Fix non-atomic write for PIPEs with data up to PIPE_BUF

kernel/src/fs/pipe.rs

@@ -217,7 +217,7 @@ mod test {
         W: Fn(Arc<PipeWriter>) + Sync + Send + 'static,
         R: Fn(Arc<PipeReader>) + Sync + Send + 'static,
     {
-        let channel = Channel::with_capacity(1);
+        let channel = Channel::with_capacity(2);
         let (writer, readr) = channel.split();
 
         let writer = PipeWriter::new(writer, StatusFlags::empty()).unwrap();
@@ -283,13 +283,13 @@ mod test {
     fn test_write_full() {
         test_blocking(
             |writer| {
-                assert_eq!(writer.write(&mut reader_from(&[1, 2])).unwrap(), 1);
+                assert_eq!(writer.write(&mut reader_from(&[1, 2, 3])).unwrap(), 2);
                 assert_eq!(writer.write(&mut reader_from(&[2])).unwrap(), 1);
             },
             |reader| {
-                let mut buf = [0; 2];
-                assert_eq!(reader.read(&mut writer_from(&mut buf)).unwrap(), 1);
-                assert_eq!(&buf[..1], &[1]);
+                let mut buf = [0; 3];
+                assert_eq!(reader.read(&mut writer_from(&mut buf)).unwrap(), 2);
+                assert_eq!(&buf[..2], &[1, 2]);
                 assert_eq!(reader.read(&mut writer_from(&mut buf)).unwrap(), 1);
                 assert_eq!(&buf[..1], &[2]);
             },
@@ -313,7 +313,7 @@ mod test {
     fn test_write_closed() {
         test_blocking(
             |writer| {
-                assert_eq!(writer.write(&mut reader_from(&[1, 2])).unwrap(), 1);
+                assert_eq!(writer.write(&mut reader_from(&[1, 2, 3])).unwrap(), 2);
                 assert_eq!(
                     writer.write(&mut reader_from(&[2])).unwrap_err().error(),
                     Errno::EPIPE
@@ -324,6 +324,24 @@ mod test {
         );
     }
 
+    #[ktest]
+    fn test_write_atomicity() {
+        test_blocking(
+            |writer| {
+                assert_eq!(writer.write(&mut reader_from(&[1])).unwrap(), 1);
+                assert_eq!(writer.write(&mut reader_from(&[1, 2])).unwrap(), 2);
+            },
+            |reader| {
+                let mut buf = [0; 3];
+                assert_eq!(reader.read(&mut writer_from(&mut buf)).unwrap(), 1);
+                assert_eq!(&buf[..1], &[1]);
+                assert_eq!(reader.read(&mut writer_from(&mut buf)).unwrap(), 2);
+                assert_eq!(&buf[..2], &[1, 2]);
+            },
+            Ordering::WriteThenRead,
+        );
+    }
+
     fn reader_from(buf: &[u8]) -> VmReader {
         VmReader::from(buf).to_fallible()
     }

kernel/src/fs/utils/channel.rs

@@ -22,6 +22,19 @@ pub struct Channel<T> {
     consumer: Consumer<T>,
 }
 
+/// Maximum number of bytes guaranteed to be written to a pipe atomically.
+///
+/// If the number of bytes to be written is less than the threshold, the write must be atomic.
+/// A non-blocking atomic write may fail with `EAGAIN`, even if there is room for a partial write.
+/// In other words, a partial write is not allowed for an atomic write.
+///
+/// For more details, see the description of `PIPE_BUF` in
+/// <https://man7.org/linux/man-pages/man7/pipe.7.html>.
+#[cfg(not(ktest))]
+const PIPE_BUF: usize = 4096;
+#[cfg(ktest)]
+const PIPE_BUF: usize = 2;
+
 impl<T> Channel<T> {
     /// Creates a new channel with the given capacity.
     ///
@@ -114,7 +127,7 @@ impl<T> Producer<T> {
     }
 
     fn update_pollee(&self) {
-        // In theory, `rb.is_full()`/`rb.is_empty()`, where the `rb` is taken from either
+        // In theory, `rb.free_len()`/`rb.is_empty()`, where the `rb` is taken from either
         // `this_end` or `peer_end`, should reflect the same state. However, we need to take the
         // correct lock when updating the events to avoid races between the state check and the
         // event update.
@@ -123,7 +136,7 @@ impl<T> Producer<T> {
         let rb = this_end.rb();
         if self.is_shutdown() {
             // The POLLOUT event is always set in this case. Don't try to remove it.
-        } else if rb.is_full() {
+        } else if rb.free_len() < PIPE_BUF {
             this_end.pollee.del_events(IoEvents::OUT);
         }
         drop(rb);
@@ -204,7 +217,7 @@ impl<T> Consumer<T> {
     }
 
     fn update_pollee(&self) {
-        // In theory, `rb.is_full()`/`rb.is_empty()`, where the `rb` is taken from either
+        // In theory, `rb.free_len()`/`rb.is_empty()`, where the `rb` is taken from either
         // `this_end` or `peer_end`, should reflect the same state. However, we need to take the
         // correct lock when updating the events to avoid races between the state check and the
         // event update.
@@ -218,7 +231,7 @@ impl<T> Consumer<T> {
 
         let peer_end = self.peer_end();
         let rb = peer_end.rb();
-        if !rb.is_full() {
+        if rb.free_len() >= PIPE_BUF {
             peer_end.pollee.add_events(IoEvents::OUT);
         }
         drop(rb);
@@ -307,6 +320,10 @@ impl<R: TRights> Fifo<u8, R> {
     #[require(R > Write)]
     pub fn write(&self, reader: &mut dyn MultiRead) -> Result<usize> {
         let mut rb = self.common.producer.rb();
+        if rb.free_len() < reader.sum_lens() && reader.sum_lens() <= PIPE_BUF {
+            // No sufficient space for an atomic write
+            return Ok(0);
+        }
         rb.write_fallible(reader)
     }
 }