Fix using SockaddrStorage to store Unix domain addresses on Linux

Since it has variable length, the user of a sockaddr_un must keep track
of its true length.  On the BSDs, this is handled by the builtin sun_len
field.  But on Linux-like operating systems it isn't.  Fix this bug by
explicitly tracking it for SockaddrStorage just like we already do for
UnixAddr.

Fixes #1866

CHANGELOG.md

@@ -6,7 +6,9 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 ## [Unreleased] - ReleaseDate
 ### Added
 
-- Add `MntFlags` and `unmount` on all of the BSDs.
+- Added `SockaddrStorage::{as_unix_addr, as_unix_addr_mut}`
+  ([#1871](https://github.com/nix-rust/nix/pull/1871))
+- Added `MntFlags` and `unmount` on all of the BSDs.
   ([#1849](https://github.com/nix-rust/nix/pull/1849))
 - Added a 'Statfs::flags' method.
   ([#1849](https://github.com/nix-rust/nix/pull/1849))
@@ -38,6 +40,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Fixed
 
+- Fixed using `SockaddrStorage` to store a Unix-domain socket address on Linux.
+  ([#1871](https://github.com/nix-rust/nix/pull/1871))
 - Fix microsecond calculation for `TimeSpec`.
   ([#1801](https://github.com/nix-rust/nix/pull/1801))
 - Fix `User::from_name` and `Group::from_name` panicking

src/sys/socket/addr.rs

@@ -1537,6 +1537,17 @@ impl SockaddrLike for SockaddrStorage {
                 let mut ss: libc::sockaddr_storage = mem::zeroed();
                 let ssp = &mut ss as *mut libc::sockaddr_storage as *mut u8;
                 ptr::copy(addr as *const u8, ssp, len as usize);
+                #[cfg(any(
+                    target_os = "android",
+                    target_os = "fuchsia",
+                    target_os = "illumos",
+                    target_os = "linux"
+                ))]
+                if i32::from(ss.ss_family) == libc::AF_UNIX {
+                    // Safe because we UnixAddr is strictly smaller than
+                    // SockaddrStorage, and we just initialized the structure.
+                    (*(&mut ss as *mut libc::sockaddr_storage as *mut UnixAddr)).sun_len = len as u8;
+                }
                 Some(Self { ss })
             }
         } else {
@@ -1598,6 +1609,21 @@ impl SockaddrLike for SockaddrStorage {
             }
         }
     }
+
+    #[cfg(any(
+        target_os = "android",
+        target_os = "fuchsia",
+        target_os = "illumos",
+        target_os = "linux"
+    ))]
+    fn len(&self) -> libc::socklen_t {
+        match self.as_unix_addr() {
+            // The UnixAddr type knows its own length
+            Some(ua) => ua.len(),
+            // For all else, we're just a boring SockaddrStorage
+            None => mem::size_of_val(self) as libc::socklen_t
+        }
+    }
 }
 
 macro_rules! accessors {
@@ -1635,6 +1661,64 @@ macro_rules! accessors {
 }
 
 impl SockaddrStorage {
+    /// Downcast to an immutable `[UnixAddr]` reference.
+    pub fn as_unix_addr(&self) -> Option<&UnixAddr> {
+        cfg_if! {
+            if #[cfg(any(target_os = "android",
+                     target_os = "fuchsia",
+                     target_os = "illumos",
+                     target_os = "linux"
+                ))]
+            {
+                let p = unsafe{ &self.ss as *const libc::sockaddr_storage };
+                // Safe because UnixAddr is strictly smaller than
+                // sockaddr_storage, and we're fully initialized
+                let len = unsafe {
+                    (*(p as *const UnixAddr )).sun_len as usize
+                };
+            } else {
+                let len = self.len() as usize;
+            }
+        }
+        // Sanity checks
+        if self.family() != Some(AddressFamily::Unix) ||
+           len < offset_of!(libc::sockaddr_un, sun_path) ||
+           len > mem::size_of::<libc::sockaddr_un>() {
+            None
+        } else {
+            Some(unsafe{&self.su})
+        }
+    }
+
+    /// Downcast to a mutable `[UnixAddr]` reference.
+    pub fn as_unix_addr_mut(&mut self) -> Option<&mut UnixAddr> {
+        cfg_if! {
+            if #[cfg(any(target_os = "android",
+                     target_os = "fuchsia",
+                     target_os = "illumos",
+                     target_os = "linux"
+                ))]
+            {
+                let p = unsafe{ &self.ss as *const libc::sockaddr_storage };
+                // Safe because UnixAddr is strictly smaller than
+                // sockaddr_storage, and we're fully initialized
+                let len = unsafe {
+                    (*(p as *const UnixAddr )).sun_len as usize
+                };
+            } else {
+                let len = self.len() as usize;
+            }
+        }
+        // Sanity checks
+        if self.family() != Some(AddressFamily::Unix) ||
+           len < offset_of!(libc::sockaddr_un, sun_path) ||
+           len > mem::size_of::<libc::sockaddr_un>() {
+            None
+        } else {
+            Some(unsafe{&mut self.su})
+        }
+    }
+
     #[cfg(any(target_os = "android", target_os = "linux"))]
     accessors! {as_alg_addr, as_alg_addr_mut, AlgAddr,
     AddressFamily::Alg, libc::sockaddr_alg, alg}
@@ -3064,6 +3148,44 @@ mod tests {
         }
     }
 
+    mod sockaddr_storage {
+        use super::*;
+
+        #[test]
+        fn from_sockaddr_un_named() {
+            let ua = UnixAddr::new("/var/run/mysock").unwrap();
+            let ptr = ua.as_ptr() as *const libc::sockaddr;
+            let ss = unsafe {
+                SockaddrStorage::from_raw(ptr, Some(ua.len()))
+            }.unwrap();
+            assert_eq!(ss.len(), ua.len());
+        }
+
+        #[cfg(any(target_os = "android", target_os = "linux"))]
+        #[test]
+        fn from_sockaddr_un_abstract_named() {
+            let name = String::from("nix\0abstract\0test");
+            let ua = UnixAddr::new_abstract(name.as_bytes()).unwrap();
+            let ptr = ua.as_ptr() as *const libc::sockaddr;
+            let ss = unsafe {
+                SockaddrStorage::from_raw(ptr, Some(ua.len()))
+            }.unwrap();
+            assert_eq!(ss.len(), ua.len());
+        }
+
+        #[cfg(any(target_os = "android", target_os = "linux"))]
+        #[test]
+        fn from_sockaddr_un_abstract_unnamed() {
+            let empty = String::new();
+            let ua = UnixAddr::new_abstract(empty.as_bytes()).unwrap();
+            let ptr = ua.as_ptr() as *const libc::sockaddr;
+            let ss = unsafe {
+                SockaddrStorage::from_raw(ptr, Some(ua.len()))
+            }.unwrap();
+            assert_eq!(ss.len(), ua.len());
+        }
+    }
+
     mod unixaddr {
         use super::*;