rightkit-daemon 0.3.0

//! OS-level peer authentication for the local transport.
//!
//! The directory mode (Unix) and the pipe DACL (Windows) already keep other
//! users out. This module is the second, independent check: after a connection
//! is accepted the kernel is asked who is on the other end, and anything that is
//! not the current user is dropped before a single byte is read.
//!
//! Fail closed: every function here returns `Ok` only when the peer was positively
//! identified as the current user. A platform that cannot answer returns
//! [`PeerError::Unsupported`]; it never falls back to "assume it is me".

use std::fmt;
use std::io;

/// Who the kernel says is on the other end of a connection.
#[derive(Debug, Clone, PartialEq, Eq, Default)]
#[non_exhaustive]
pub struct PeerIdentity {
    /// Unix: the peer's effective uid.
    pub uid: Option<u32>,
    /// Peer process id when the platform reports it (Linux, macOS, Windows; not the BSDs).
    pub pid: Option<u32>,
    /// Windows: the peer process token's user SID in `S-1-...` form.
    pub sid: Option<String>,
    /// macOS: the peer's audit token (`audit_token_t`, eight words: pid at index 5, pid version
    /// at index 7) as the kernel captured it when the connection was made (`LOCAL_PEERTOKEN`).
    /// The race-free handle for a code-signature check of the peer. `None` elsewhere, or when
    /// the kernel did not provide one.
    pub audit_token: Option<[u32; 8]>,
    /// The peer process's start marker as read when the connection was accepted (Unix: start
    /// time from the OS; Windows: creation time). Pins the pid to one process, so a pid reused
    /// by another process after the peer exited is told apart at the server-side executable
    /// check. `None` when it could not be read; that check then fails closed.
    pub started: Option<(u64, u64)>,
}

/// Why a peer was not accepted as the current user.
#[derive(Debug)]
#[non_exhaustive]
pub enum PeerError {
    /// This platform cannot authenticate the peer, so no peer is accepted.
    Unsupported(&'static str),
    /// The peer is a different user.
    Mismatch(PeerIdentity),
    /// The OS could not tell who the peer is (including a peer that already left).
    Query(io::Error),
}

impl fmt::Display for PeerError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            PeerError::Unsupported(why) => write!(f, "peer authentication unsupported: {why}"),
            PeerError::Mismatch(id) => {
                write!(f, "peer is not the current user")?;
                if let Some(uid) = id.uid {
                    write!(f, " (uid {uid})")?;
                }
                if let Some(pid) = id.pid {
                    write!(f, " (pid {pid})")?;
                }
                Ok(())
            }
            PeerError::Query(e) => write!(f, "could not identify peer: {e}"),
        }
    }
}

impl std::error::Error for PeerError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match self {
            PeerError::Query(e) => Some(e),
            _ => None,
        }
    }
}

impl From<PeerError> for io::Error {
    fn from(e: PeerError) -> Self {
        let kind = match &e {
            PeerError::Unsupported(_) => io::ErrorKind::Unsupported,
            PeerError::Mismatch(_) => io::ErrorKind::PermissionDenied,
            PeerError::Query(q) => q.kind(),
        };
        io::Error::new(kind, e.to_string())
    }
}

impl PeerError {
    /// The peer disconnected before it could be identified (a probe, or `wake`).
    /// Not an attack; the connection is still dropped, but no rejection is logged.
    pub(crate) fn peer_gone(&self) -> bool {
        let PeerError::Query(e) = self else {
            return false;
        };
        if matches!(
            e.kind(),
            io::ErrorKind::NotConnected
                | io::ErrorKind::BrokenPipe
                | io::ErrorKind::ConnectionReset
                | io::ErrorKind::UnexpectedEof
        ) {
            return true;
        }
        // ERROR_BROKEN_PIPE 109, ERROR_NO_DATA 232, ERROR_PIPE_NOT_CONNECTED 233.
        cfg!(windows) && matches!(e.raw_os_error(), Some(109 | 232 | 233))
    }
}

/// `D:P(A;;GA;;;<sid>)`: protected DACL (no inheritance), one ACE granting
/// GENERIC_ALL to exactly this SID.
#[cfg(any(windows, test))]
pub(crate) fn sddl_for_sid(sid: &str) -> String {
    format!("D:P(A;;GA;;;{sid})")
}

// ---------------------------------------------------------------------------
// Unix
// ---------------------------------------------------------------------------

#[cfg(unix)]
mod unix {
    use super::*;
    use std::os::fd::AsRawFd;
    use std::os::unix::net::UnixStream;

    extern "C" {
        fn geteuid() -> u32;
    }

    #[cfg(any(
        target_os = "macos",
        target_os = "ios",
        target_os = "freebsd",
        target_os = "openbsd",
        target_os = "netbsd",
        target_os = "dragonfly"
    ))]
    extern "C" {
        fn getpeereid(fd: i32, euid: *mut u32, egid: *mut u32) -> i32;
    }

    // Linux/Android generic ABI: SOL_SOCKET = 1, SO_PEERCRED = 17. The mips, powerpc
    // and sparc families number these differently and are deliberately left Unsupported
    // rather than guessed.
    #[cfg(all(
        any(target_os = "linux", target_os = "android"),
        not(any(
            target_arch = "mips",
            target_arch = "mips32r6",
            target_arch = "mips64",
            target_arch = "mips64r6",
            target_arch = "powerpc",
            target_arch = "powerpc64",
            target_arch = "sparc",
            target_arch = "sparc64"
        ))
    ))]
    mod linux {
        pub const SOL_SOCKET: i32 = 1;
        pub const SO_PEERCRED: i32 = 17;
        #[repr(C)]
        #[derive(Default)]
        pub struct Ucred {
            pub pid: i32,
            pub uid: u32,
            pub gid: u32,
        }
        extern "C" {
            pub fn getsockopt(
                fd: i32,
                level: i32,
                name: i32,
                val: *mut core::ffi::c_void,
                len: *mut u32,
            ) -> i32;
        }
    }

    pub(crate) fn current_euid() -> u32 {
        unsafe { geteuid() }
    }

    #[cfg(any(
        target_os = "macos",
        target_os = "ios",
        target_os = "freebsd",
        target_os = "openbsd",
        target_os = "netbsd",
        target_os = "dragonfly"
    ))]
    fn peer_identity(s: &UnixStream) -> Result<PeerIdentity, PeerError> {
        let (mut uid, mut gid) = (u32::MAX, u32::MAX);
        // SAFETY: valid fd borrowed from `s`; out-pointers are valid locals.
        let rc = unsafe { getpeereid(s.as_raw_fd(), &mut uid, &mut gid) };
        if rc != 0 {
            return Err(PeerError::Query(io::Error::last_os_error()));
        }
        Ok(PeerIdentity {
            uid: Some(uid),
            pid: peer_pid(s),
            sid: None,
            audit_token: crate::host_identity::peer_audit_token(s),
            started: peer_pid(s).and_then(crate::host_identity::process_start_marker),
        })
    }

    /// The peer's process id from the `LOCAL_PEERPID` socket option. `None` when the option
    /// fails: the peer is still authenticated by its uid, the pid is only extra detail.
    #[cfg(any(target_os = "macos", target_os = "ios"))]
    fn peer_pid(s: &UnixStream) -> Option<u32> {
        const SOL_LOCAL: i32 = 0;
        const LOCAL_PEERPID: i32 = 0x002;
        extern "C" {
            fn getsockopt(
                fd: i32,
                level: i32,
                name: i32,
                val: *mut core::ffi::c_void,
                len: *mut u32,
            ) -> i32;
        }
        let mut pid: i32 = 0;
        let mut len = std::mem::size_of::<i32>() as u32;
        // SAFETY: valid fd borrowed from `s`; `pid` and `len` are valid locals of the size
        // the option writes (`pid_t`, `socklen_t`).
        let rc = unsafe {
            getsockopt(
                s.as_raw_fd(),
                SOL_LOCAL,
                LOCAL_PEERPID,
                (&mut pid as *mut i32).cast(),
                &mut len,
            )
        };
        if rc != 0 {
            return None;
        }
        u32::try_from(pid).ok().filter(|p| *p != 0)
    }

    /// The BSDs have no peer-pid socket option of this shape.
    #[cfg(any(
        target_os = "freebsd",
        target_os = "openbsd",
        target_os = "netbsd",
        target_os = "dragonfly"
    ))]
    fn peer_pid(_s: &UnixStream) -> Option<u32> {
        None
    }

    #[cfg(all(
        any(target_os = "linux", target_os = "android"),
        not(any(
            target_arch = "mips",
            target_arch = "mips32r6",
            target_arch = "mips64",
            target_arch = "mips64r6",
            target_arch = "powerpc",
            target_arch = "powerpc64",
            target_arch = "sparc",
            target_arch = "sparc64"
        ))
    ))]
    fn peer_identity(s: &UnixStream) -> Result<PeerIdentity, PeerError> {
        let mut cred = linux::Ucred::default();
        let mut len = std::mem::size_of::<linux::Ucred>() as u32;
        // SAFETY: valid fd; `cred` and `len` describe a writable ucred.
        let rc = unsafe {
            linux::getsockopt(
                s.as_raw_fd(),
                linux::SOL_SOCKET,
                linux::SO_PEERCRED,
                &mut cred as *mut linux::Ucred as *mut core::ffi::c_void,
                &mut len,
            )
        };
        if rc != 0 {
            return Err(PeerError::Query(io::Error::last_os_error()));
        }
        if (len as usize) < std::mem::size_of::<linux::Ucred>() {
            return Err(PeerError::Query(io::Error::new(
                io::ErrorKind::InvalidData,
                "short SO_PEERCRED reply",
            )));
        }
        Ok(PeerIdentity {
            uid: Some(cred.uid),
            pid: u32::try_from(cred.pid).ok().filter(|p| *p != 0),
            sid: None,
            audit_token: None,
            started: u32::try_from(cred.pid)
                .ok()
                .filter(|p| *p != 0)
                .and_then(crate::host_identity::process_start_marker),
        })
    }

    #[cfg(not(any(
        target_os = "macos",
        target_os = "ios",
        target_os = "freebsd",
        target_os = "openbsd",
        target_os = "netbsd",
        target_os = "dragonfly",
        all(
            any(target_os = "linux", target_os = "android"),
            not(any(
                target_arch = "mips",
                target_arch = "mips32r6",
                target_arch = "mips64",
                target_arch = "mips64r6",
                target_arch = "powerpc",
                target_arch = "powerpc64",
                target_arch = "sparc",
                target_arch = "sparc64"
            ))
        )
    )))]
    fn peer_identity(_s: &UnixStream) -> Result<PeerIdentity, PeerError> {
        Err(PeerError::Unsupported(
            "no peer-credential query is implemented for this Unix platform",
        ))
    }

    /// The pure decision: the peer is accepted only when its uid is known and equals `me`.
    pub(crate) fn accept_uid(id: PeerIdentity, me: u32) -> Result<PeerIdentity, PeerError> {
        if id.uid == Some(me) {
            Ok(id)
        } else {
            Err(PeerError::Mismatch(id))
        }
    }

    /// Verify the peer of a connected Unix stream is the current user.
    ///
    /// macOS and the BSDs use `getpeereid`; Linux and Android use `SO_PEERCRED`. The
    /// peer's effective uid must equal this process's `geteuid`. Any other Unix
    /// returns [`PeerError::Unsupported`]: the answer is never `Ok` on a platform that
    /// cannot check.
    ///
    /// The credentials are those the kernel captured when the connection was made, so a
    /// peer that connects and then exits is still identified correctly.
    pub fn verify_peer_is_current_user(stream: &UnixStream) -> Result<PeerIdentity, PeerError> {
        accept_uid(peer_identity(stream)?, current_euid())
    }
}

#[cfg(unix)]
pub(crate) use unix::current_euid;
#[cfg(unix)]
pub use unix::verify_peer_is_current_user;

// ---------------------------------------------------------------------------
// Windows
// ---------------------------------------------------------------------------

#[cfg(windows)]
mod win {
    use super::*;
    use std::os::windows::io::RawHandle;
    use windows::core::PWSTR;
    use windows::Win32::Foundation::{CloseHandle, LocalFree, HANDLE, HLOCAL};
    use windows::Win32::Security::Authorization::{
        ConvertSidToStringSidW, ConvertStringSecurityDescriptorToSecurityDescriptorW,
        SDDL_REVISION_1,
    };
    use windows::Win32::Security::{
        GetTokenInformation, TokenUser, PSECURITY_DESCRIPTOR, PSID, SECURITY_ATTRIBUTES,
        TOKEN_QUERY, TOKEN_USER,
    };
    use windows::Win32::System::Pipes::{GetNamedPipeClientProcessId, GetNamedPipeServerProcessId};
    use windows::Win32::System::Threading::{
        GetCurrentProcess, OpenProcess, OpenProcessToken, PROCESS_QUERY_LIMITED_INFORMATION,
    };

    /// Win32 error as an `io::Error` that keeps the code, so `peer_gone` can see it.
    fn win_err(e: windows::core::Error) -> io::Error {
        io::Error::from_raw_os_error((e.code().0 as u32 & 0xFFFF) as i32)
    }

    struct Owned(HANDLE);
    impl Drop for Owned {
        fn drop(&mut self) {
            unsafe {
                let _ = CloseHandle(self.0);
            }
        }
    }

    fn sid_to_string(sid: PSID) -> io::Result<String> {
        unsafe {
            let mut p = PWSTR::null();
            ConvertSidToStringSidW(sid, &mut p).map_err(win_err)?;
            let s = p.to_string().map_err(io::Error::other);
            let _ = LocalFree(Some(HLOCAL(p.0 as *mut _)));
            s
        }
    }

    /// The user SID string of an access token opened with TOKEN_QUERY.
    fn token_user_sid(tok: HANDLE) -> io::Result<String> {
        unsafe {
            let mut need = 0u32;
            let _ = GetTokenInformation(tok, TokenUser, None, 0, &mut need);
            if need == 0 {
                return Err(io::Error::other("token user size query returned 0"));
            }
            // u64 storage keeps the TOKEN_USER header pointer-aligned.
            let mut buf = vec![0u64; (need as usize).div_ceil(8)];
            GetTokenInformation(
                tok,
                TokenUser,
                Some(buf.as_mut_ptr() as *mut _),
                (buf.len() * 8) as u32,
                &mut need,
            )
            .map_err(win_err)?;
            let user = &*(buf.as_ptr() as *const TOKEN_USER);
            sid_to_string(user.User.Sid)
        }
    }

    /// The SID string of the user running this process.
    pub(crate) fn current_user_sid_string() -> io::Result<String> {
        unsafe {
            let mut tok = HANDLE::default();
            OpenProcessToken(GetCurrentProcess(), TOKEN_QUERY, &mut tok).map_err(win_err)?;
            let tok = Owned(tok);
            token_user_sid(tok.0)
        }
    }

    /// The SID string of the user running process `pid`. Fails closed: a process that
    /// cannot be opened or queried yields an error, never a guess.
    fn process_user_sid_string(pid: u32) -> io::Result<String> {
        unsafe {
            let proc =
                Owned(OpenProcess(PROCESS_QUERY_LIMITED_INFORMATION, false, pid).map_err(win_err)?);
            let mut tok = HANDLE::default();
            OpenProcessToken(proc.0, TOKEN_QUERY, &mut tok).map_err(win_err)?;
            let tok = Owned(tok);
            token_user_sid(tok.0)
        }
    }

    /// Compare a peer process's user SID with ours.
    fn verify_pid(pid: u32) -> Result<PeerIdentity, PeerError> {
        if pid == 0 {
            return Err(PeerError::Query(io::Error::other("peer process id is 0")));
        }
        let me = current_user_sid_string().map_err(PeerError::Query)?;
        let theirs = process_user_sid_string(pid).map_err(PeerError::Query)?;
        let id = PeerIdentity {
            uid: None,
            pid: Some(pid),
            sid: Some(theirs.clone()),
            audit_token: None,
            started: crate::host_identity::process_start_marker(pid),
        };
        // SID strings are canonical (`S-1-5-21-...`), so equality of the strings is
        // equality of the SIDs.
        if theirs.eq_ignore_ascii_case(&me) {
            Ok(id)
        } else {
            Err(PeerError::Mismatch(id))
        }
    }

    /// Server side: verify that the client connected to `handle` (a server pipe instance
    /// on which `ConnectNamedPipe` has completed) runs as the current user.
    ///
    /// `GetNamedPipeClientProcessId`, then the process token's user SID against ours. Call
    /// it before reading any request byte. Fails closed: a client that cannot be opened or
    /// queried is rejected. The pid is the one the kernel recorded at connect time; the
    /// pipe DACL (see [`owner_only_security_attributes`]) is the primary boundary and this
    /// is the independent second check.
    ///
    /// `handle` must be a valid, connected named-pipe server handle for the duration of
    /// the call.
    pub fn verify_pipe_peer_is_current_user(handle: RawHandle) -> Result<PeerIdentity, PeerError> {
        let mut pid = 0u32;
        // SAFETY: the caller guarantees a valid pipe handle; `pid` is a valid out-pointer.
        unsafe { GetNamedPipeClientProcessId(HANDLE(handle), &mut pid) }
            .map_err(|e| PeerError::Query(win_err(e)))?;
        verify_pid(pid)
    }

    /// Client side: verify that the server behind `handle` (a pipe opened with
    /// `CreateFile`) runs as the current user, via `GetNamedPipeServerProcessId`. Catches
    /// a different user squatting on the pipe name. Open the pipe with
    /// `SECURITY_SQOS_PRESENT | SECURITY_IDENTIFICATION` as well, so the server can
    /// identify but never impersonate the caller.
    pub fn verify_pipe_server_is_current_user(
        handle: RawHandle,
    ) -> Result<PeerIdentity, PeerError> {
        let mut pid = 0u32;
        // SAFETY: as above, for a client pipe handle.
        unsafe { GetNamedPipeServerProcessId(HANDLE(handle), &mut pid) }
            .map_err(|e| PeerError::Query(win_err(e)))?;
        verify_pid(pid)
    }

    /// A security descriptor granting `GENERIC_ALL` to the current user and nobody else,
    /// plus a `SECURITY_ATTRIBUTES` that points at it.
    ///
    /// SDDL: `D:P(A;;GA;;;<current user SID>)`, built from the process token's real SID
    /// (never a literal such as `OW` or `BA`). The DACL is protected (`P`), so it inherits
    /// nothing, and `bInheritHandle` is false.
    ///
    /// # Lifetime
    ///
    /// The descriptor is `LocalAlloc`ed and freed when this guard drops. The pointer from
    /// [`as_ptr`](Self::as_ptr) is valid exactly as long as the guard is alive (the
    /// attributes live on the heap, so moving the guard does not invalidate it). Windows
    /// copies the descriptor into the kernel object inside `CreateNamedPipeW`,
    /// `CreateMutexW`, `CreateEventW` and similar calls, so a single create call needs the
    /// guard only until it returns. Keep the guard alive across every create call that
    /// uses it (for example each new pipe instance).
    pub struct OwnerOnlySecurityAttributes {
        attrs: Box<SECURITY_ATTRIBUTES>,
        sd: PSECURITY_DESCRIPTOR,
        sddl: String,
    }

    // SAFETY: the descriptor and attributes are written once at construction and only
    // read afterwards.
    unsafe impl Send for OwnerOnlySecurityAttributes {}
    unsafe impl Sync for OwnerOnlySecurityAttributes {}

    impl OwnerOnlySecurityAttributes {
        /// Pointer to the `SECURITY_ATTRIBUTES`, for the `lpSecurityAttributes` argument of
        /// `CreateNamedPipeW`, `CreateMutexW`, `CreateEventW`, ... Cast it to the
        /// `*const SECURITY_ATTRIBUTES` of whichever Windows binding crate you use. Valid
        /// while `self` is alive; null is never returned.
        pub fn as_ptr(&self) -> *const core::ffi::c_void {
            &*self.attrs as *const SECURITY_ATTRIBUTES as *const core::ffi::c_void
        }

        /// The SDDL string the descriptor was built from.
        pub fn sddl(&self) -> &str {
            &self.sddl
        }

        pub(crate) fn raw(&self) -> &SECURITY_ATTRIBUTES {
            &self.attrs
        }
    }

    impl Drop for OwnerOnlySecurityAttributes {
        fn drop(&mut self) {
            unsafe {
                let _ = LocalFree(Some(HLOCAL(self.sd.0)));
            }
        }
    }

    /// Build owner-only [`OwnerOnlySecurityAttributes`] for the current user. Usable for
    /// named pipes, mutexes and events.
    pub fn owner_only_security_attributes() -> io::Result<OwnerOnlySecurityAttributes> {
        let sddl = sddl_for_sid(&current_user_sid_string()?);
        let wide: Vec<u16> = sddl.encode_utf16().chain(std::iter::once(0)).collect();
        let mut sd = PSECURITY_DESCRIPTOR::default();
        unsafe {
            ConvertStringSecurityDescriptorToSecurityDescriptorW(
                windows::core::PCWSTR(wide.as_ptr()),
                SDDL_REVISION_1,
                &mut sd,
                None,
            )
            .map_err(win_err)?;
        }
        let attrs = Box::new(SECURITY_ATTRIBUTES {
            nLength: std::mem::size_of::<SECURITY_ATTRIBUTES>() as u32,
            lpSecurityDescriptor: sd.0,
            bInheritHandle: false.into(),
        });
        Ok(OwnerOnlySecurityAttributes { attrs, sd, sddl })
    }
}

#[cfg(all(windows, test))]
pub(crate) use win::current_user_sid_string;
#[cfg(windows)]
pub use win::{
    owner_only_security_attributes, verify_pipe_peer_is_current_user,
    verify_pipe_server_is_current_user, OwnerOnlySecurityAttributes,
};

/// The SID string (`S-1-...`) of the user running this process, read from the process
/// token. For callers building their own owner-only DACLs or comparing a peer's
/// [`PeerIdentity::sid`] against the current user outside a pipe check.
#[cfg(windows)]
pub fn current_user_sid() -> io::Result<String> {
    win::current_user_sid_string()
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn sddl_is_built_from_the_given_sid_with_a_protected_single_ace_dacl() {
        assert_eq!(
            sddl_for_sid("S-1-5-21-1-2-3-1001"),
            "D:P(A;;GA;;;S-1-5-21-1-2-3-1001)"
        );
    }

    #[test]
    fn peer_error_maps_to_typed_io_kinds() {
        let k = |e: PeerError| io::Error::from(e).kind();
        assert_eq!(k(PeerError::Unsupported("x")), io::ErrorKind::Unsupported);
        assert_eq!(
            k(PeerError::Mismatch(PeerIdentity::default())),
            io::ErrorKind::PermissionDenied
        );
    }

    #[test]
    fn a_vanished_peer_is_not_an_attack_but_a_mismatch_always_is() {
        assert!(PeerError::Query(io::Error::from(io::ErrorKind::NotConnected)).peer_gone());
        assert!(!PeerError::Query(io::Error::from(io::ErrorKind::PermissionDenied)).peer_gone());
        assert!(!PeerError::Mismatch(PeerIdentity::default()).peer_gone());
        assert!(!PeerError::Unsupported("x").peer_gone());
    }

    #[cfg(unix)]
    mod unix_tests {
        use super::*;
        use std::os::unix::net::UnixStream;

        #[test]
        fn same_user_socketpair_is_accepted_with_our_uid() {
            let (a, b) = UnixStream::pair().unwrap();
            for s in [&a, &b] {
                match verify_peer_is_current_user(s) {
                    Ok(id) => assert_eq!(id.uid, Some(current_euid())),
                    // A Unix with no credential query must refuse, never accept.
                    Err(PeerError::Unsupported(_)) => {}
                    Err(e) => panic!("same-user socketpair rejected: {e}"),
                }
            }
        }

        #[test]
        fn a_different_uid_is_a_mismatch_and_an_unknown_uid_fails_closed() {
            let me = current_euid();
            let other = PeerIdentity {
                uid: Some(me.wrapping_add(1)),
                ..Default::default()
            };
            assert!(matches!(
                super::super::unix::accept_uid(other, me),
                Err(PeerError::Mismatch(id)) if id.uid == Some(me.wrapping_add(1))
            ));
            assert!(matches!(
                super::super::unix::accept_uid(PeerIdentity::default(), me),
                Err(PeerError::Mismatch(_))
            ));
            assert!(super::super::unix::accept_uid(
                PeerIdentity {
                    uid: Some(me),
                    ..Default::default()
                },
                me
            )
            .is_ok());
        }

        // Cross-user (a real second account on the other end of a socket) cannot be
        // exercised without privileges: it needs root to `setuid` a helper, or a second
        // account. That branch is covered by the `accept_uid` test above, which feeds the
        // decision function a foreign uid. Run a manual check as root with:
        //   sudo -u nobody <test binary> --ignored cross_user
        #[test]
        #[ignore = "needs a second OS account; see comment"]
        fn cross_user_socket_is_rejected() {
            let path = std::env::var("RK_SERVICE_FOREIGN_SOCKET")
                .expect("set RK_SERVICE_FOREIGN_SOCKET to a socket served by another user");
            let s = UnixStream::connect(path).unwrap();
            assert!(matches!(
                verify_peer_is_current_user(&s),
                Err(PeerError::Mismatch(_))
            ));
        }
    }

    #[cfg(windows)]
    mod windows_tests {
        use super::*;

        #[test]
        fn the_public_sid_helper_reports_this_processs_user() {
            let sid = current_user_sid().unwrap();
            assert!(sid.starts_with("S-1-"), "{sid}");
            assert_eq!(sid, current_user_sid_string().unwrap());
        }

        #[test]
        fn owner_only_attributes_use_the_real_current_sid() {
            let sid = current_user_sid_string().unwrap();
            assert!(sid.starts_with("S-1-"), "{sid}");
            let a = owner_only_security_attributes().unwrap();
            assert_eq!(a.sddl(), format!("D:P(A;;GA;;;{sid})"));
            assert!(!a.as_ptr().is_null());
            assert!(!a.raw().lpSecurityDescriptor.is_null());
            assert!(!a.raw().bInheritHandle.as_bool());
        }
    }
}