sys/socket: add UnixAddr abstract name getter

This introduces an `as_abstract()` getter to `UnixAddr` in order to
retrieve the name of an abstract unix socket.
This also adds tests around abstract addresses and clarify docs,
adding explicit semantics.

Author: lucab

CHANGELOG.md

@@ -36,6 +36,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#768](https:://github.com/nix-rust/nix/pull/768))
 - Added `nix::unistd::mkfifo`.
   ([#602](https://github.com/nix-rust/nix/pull/774))
+- Added `nix::sys::socket::UnixAddr::as_abstract` for Linux.
+  ([#785](https://github.com/nix-rust/nix/pull/785))
 
 ### Changed
 - Renamed existing `ptrace` wrappers to encourage namespacing ([#692](https://github.com/nix-rust/nix/pull/692))
@@ -64,6 +66,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Moved constants ptrace request, event and options to enums and updated ptrace functions and argument types accordingly. 
   ([#749](https://github.com/nix-rust/nix/pull/749))
 - `AioCb::Drop` will now panic if the `AioCb` is still in-progress ([#715](https://github.com/nix-rust/nix/pull/715))
+- Restricted `nix::sys::socket::UnixAddr::new_abstract` to Linux only.
+  ([#785](https://github.com/nix-rust/nix/pull/785))
+
 
 ### Fixed
 - Fix compilation and tests for OpenBSD targets

src/sys/socket/addr.rs

@@ -521,7 +521,9 @@ impl fmt::Display for Ipv6Addr {
  *
  */
 
-/// A wrapper around `sockaddr_un`. We track the length of `sun_path` (excluding
+/// A wrapper around `sockaddr_un`.
+///
+/// This also tracks the length of `sun_path` address (excluding
 /// a terminating null), because it may not be null-terminated.  For example,
 /// unconnected and Linux abstract sockets are never null-terminated, and POSIX
 /// does not require that `sun_len` include the terminating null even for normal
@@ -555,10 +557,13 @@ impl UnixAddr {
         }))
     }
 
-    /// Create a new sockaddr_un representing an address in the
-    /// "abstract namespace". This is a Linux-specific extension,
-    /// primarily used to allow chrooted processes to communicate with
-    /// specific daemons.
+    /// Create a new `sockaddr_un` representing an address in the "abstract namespace".
+    ///
+    /// The leading null byte for the abstract namespace is automatically added;
+    /// thus the input `path` is expected to be the bare name, not null-prefixed.
+    /// This is a Linux-specific extension, primarily used to allow chrooted
+    /// processes to communicate with processes having a different filesystem view.
+    #[cfg(target_os = "linux")]
     pub fn new_abstract(path: &[u8]) -> Result<UnixAddr> {
         unsafe {
             let mut ret = libc::sockaddr_un {
@@ -587,7 +592,7 @@ impl UnixAddr {
     /// If this address represents a filesystem path, return that path.
     pub fn path(&self) -> Option<&Path> {
         if self.1 == 0 || self.0.sun_path[0] == 0 {
-            // unbound or abstract
+            // unnamed or abstract
             None
         } else {
             let p = self.sun_path();
@@ -600,6 +605,20 @@ impl UnixAddr {
             Some(Path::new(<OsStr as OsStrExt>::from_bytes(&p[..reallen])))
         }
     }
+
+    /// If this address represents an abstract socket, return its name.
+    ///
+    /// For abstract sockets only the bare name is returned, without the
+    /// leading null byte. `None` is returned for unnamed or path-backed sockets.
+    #[cfg(target_os = "linux")]
+    pub fn as_abstract(&self) -> Option<&[u8]> {
+        if self.1 >= 1 && self.0.sun_path[0] == 0 {
+            Some(&self.sun_path()[1..])
+        } else {
+            // unnamed or filesystem path
+            None
+        }
+    }
 }
 
 impl PartialEq for UnixAddr {

test/sys/test_socket.rs

@@ -64,6 +64,28 @@ pub fn test_path_to_sock_addr() {
     assert_eq!(addr.path(), Some(actual));
 }
 
+// Test getting/setting abstract addresses (without unix socket creation)
+#[cfg(target_os = "linux")]
+#[test]
+pub fn test_abstract_uds_addr() {
+    let empty = String::new();
+    let addr = UnixAddr::new_abstract(empty.as_bytes()).unwrap();
+    assert_eq!(addr.as_abstract(), Some(empty.as_bytes()));
+
+    let name = String::from("nix\0abstract\0test");
+    let addr = UnixAddr::new_abstract(name.as_bytes()).unwrap();
+    assert_eq!(addr.as_abstract(), Some(name.as_bytes()));
+    assert_eq!(addr.path(), None);
+
+    // Internally, name is null-prefixed (abstract namespace)
+    let internal: &[u8] = unsafe {
+        slice::from_raw_parts(addr.0.sun_path.as_ptr() as *const u8, addr.1)
+    };
+    let mut abstract_name = name.clone();
+    abstract_name.insert(0, '\0');
+    assert_eq!(internal, abstract_name.as_bytes());
+}
+
 #[test]
 pub fn test_getsockname() {
     use nix::sys::socket::{socket, AddressFamily, SockType, SockFlag};