msg_socket: add specialized error for zero sized recv

Receiving no data can be considered a special case for MsgOnSocket. It
doesn't necessarily indicate EOF because usually messages are sent over
a datagram channel and some fixed sized messages are actually zero
sized. This change handles this by adding a specific result for
receiving no data when at least some was expected. This will also
simplify some error handling that had to unpack the `BadRecvSize`
variant.

TEST=cargo test -p msg_socket
BUG=None

Change-Id: I16feeabc1481de93a2b189989d095434b5b417b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2079178
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Zach Reizner <zachr@chromium.org>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Auto-Submit: Zach Reizner <zachr@chromium.org>

2 files changed, 17 insertions, 0 deletions

diff --git a/msg_socket/src/lib.rs b/msg_socket/src/lib.rs
index 5871735..c674f37 100644
--- a/msg_socket/src/lib.rs
+++ b/msg_socket/src/lib.rs
@@ -164,6 +164,20 @@ pub trait MsgReceiver: AsRef<UnixSeqpacket> {
                 )
             }
         };
+
+        if msg_buffer.len() == 0 && Self::M::fixed_size() != Some(0) {
+            return Err(MsgError::RecvZero);
+        }
+
+        if let Some(fixed_size) = Self::M::fixed_size() {
+            if fixed_size != msg_buffer.len() {
+                return Err(MsgError::BadRecvSize {
+                    expected: fixed_size,
+                    actual: msg_buffer.len(),
+                });
+            }
+        }
+
         // Safe because fd buffer is read from socket.
         let (v, read_fd_size) = unsafe { Self::M::read_from_buffer(&msg_buffer, &fd_buffer)? };
         if fd_buffer.len() != read_fd_size {
diff --git a/msg_socket/src/msg_on_socket.rs b/msg_socket/src/msg_on_socket.rs
index 3f34019..b925630 100644
--- a/msg_socket/src/msg_on_socket.rs
+++ b/msg_socket/src/msg_on_socket.rs
@@ -28,6 +28,8 @@ pub enum MsgError {
     /// There was not the expected amount of data when receiving a message. The inner
     /// value is how much data is expected and how much data was actually received.
     BadRecvSize { expected: usize, actual: usize },
+    /// There was no data received when the socket `recv`-ed.
+    RecvZero,
     /// There was no associated file descriptor received for a request that expected it.
     ExpectFd,
     /// There was some associated file descriptor received but not used when deserialize.
@@ -58,6 +60,7 @@ impl Display for MsgError {
                 "wrong amount of data received; expected {} bytes; got {} bytes",
                 expected, actual
             ),
+            RecvZero => write!(f, "received zero data"),
             ExpectFd => write!(f, "missing associated file descriptor for request"),
             NotExpectFd => write!(f, "unexpected file descriptor is unused"),
             SettingFdFlags(e) => write!(f, "failed setting flags on the message FD: {}", e),