Add types for inspecting control messages in Unix socket ancillary data

library/std/src/os/unix/net/cmsg.rs

@@ -0,0 +1,289 @@
+use crate::ffi::c_int;
+use crate::mem::{size_of, MaybeUninit};
+
+// Wrapper around `libc::CMSG_LEN` to safely decouple from OS-specific ints.
+//
+// https://github.com/rust-lang/libc/issues/3240
+#[inline]
+const fn CMSG_LEN(len: usize) -> usize {
+    let c_len = len & 0x7FFFFFFF;
+    let padding = (unsafe { libc::CMSG_LEN(c_len as _) } as usize) - c_len;
+    len + padding
+}
+
+// Wrapper around `libc::CMSG_SPACE` to safely decouple from OS-specific ints.
+//
+// https://github.com/rust-lang/libc/issues/3240
+#[inline]
+const fn CMSG_SPACE(len: usize) -> usize {
+    let c_len = len & 0x7FFFFFFF;
+    let padding = (unsafe { libc::CMSG_SPACE(c_len as _) } as usize) - c_len;
+    len + padding
+}
+
+/// A socket control message with borrowed data.
+///
+/// This type is semantically equivalent to POSIX `struct cmsghdr`, but is
+/// not guaranteed to have the same internal representation.
+#[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub struct ControlMessage<'a> {
+    cmsg_len: usize,
+    cmsg_level: c_int,
+    cmsg_type: c_int,
+    data: &'a [u8],
+}
+
+impl<'a> ControlMessage<'a> {
+    /// Creates a `ControlMessage` with the given level, type, and data.
+    ///
+    /// The semantics of a control message "level" and "type" are OS-specific,
+    /// but generally the level is a sort of general category of socket and the
+    /// type identifies a specific control message data layout.
+    #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+    pub fn new(cmsg_level: c_int, cmsg_type: c_int, data: &'a [u8]) -> ControlMessage<'a> {
+        let cmsg_len = CMSG_LEN(data.len());
+        ControlMessage { cmsg_len, cmsg_level, cmsg_type, data }
+    }
+}
+
+impl ControlMessage<'_> {
+    /// Returns the control message's level, an OS-specific value.
+    ///
+    /// POSIX describes this field as the "originating protocol".
+    #[inline]
+    #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+    pub fn cmsg_level(&self) -> c_int {
+        self.cmsg_level
+    }
+
+    /// Returns the control message's type, an OS-specific value.
+    ///
+    /// POSIX describes this field as the "protocol-specific type".
+    #[inline]
+    #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+    pub fn cmsg_type(&self) -> c_int {
+        self.cmsg_type
+    }
+
+    /// Returns the control message's type-specific data.
+    ///
+    /// The returned slice is equivalent to the result of C macro `CMSG_DATA()`.
+    /// Control message data is not guaranteed to be aligned, so code that needs
+    /// to inspect it should first copy the data to a properly-aligned location.
+    #[inline]
+    #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+    pub fn data(&self) -> &[u8] {
+        self.data
+    }
+
+    /// Returns `true` if the control message data is truncated.
+    ///
+    /// The kernel may truncate a control message if its data is too large to
+    /// fit into the capacity of the userspace buffer.
+    ///
+    /// The semantics of truncated control messages are OS- and type-specific.
+    #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+    pub fn truncated(&self) -> bool {
+        self.cmsg_len > CMSG_LEN(self.data.len())
+    }
+
+    #[inline]
+    pub(super) fn cmsg_space(&self) -> usize {
+        CMSG_SPACE(self.data.len())
+    }
+
+    #[allow(dead_code)] // currently the only use is in the test suite
+    pub(super) fn copy_to_slice<'a>(&self, dst: &'a mut [MaybeUninit<u8>]) -> &'a [u8] {
+        assert_eq!(dst.len(), self.cmsg_space());
+
+        // SAFETY: C type `struct cmsghdr` is safe to zero-initialize.
+        let mut hdr: libc::cmsghdr = unsafe { core::mem::zeroed() };
+
+        // Write `cmsg.cmsg_len` instead of `CMSG_LEN(data.len())` so that
+        // truncated control messages are preserved as-is.
+        hdr.cmsg_len = self.cmsg_len as _;
+        hdr.cmsg_level = self.cmsg_level;
+        hdr.cmsg_type = self.cmsg_type;
+
+        #[inline]
+        unsafe fn sized_to_slice<T: Sized>(t: &T) -> &[u8] {
+            let t_ptr = (t as *const T).cast::<u8>();
+            crate::slice::from_raw_parts(t_ptr, size_of::<T>())
+        }
+
+        let (hdr_dst, after_hdr) = dst.split_at_mut(size_of::<libc::cmsghdr>());
+        let (data_dst, padding_dst) = after_hdr.split_at_mut(self.data.len());
+
+        // SAFETY: C type `struct cmsghdr` is safe to bitwise-copy from.
+        MaybeUninit::write_slice(hdr_dst, unsafe { sized_to_slice(&hdr) });
+
+        // See comment in `ControlMessagesIter` regarding `CMSG_DATA()`.
+        MaybeUninit::write_slice(data_dst, self.data());
+
+        if padding_dst.len() > 0 {
+            for byte in padding_dst.iter_mut() {
+                byte.write(0);
+            }
+        }
+
+        // SAFETY: Every byte in `dst` has been initialized.
+        unsafe { MaybeUninit::slice_assume_init_ref(dst) }
+    }
+}
+
+/// A borrowed reference to a `&[u8]` slice containing control messages.
+///
+/// Note that this type does not guarantee the control messages are valid, or
+/// even well-formed. Code that uses control messages to implement (for example)
+/// access control or file descriptor passing should maintain a chain of custody
+/// to verify that the `&ControlMessages` came from a trusted source, such as
+/// a syscall.
+#[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+pub struct ControlMessages {
+    bytes: [u8],
+}
+
+impl ControlMessages {
+    /// Creates a `ControlMessages` wrapper from a `&[u8]` slice containing
+    /// encoded control messages.
+    ///
+    /// This method does not attempt to verify that the provided bytes represent
+    /// valid control messages.
+    #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+    pub fn from_bytes(bytes: &[u8]) -> &ControlMessages {
+        // SAFETY: casting `&[u8]` to `&ControlMessages` is safe because its
+        // internal representation is `[u8]`.
+        unsafe { &*(bytes as *const [u8] as *const ControlMessages) }
+    }
+
+    /// Returns a `&[u8]` slice containing encoded control messages.
+    #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+    pub fn as_bytes(&self) -> &[u8] {
+        &self.bytes
+    }
+
+    /// Returns `true` if `self.as_bytes()` is an empty slice.
+    #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+    pub fn is_empty(&self) -> bool {
+        self.bytes.is_empty()
+    }
+
+    /// Returns an iterator over the control messages.
+    #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+    pub fn iter(&self) -> ControlMessagesIter<'_> {
+        ControlMessagesIter { bytes: &self.bytes }
+    }
+}
+
+#[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+impl<'a> IntoIterator for &'a ControlMessages {
+    type Item = ControlMessage<'a>;
+    type IntoIter = ControlMessagesIter<'a>;
+
+    fn into_iter(self) -> ControlMessagesIter<'a> {
+        self.iter()
+    }
+}
+
+/// An iterator over the content of a [`ControlMessages`].
+///
+/// Each control message starts with a header describing its own length. This
+/// iterator is safe even if the header lengths are incorrect, but the returned
+/// control messages may contain incorrect data.
+///
+/// Iteration ends when the remaining data is smaller than the size of a single
+/// control message header.
+#[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+pub struct ControlMessagesIter<'a> {
+    bytes: &'a [u8],
+}
+
+impl<'a> ControlMessagesIter<'a> {
+    /// Returns a `&[u8]` slice containing any remaining data.
+    ///
+    /// Even if `next()` returns `None`, this method may return a non-empty
+    /// slice if the original `ControlMessages` was truncated in the middle
+    /// of a control message header.
+    #[inline]
+    #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+    pub fn into_bytes(self) -> &'a [u8] {
+        self.bytes
+    }
+}
+
+#[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+impl<'a> Iterator for ControlMessagesIter<'a> {
+    type Item = ControlMessage<'a>;
+
+    fn next(&mut self) -> Option<ControlMessage<'a>> {
+        const CMSGHDR_SIZE: usize = size_of::<libc::cmsghdr>();
+
+        if CMSGHDR_SIZE > self.bytes.len() {
+            return None;
+        }
+
+        // SAFETY: C type `struct cmsghdr` is safe to bitwise-copy from.
+        let hdr = unsafe {
+            let mut hdr = MaybeUninit::<libc::cmsghdr>::uninit();
+            hdr.as_mut_ptr().cast::<u8>().copy_from(self.bytes.as_ptr(), CMSGHDR_SIZE);
+            hdr.assume_init()
+        };
+
+        // `cmsg_bytes` contains the full content of the control message,
+        // which may have been truncated if there was insufficient capacity.
+        let cmsg_bytes;
+        let hdr_cmsg_len = hdr.cmsg_len as usize;
+        if hdr_cmsg_len >= self.bytes.len() {
+            cmsg_bytes = self.bytes;
+        } else {
+            cmsg_bytes = &self.bytes[..hdr_cmsg_len];
+        }
+
+        // `cmsg_data` is the portion of the control message that contains
+        // type-specific content (file descriptors, etc).
+        //
+        // POSIX specifies that a pointer to this data should be obtained with
+        // macro `CMSG_DATA()`, but its definition is problematic for Rust:
+        //
+        //   1. The macro may in principle read fields of `cmsghdr`. To avoid
+        //      unaligned reads this code would call it as `CMSG_DATA(&hdr)`.
+        //      But the resulting pointer would be relative to the stack value
+        //      `hdr`, not the actual message data contained in `cmsg_bytes`.
+        //
+        //   2. `CMSG_DATA()` is implemented with `pointer::offset()`, which
+        //      causes undefined behavior if its result is outside the original
+        //      allocated object. The POSIX spec allows control messages to
+        //      have padding between the header and data, in which case
+        //      `CMSG_DATA(&hdr)` is UB.
+        //
+        //   3. The control message may have been truncated. We know there's
+        //      at least `CMSGHDR_SIZE` bytes available, but anything past that
+        //      isn't guaranteed. Again, possible UB in the presence of padding.
+        //
+        // Therefore, this code obtains `cmsg_data` by assuming it directly
+        // follows the header (with no padding, and no header field dependency).
+        // This is true on all target OSes currently supported by Rust.
+        //
+        // If in the future support is added for an OS with cmsg data padding,
+        // then this implementation will cause unit test failures rather than
+        // risking silent UB.
+        let cmsg_data = &cmsg_bytes[CMSGHDR_SIZE..];
+
+        // `cmsg_space` is the length of the control message plus any padding
+        // necessary to align the next message.
+        let cmsg_space = CMSG_SPACE(cmsg_data.len());
+        if cmsg_space >= self.bytes.len() {
+            self.bytes = &[];
+        } else {
+            self.bytes = &self.bytes[cmsg_space..];
+        }
+
+        Some(ControlMessage {
+            cmsg_len: hdr_cmsg_len,
+            cmsg_level: hdr.cmsg_level,
+            cmsg_type: hdr.cmsg_type,
+            data: cmsg_data,
+        })
+    }
+}

library/std/src/os/unix/net/mod.rs

@@ -7,6 +7,7 @@ mod addr;
 #[doc(cfg(any(target_os = "android", target_os = "linux")))]
 #[cfg(any(doc, target_os = "android", target_os = "linux"))]
 mod ancillary;
+mod cmsg;
 mod datagram;
 mod listener;
 mod stream;
@@ -18,6 +19,8 @@ pub use self::addr::*;
 #[cfg(any(doc, target_os = "android", target_os = "linux"))]
 #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
 pub use self::ancillary::*;
+#[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+pub use self::cmsg::*;
 #[stable(feature = "unix_socket", since = "1.10.0")]
 pub use self::datagram::*;
 #[stable(feature = "unix_socket", since = "1.10.0")]