     fn check_buffer_length(&self) -> Result<(), DecodeError> {
         let len = self.buffer.as_ref().len();
-        if len < STATUS_MESSAGE_LEN {
+        if len < MINIMAL_STATUS_MESSAGE_LEN {
             return Err(format!(
                 "invalid StatusMessageBuffer buffer: length is {len} \
-                instead of {STATUS_MESSAGE_LEN}"
+                instead of at least {MINIMAL_STATUS_MESSAGE_LEN}"
             )
             .into());
         }
@@ -118,12 +122,22 @@ impl<T: AsRef<[u8]>> StatusMessageBuffer<T> {
         NativeEndian::read_u32(&self.buffer.as_ref()[BACKLOG])
     }
 
-    pub fn feature_bitmap(&self) -> u32 {
-        NativeEndian::read_u32(&self.buffer.as_ref()[FEATURE_BITMAP])
+    pub fn feature_bitmap(&self) -> Option<u32> {
+        let buf = self.buffer.as_ref();
+        if buf.len() < FEATURE_BITMAP.end {
+            None
+        } else {
+            Some(NativeEndian::read_u32(&buf[FEATURE_BITMAP]))
+        }
     }
 
-    pub fn backlog_wait_time(&self) -> u32 {
-        NativeEndian::read_u32(&self.buffer.as_ref()[BACKLOG_WAIT_TIME])
+    pub fn backlog_wait_time(&self) -> Option<u32> {
+        let buf = self.buffer.as_ref();
+        if buf.len() < BACKLOG_WAIT_TIME.end {
+            None
+        } else {
+            Some(NativeEndian::read_u32(&buf[BACKLOG_WAIT_TIME]))
+        }
     }
 }
 
@@ -161,17 +175,17 @@ impl<T: AsRef<[u8]> + AsMut<[u8]>> StatusMessageBuffer<T> {
     }
 
     pub fn set_feature_bitmap(&mut self, value: u32) {
-        NativeEndian::write_u32(
-            &mut self.buffer.as_mut()[FEATURE_BITMAP],
-            value,
-        )
+        let buf = &mut self.buffer.as_mut();
+        if buf.len() >= FEATURE_BITMAP.end {
+            NativeEndian::write_u32(&mut buf[FEATURE_BITMAP], value)
+        }
     }
 
     pub fn set_backlog_wait_time(&mut self, value: u32) {
-        NativeEndian::write_u32(
-            &mut self.buffer.as_mut()[BACKLOG_WAIT_TIME],
-            value,
-        )
+        let buf = &mut self.buffer.as_mut();
+        if buf.len() >= BACKLOG_WAIT_TIME.end {
+            NativeEndian::write_u32(&mut buf[BACKLOG_WAIT_TIME], value)
+        }
     }
 }
 
@@ -195,7 +209,7 @@ impl<T: AsRef<[u8]>> Parseable<StatusMessageBuffer<T>> for StatusMessage {
 
 impl Emitable for StatusMessage {
     fn buffer_len(&self) -> usize {
-        STATUS_MESSAGE_LEN
+        MAXIMAL_STATUS_MESSAGE_LEN
     }
 
     fn emit(&self, buffer: &mut [u8]) {
@@ -208,7 +222,11 @@ impl Emitable for StatusMessage {
         buffer.set_backlog_limit(self.backlog_limit);
         buffer.set_lost(self.lost);
         buffer.set_backlog(self.backlog);
-        buffer.set_feature_bitmap(self.feature_bitmap);
-        buffer.set_backlog_wait_time(self.backlog_wait_time);
+        if let Some(feature_bitmap) = self.feature_bitmap {
+            buffer.set_feature_bitmap(feature_bitmap);
+        }
+        if let Some(backlog_wait_time) = self.backlog_wait_time {
+            buffer.set_backlog_wait_time(backlog_wait_time);
+        }
     }
 }