diff --git a/redox-rt/src/arch/x86_64.rs b/redox-rt/src/arch/x86_64.rs
index cade85a186f1ca0ae78ccd08e1ce0a59779a1afa..6c12a9642f69535c27fb35878b05670782a2b538 100644
--- a/redox-rt/src/arch/x86_64.rs
+++ b/redox-rt/src/arch/x86_64.rs
@@ -78,7 +78,11 @@ asmfunction!(__relibc_internal_fork_wrapper -> usize: ["
 
     mov rdi, rsp
     call __relibc_internal_fork_impl
-    jmp 2f
+
+    add rsp, 80
+
+    pop rbp
+    ret
 
 "] <= []);
 asmfunction!(__relibc_internal_fork_ret: ["
@@ -91,8 +95,6 @@ asmfunction!(__relibc_internal_fork_ret: ["
 
     xor rax, rax
 
-    .p2align 4
-2:
     add rsp, 32
     pop r15
     pop r14
diff --git a/redox-rt/src/signal.rs b/redox-rt/src/signal.rs
index e45cfa4c4c84c30699de8ad7eeacfae5f348663c..03da284d261ab277e644e47e9240b8d032e451ef 100644
--- a/redox-rt/src/signal.rs
+++ b/redox-rt/src/signal.rs
@@ -129,16 +129,17 @@ pub fn sigaction(signal: u8, new: Option<&Sigaction>, old: Option<&mut Sigaction
 
     let _sigguard = tmp_disable_signals();
     let ctl = current_sigctl();
-    let guard = SIGACTIONS.lock();
+    let mut guard = SIGACTIONS.lock();
     let old_ignmask = IGNMASK.load(Ordering::Relaxed);
 
     if let Some(old) = old {
-        // TODO
+        *old = guard[usize::from(signal)];
     }
 
     let Some(new) = new else {
         return Ok(());
     };
+    guard[usize::from(signal)] = *new;
 
     let sig_group = usize::from(signal) / 32;
     let sig_bit32 = 1 << ((signal - 1) % 32);
diff --git a/redox-rt/src/sync.rs b/redox-rt/src/sync.rs
index 713077842987f9925d39d0f94d2c5aee6373c65f..172bd37b1f1c0d49af5e8b653387ce32f6b879ac 100644
--- a/redox-rt/src/sync.rs
+++ b/redox-rt/src/sync.rs
@@ -2,7 +2,7 @@
 
 use core::cell::UnsafeCell;
 use core::ops::{Deref, DerefMut};
-use core::sync::atomic::AtomicU32;
+use core::sync::atomic::{AtomicU32, Ordering};
 
 pub struct Mutex<T> {
     pub lockword: AtomicU32,
@@ -24,6 +24,9 @@ impl<T> Mutex<T> {
         }
     }
     pub fn lock(&self) -> MutexGuard<'_, T> {
+        while self.lockword.compare_exchange(UNLOCKED, LOCKED, Ordering::Acquire, Ordering::Relaxed).is_err() {
+            core::hint::spin_loop();
+        }
         MutexGuard { lock: self }
     }
 }
@@ -44,5 +47,6 @@ impl<T> DerefMut for MutexGuard<'_, T> {
 }
 impl<T> Drop for MutexGuard<'_, T> {
     fn drop(&mut self) {
+        self.lock.lockword.store(UNLOCKED, Ordering::Release);
     }
 }
diff --git a/src/ld_so/tcb.rs b/src/ld_so/tcb.rs
index ae1b9eaf4c025ed2484c6cbe0a064cc95f866521..a7c0457b25b5d0cd2bcca0b032b7311f19b166f2 100644
--- a/src/ld_so/tcb.rs
+++ b/src/ld_so/tcb.rs
@@ -56,6 +56,13 @@ pub struct Tcb {
     pub pthread: Pthread,
 }
 
+#[cfg(target_os = "redox")]
+const _: () = {
+    if mem::size_of::<Tcb>() > syscall::PAGE_SIZE {
+        panic!("too large TCB!");
+    }
+};
+
 impl Tcb {
     /// Create a new TCB
     pub unsafe fn new(size: usize) -> Result<&'static mut Self> {
@@ -223,7 +230,7 @@ impl Tcb {
         syscall!(ARCH_PRCTL, ARCH_SET_FS, tls_end);
     }
 
-    #[cfg(all(target_os = "redox"))]
+    #[cfg(target_os = "redox")]
     unsafe fn os_arch_activate(tls_end: usize, tls_len: usize) {
         redox_rt::tcb_activate(tls_end, tls_len)
     }