From ed9aabfa28b831c5e404847c7bdaacab63e9c2ae Mon Sep 17 00:00:00 2001
From: Maarten de Vries <maarten@de-vri.es>
Date: Wed, 15 Nov 2023 13:44:49 +0100
Subject: [PATCH] Ensure proper alignment of control message buffer in writer.

---
 CHANGELOG.md            |  3 +++
 src/ancillary/writer.rs | 17 +++++++++++++++++
 src/sys.rs              |  2 +-
 tests/ancillary_fds.rs  | 27 +++++++++++++++++++++++++++
 4 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 538cf6f..9f780dd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,6 @@
+Unreleased:
+  * Ensure proper alginment of control message buffer in the writer.
+
 v0.7.0 - 2023-03-03:
   * Fix `OwnedFileDescriptors` iteration.
 
diff --git a/src/ancillary/writer.rs b/src/ancillary/writer.rs
index 181d4ab..9174761 100644
--- a/src/ancillary/writer.rs
+++ b/src/ancillary/writer.rs
@@ -40,8 +40,14 @@ pub struct AncillaryMessageWriter<'a> {
 pub struct AddControlMessageError(());
 
 impl<'a> AncillaryMessageWriter<'a> {
+	/// Alignment requirement for the control messages added to the buffer.
+	pub const BUFFER_ALIGN: usize = std::mem::align_of::<libc::cmsghdr>();
+
 	/// Create an ancillary data with the given buffer.
 	///
+	/// Some bytes at the start of the buffer may be left unused to enforce alignment to [`Self::BUFFER_ALIGN`].
+	/// You can use [`Self::capacity()`] to check how much of the buffer can be used for control messages.
+	///
 	/// # Example
 	///
 	/// ```no_run
@@ -51,6 +57,7 @@ impl<'a> AncillaryMessageWriter<'a> {
 	/// let mut ancillary = AncillaryMessageWriter::new(&mut ancillary_buffer);
 	/// ```
 	pub fn new(buffer: &'a mut [u8]) -> Self {
+		let buffer = align_buffer_mut(buffer, Self::BUFFER_ALIGN);
 		Self { buffer, length: 0 }
 	}
 
@@ -227,3 +234,13 @@ fn reserve_ancillary_data<'a>(
 		Ok(std::slice::from_raw_parts_mut(data, additional_space))
 	}
 }
+
+/// Align a buffer to the given alignment.
+fn align_buffer_mut(buffer: &mut [u8], align: usize) -> &mut [u8] {
+	let offset = buffer.as_ptr().align_offset(align);
+	if offset > buffer.len() {
+		&mut []
+	} else {
+		&mut buffer[offset..]
+	}
+}
diff --git a/src/sys.rs b/src/sys.rs
index b827909..79c5e17 100644
--- a/src/sys.rs
+++ b/src/sys.rs
@@ -267,7 +267,7 @@ fn path_to_sockaddr(path: &Path) -> std::io::Result<(libc::sockaddr_un, usize)>
 
 /// Get the Unix path of a socket address.
 ///
-/// An error is retuend if the address is not a Unix address, or if it is an unnamed or abstract.
+/// An error is returned if the address is not a Unix address, or if it is an unnamed or abstract.
 fn sockaddr_to_path(address: &libc::sockaddr_un, len: libc::socklen_t) -> std::io::Result<&std::path::Path> {
 	use std::ffi::OsStr;
 	use std::os::unix::ffi::OsStrExt;
diff --git a/tests/ancillary_fds.rs b/tests/ancillary_fds.rs
index f6f2b32..e5a99f4 100644
--- a/tests/ancillary_fds.rs
+++ b/tests/ancillary_fds.rs
@@ -48,3 +48,30 @@ async fn can_take_ownership_of_received_fds() {
 	assert!(let Ok(_) = file.read_to_end(&mut contents));
 	assert!(contents == b"Wie dit leest is gek.");
 }
+
+#[tokio::test]
+async fn pass_fd_unaligned_buffer() {
+	use tokio_seqpacket::ancillary::AncillaryMessageWriter;
+
+	// Receive a file descriptor
+	let mut ancillary_buffer = [0; 64];
+	// But use a purposefully misaligned ancillary buffer.
+	let align = ancillary_buffer.as_ptr().align_offset(AncillaryMessageWriter::BUFFER_ALIGN);
+	let ancillary = receive_file_descriptor(&mut ancillary_buffer[align + 1..]).await;
+
+	// Check that we got exactly one control message containing file descriptors.
+	let mut messages = ancillary.messages();
+	let_assert!(Some(AncillaryMessage::FileDescriptors(mut fds)) = messages.next());
+	assert!(let None = messages.next());
+
+	// Check that we got exactly one file descriptor in the first control message.
+	let_assert!(Some(fd) = fds.next());
+	assert!(let None = fds.next());
+
+	// Check that we can retrieve the message from the attached file.
+	let_assert!(Ok(fd) = fd.try_clone_to_owned());
+	let mut file = std::fs::File::from(fd);
+	let mut contents = Vec::new();
+	assert!(let Ok(_) = file.read_to_end(&mut contents));
+	assert!(contents == b"Wie dit leest is gek.");
+}