Updated to 2.5.49, which involved fixing the calls to do_fork.

arch/um/kernel/process_kern.c

@@ -190,7 +190,7 @@ int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
 
 	current->thread.request.u.thread.proc = fn;
 	current->thread.request.u.thread.arg = arg;
-	p = do_fork(CLONE_VM | flags, 0, NULL, 0, NULL);
+	p = do_fork(CLONE_VM | flags, 0, NULL, 0, NULL, NULL);
 	if(IS_ERR(p)) panic("do_fork failed in kernel_thread");
 	return(p->pid);
 }

arch/um/kernel/syscall_kern.c

@@ -33,7 +33,7 @@ long sys_fork(void)
 	struct task_struct *p;
 
 	current->thread.forking = 1;
-        p = do_fork(SIGCHLD, 0, NULL, 0, NULL);
+        p = do_fork(SIGCHLD, 0, NULL, 0, NULL, NULL);
 	current->thread.forking = 0;
 	return(IS_ERR(p) ? PTR_ERR(p) : p->pid);
 }
@@ -43,7 +43,7 @@ long sys_clone(unsigned long clone_flags, unsigned long newsp)
 	struct task_struct *p;
 
 	current->thread.forking = 1;
-	p = do_fork(clone_flags, newsp, NULL, 0, NULL);
+	p = do_fork(clone_flags, newsp, NULL, 0, NULL, NULL);
 	current->thread.forking = 0;
 	return(IS_ERR(p) ? PTR_ERR(p) : p->pid);
 }
@@ -53,7 +53,7 @@ long sys_vfork(void)
 	struct task_struct *p;
 
 	current->thread.forking = 1;
-	p = do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, 0, NULL, 0, NULL);
+	p = do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, 0, NULL, 0, NULL, NULL);
 	current->thread.forking = 0;
 	return(IS_ERR(p) ? PTR_ERR(p) : p->pid);
 }