block_util: Align buffers for O_DIRECT

Whenever the backing file of our virtio-block device is opened with
O_DIRECT, there's a requirement about the buffer address and size to be
aligned to the sector size.

We know virtio-block requests are sector aligned in terms of size, but
we must still check if the buffer address is. In case it's not, we
create an intermediate buffer that will be passed through the system
call. In case of a write operation, the content of the non-aligned
buffer must be copied beforehand, and in case of a read operation, the
content of the aligned buffer must be copied to the non-aligned one
after the operation has been completed.

Fixes #3587

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

virtio-devices/src/block.rs

@@ -57,6 +57,8 @@ pub enum Error {
     RequestParsing(block_util::Error),
     /// Failed to execute the request.
     RequestExecuting(block_util::ExecuteError),
+    /// Failed to complete the request.
+    RequestCompleting(block_util::Error),
     /// Missing the expected entry in the list of requests.
     MissingEntryRequestList,
     /// The asynchronous request returned with failure.
@@ -188,10 +190,11 @@ impl BlockEpollHandler {
         let completion_list = self.disk_image.complete();
         for (user_data, result) in completion_list {
             let desc_index = user_data as u16;
-            let request = self
+            let mut request = self
                 .request_list
                 .remove(&desc_index)
                 .ok_or(Error::MissingEntryRequestList)?;
+            request.complete_async().map_err(Error::RequestCompleting)?;
 
             let (status, len) = if result >= 0 {
                 match request.request_type {