util, pasta: Add do_clone() wrapper around __clone2() and clone()

Spotted in Debian's buildd logs: on ia64, clone(2) is not available:
the glibc wrapper is named __clone2() and it takes, additionally,
the size of the stack area passed by the caller.

Add a do_clone() wrapper handling the different cases, and also
taking care of pointing the child's stack in the middle of the
allocated area: on PA-RISC (hppa), handled by clone(), the stack
grows up, and on ia64 the stack grows down, but the register backing
store grows up -- and I think it might be actually used here.

Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

pasta.c

@@ -226,8 +226,8 @@ void pasta_start_ns(struct ctx *c, uid_t uid, gid_t gid,
 		arg.argv = sh_argv;
 	}
 
-	pasta_child_pid = clone(pasta_spawn_cmd,
-				ns_fn_stack + sizeof(ns_fn_stack) / 2,
+	pasta_child_pid = do_clone(pasta_spawn_cmd, ns_fn_stack,
+				   sizeof(ns_fn_stack),
 				   CLONE_NEWIPC | CLONE_NEWPID | CLONE_NEWNET |
 				   CLONE_NEWUTS,
 				   (void *)&arg);

util.c

@@ -482,3 +482,24 @@ int write_file(const char *path, const char *buf)
 	close(fd);
 	return len == 0 ? 0 : -1;
 }
+
+/**
+ * do_clone() - Wrapper of __clone2() for ia64, clone() for other architectures
+ * @fn:		Entry point for child
+ * @stack_area:	Stack area for child: we'll point callees to the middle of it
+ * @stack_size:	Total size of stack area, passed to callee divided by two
+ * @flags:	clone() system call flags
+ * @arg:	Argument to @fn
+ *
+ * Return: thread ID of child, -1 on failure
+ */
+int do_clone(int (*fn)(void *), char *stack_area, size_t stack_size, int flags,
+	     void *arg)
+{
+#ifdef __ia64__
+	return __clone2(fn, stack_area + stack_size / 2, stack_size / 2,
+			flags, arg);
+#else
+	return clone(fn, stack_area + stack_size / 2, flags, arg);
+#endif
+}

util.h

@@ -81,11 +81,13 @@
 	(((struct in_addr *)(a))->s_addr == ((struct in_addr *)b)->s_addr)
 
 #define NS_FN_STACK_SIZE	(RLIMIT_STACK_VAL * 1024 / 8)
+int do_clone(int (*fn)(void *), char *stack_area, size_t stack_size, int flags,
+	     void *arg);
 #define NS_CALL(fn, arg)						\
 	do {								\
 		char ns_fn_stack[NS_FN_STACK_SIZE];			\
 									\
-		clone((fn), ns_fn_stack + sizeof(ns_fn_stack) / 2,	\
+		do_clone((fn), ns_fn_stack, sizeof(ns_fn_stack),	\
 			 CLONE_VM | CLONE_VFORK | CLONE_FILES | SIGCHLD,\
 			 (void *)(arg));				\
 	} while (0)