Make clone unsafe

There are many features of `clone` that may cause memory unsafety when
called. This documents one of them and references `fork()`, which is
already unsafe to call.

src/sched.rs

@@ -95,7 +95,17 @@ mod sched_linux_like {
     /// address need not be the highest address of the region.  Nix will take
     /// care of that requirement.  The user only needs to provide a reference to
     /// a normally allocated buffer.
-    pub fn clone(
+    ///
+    /// # Safety
+    ///
+    /// Because `clone` creates a child process with its stack located in
+    /// `stack` without specifying the size of the stack, special care must be
+    /// taken to ensure that the child process does not overflow the provided
+    /// stack space.
+    ///
+    /// See [`fork`](crate::unistd::fork) for additional safety concerns related
+    /// to executing child processes.
+    pub unsafe fn clone(
         mut cb: CloneCb,
         stack: &mut [u8],
         flags: CloneFlags,
@@ -106,20 +116,18 @@ mod sched_linux_like {
             (*cb)() as c_int
         }
 
-        let res = unsafe {
-            let combined = flags.bits() | signal.unwrap_or(0);
-            let ptr = stack.as_mut_ptr().add(stack.len());
-            let ptr_aligned = ptr.sub(ptr as usize % 16);
-            libc::clone(
-                mem::transmute(
-                    callback
-                        as extern "C" fn(*mut Box<dyn FnMut() -> isize>) -> i32,
-                ),
-                ptr_aligned as *mut c_void,
-                combined,
-                &mut cb as *mut _ as *mut c_void,
-            )
-        };
+        let combined = flags.bits() | signal.unwrap_or(0);
+        let ptr = stack.as_mut_ptr().add(stack.len());
+        let ptr_aligned = ptr.sub(ptr as usize % 16);
+        let res = libc::clone(
+            mem::transmute(
+                callback
+                    as extern "C" fn(*mut Box<dyn FnMut() -> isize>) -> i32,
+            ),
+            ptr_aligned as *mut c_void,
+            combined,
+            &mut cb as *mut _ as *mut c_void,
+        );
 
         Errno::result(res).map(Pid::from_raw)
     }