Make special devices go to the FileIo fast path

kernel/src/device/null.rs

@@ -16,6 +16,10 @@ impl Device for Null {
         // Same value with Linux
         DeviceId::new(1, 3)
     }
+
+    fn open(&self) -> Result<Option<Arc<dyn FileIo>>> {
+        Ok(Some(Arc::new(Null)))
+    }
 }
 
 impl FileIo for Null {

kernel/src/device/random.rs

@@ -31,6 +31,10 @@ impl Device for Random {
         // The same value as Linux
         DeviceId::new(1, 8)
     }
+
+    fn open(&self) -> Result<Option<Arc<dyn FileIo>>> {
+        Ok(Some(Arc::new(Random)))
+    }
 }
 
 impl FileIo for Random {

kernel/src/device/urandom.rs

@@ -31,6 +31,10 @@ impl Device for Urandom {
         // The same value as Linux
         DeviceId::new(1, 9)
     }
+
+    fn open(&self) -> Result<Option<Arc<dyn FileIo>>> {
+        Ok(Some(Arc::new(Urandom)))
+    }
 }
 
 impl FileIo for Urandom {

kernel/src/device/zero.rs

@@ -16,6 +16,10 @@ impl Device for Zero {
         // Same value with Linux
         DeviceId::new(1, 5)
     }
+
+    fn open(&self) -> Result<Option<Arc<dyn FileIo>>> {
+        Ok(Some(Arc::new(Zero)))
+    }
 }
 
 impl FileIo for Zero {

kernel/src/fs/inode_handle/mod.rs

@@ -98,12 +98,13 @@ impl InodeHandle_ {
             todo!("support write_at for FileIo");
         }
 
-        if self.status_flags().contains(StatusFlags::O_APPEND) {
+        let status_flags = self.status_flags();
+        if status_flags.contains(StatusFlags::O_APPEND) {
             // If the file has the O_APPEND flag, the offset is ignored
             offset = self.dentry.size();
         }
 
-        if self.status_flags().contains(StatusFlags::O_DIRECT) {
+        if status_flags.contains(StatusFlags::O_DIRECT) {
             self.dentry.inode().write_direct_at(offset, reader)
         } else {
             self.dentry.inode().write_at(offset, reader)