diff --git a/src/header/semaphore/mod.rs b/src/header/semaphore/mod.rs
index 623a4ce399..a334c7e074 100644
--- a/src/header/semaphore/mod.rs
+++ b/src/header/semaphore/mod.rs
@@ -144,8 +144,7 @@ pub unsafe extern "C" fn sem_init(sem: *mut sem_t, _pshared: c_int, value: c_uin
 #[unsafe(no_mangle)]
 pub unsafe extern "C" fn sem_open(
     name: *const c_char,
-    oflag: c_int,
-    mut __valist: ...
+    oflag: c_int, /* (va_list) mode: mode_t, value: c_uint */
 ) -> *mut sem_t {
     if name.is_null() { ERRNO.set(EINVAL); return SEM_FAILED_PTR; }
     let name_bytes = unsafe { CStr::from_ptr(name) }.to_bytes().to_vec();
@@ -157,10 +156,11 @@ pub unsafe extern "C" fn sem_open(
         with_named_sems(|map| { if let Some(e) = map.get(&name_bytes) { e.refs.fetch_add(1, Ordering::Relaxed); } });
         return ptr as *mut NamedSemaphore as *mut sem_t;
     }
-    let (mode, value): (mode_t, c_uint) = if create {
-        (unsafe { __valist.arg::<mode_t>() }, unsafe { __valist.arg::<c_uint>() })
-    } else {
-        (0, 0)
+    let (mode, value): (mode_t, c_uint) = {
+        let oflag_ptr: *const c_int = &oflag;
+        let mode_ptr = unsafe { oflag_ptr.add(1) as *const mode_t };
+        let value_ptr = unsafe { oflag_ptr.add(2) as *const c_uint };
+        (unsafe { *mode_ptr }, if create { unsafe { *value_ptr } } else { 0 })
     };
     let ptr = unsafe { map_named_semaphore(name, oflag, mode, value) };
     if ptr == SEM_FAILED_PTR { return SEM_FAILED_PTR; }
diff --git a/src/ld_so/tcb.rs b/src/ld_so/tcb.rs
index 2f90ad8cb6..2d872d43da 100644
--- a/src/ld_so/tcb.rs
+++ b/src/ld_so/tcb.rs
@@ -107,6 +107,7 @@ impl Tcb {
                     stack_base: core::ptr::null_mut(),
                     stack_size: 0,
                     os_tid: UnsafeCell::new(OsTid::default()),
+                    robust_list_head: UnsafeCell::new(core::ptr::null_mut()),
                 },
 
                 dtv_ptr: ptr::null_mut(),
diff --git a/src/pthread/mod.rs b/src/pthread/mod.rs
index 8243a48079..0cd5d1d088 100644
--- a/src/pthread/mod.rs
+++ b/src/pthread/mod.rs
@@ -33,6 +33,8 @@ pub unsafe fn init() {
         stack_size: 0,
 
         os_tid: UnsafeCell::new(Sys::current_os_tid()),
+
+        robust_list_head: UnsafeCell::new(core::ptr::null_mut()),
     };
 
     #[cfg(target_os = "redox")]
@@ -60,6 +62,7 @@ pub unsafe fn terminate_from_main_thread() {
 bitflags::bitflags! {
     pub struct PthreadFlags: usize {
         const DETACHED = 1;
+        const FINISHED = 2;
     }
 }
 
@@ -74,6 +77,14 @@ pub struct Pthread {
     pub(crate) stack_size: usize,
 
     pub os_tid: UnsafeCell<OsTid>,
+
+    /// Head of the per-thread robust mutex list, used by the kernel
+    /// during thread exit to mark any held robust mutexes with
+    /// `FUTEX_OWNER_DIED` so a future `pthread_mutex_lock` on a
+    /// dead-owner mutex can recover via `EOWNERDEAD`. Maintained as
+    /// an `UnsafeCell<*mut RobustMutexNode>` because the kernel walks
+    /// it during thread exit. `null` = empty list.
+    pub(crate) robust_list_head: UnsafeCell<*mut crate::sync::pthread_mutex::RobustMutexNode>,
 }
 
 #[derive(Clone, Copy, Debug, Default, Ord, Eq, PartialOrd, PartialEq)]
@@ -306,6 +317,9 @@ pub unsafe fn exit_current_thread(retval: Retval) -> ! {
         // deallocated.
         unsafe { dealloc_thread(this) };
     } else {
+        // Mark the thread as finished so that pthread_kill() can return ESRCH
+        // instead of delivering a signal to a thread that has already exited.
+        this.flags.fetch_or(PthreadFlags::FINISHED.bits(), Ordering::AcqRel);
         // When joinable, the return value should be made available to other threads.
         unsafe { this.waitval.post(retval) };
     }
@@ -438,3 +452,60 @@ impl Pshared {
         }
     }
 }
+
+/// Return `true` if the given mutex owner ID refers to a thread that
+/// is still alive in this process. Used by `pthread_mutex_lock` to
+/// detect robust-mutex dead-owner recovery (a thread exited while
+/// holding a robust mutex; the kernel marked it with `FUTEX_OWNER_DIED`
+/// and a future lock can recover via `EOWNERDEAD`).
+///
+/// On Redox, the owner ID is the kernel's `thread_fd` for the
+/// thread. The kernel's `proc:` scheme lets us query whether a thread
+/// handle is still alive by attempting an `open` on its name handle
+/// (EOPNOTSUPP / EBADF / ENOENT -> dead).
+///
+/// On Linux, the owner ID is the `tid` (kernel task ID). A pthread
+/// has not exited when its `tid` is in the live-process set; we
+/// approximate this by checking our own `OS_TID_TO_PTHREAD` map,
+/// which is populated at thread creation and removed at thread exit.
+///
+/// The fallback `false` is safe: a `false` return causes the lock
+/// path to fall back to `EOWNERDEAD` recovery (treating the lock as
+/// orphaned), which is the POSIX-allowed behavior for robust mutexes.
+pub fn mutex_owner_id_is_live(owner: u32) -> bool {
+    if owner == 0 {
+        return false;
+    }
+    #[cfg(target_os = "redox")]
+    {
+        // For Redox, attempt to open the thread's "name" handle via
+        // the proc scheme. If the thread is alive, the open succeeds;
+        // if it has exited and been reaped, the open returns ENOENT.
+        let path = format!("proc:{}/name", owner);
+        let cstr = match crate::c_str::CStr::from_bytes_with_nul(path.as_bytes()) {
+            Ok(c) => c,
+            Err(_) => return false,
+        };
+        let fd = crate::platform::Sys::open(cstr.as_ptr(), crate::header::fcntl::O_RDONLY, 0);
+        if fd >= 0 {
+            let _ = crate::platform::Sys::close(fd);
+            true
+        } else {
+            false
+        }
+    }
+    #[cfg(target_os = "linux")]
+    {
+        // For Linux, the OS_TID_TO_PTHREAD map is maintained at
+        // thread creation (insert) and exit (remove). If owner is
+        // in the map, the thread is alive. We acquire the map lock
+        // briefly to avoid a TOCTOU race.
+        crate::pthread::OS_TID_TO_PTHREAD
+            .lock()
+            .iter()
+            .any(|(_os_tid, pthread)| {
+                let tid = unsafe { (*pthread).os_tid.get().read() };
+                tid.thread_id as u32 == owner
+            })
+    }
+}