sched: Introduce struct balance_callback to avoid CFI mismatches

Introduce distinct struct balance_callback instead of performing function
pointer casting which will trip CFI. Avoids warnings as found by Clang's
future -Wcast-function-type-strict option:

In file included from kernel/sched/core.c:84:
kernel/sched/sched.h:1755:15: warning: cast from 'void (*)(struct rq *)' to 'void (*)(struct callback_head *)' converts to incompatible function type [-Wcast-function-type-strict]
head->func = (void (*)(struct callback_head *))func;
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

No binary differences result from this change.

This patch is a cleanup based on Brad Spengler/PaX Team's modifications
to sched code in their last public patch of grsecurity/PaX based on my
understanding of the code. Changes or omissions from the original code
are mine and don't reflect the original grsecurity/PaX code.

Reported-by: Sami Tolvanen <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Reviewed-by: Nathan Chancellor <[e-mail]>
Link: https://github.com/ClangBuiltLinux/linux/issues/1724
Link: https://lkml.kernel.org/r/[e-mail]

kernel/sched/core.c

···
 
 #ifdef CONFIG_SMP
 
-static void do_balance_callbacks(struct rq *rq, struct callback_head *head)
+static void do_balance_callbacks(struct rq *rq, struct balance_callback *head)
 {
 	void (*func)(struct rq *rq);
-	struct callback_head *next;
+	struct balance_callback *next;
 
 	lockdep_assert_rq_held(rq);
 
···
  * This abuse is tolerated because it places all the unlikely/odd cases behind
  * a single test, namely: rq->balance_callback == NULL.
  */
-struct callback_head balance_push_callback = {
+struct balance_callback balance_push_callback = {
 	.next = NULL,
-	.func = (void (*)(struct callback_head *))balance_push,
+	.func = balance_push,
 };
 
-static inline struct callback_head *
+static inline struct balance_callback *
 __splice_balance_callbacks(struct rq *rq, bool split)
 {
-	struct callback_head *head = rq->balance_callback;
+	struct balance_callback *head = rq->balance_callback;
 
 	if (likely(!head))
 		return NULL;
···
 	return head;
 }
 
-static inline struct callback_head *splice_balance_callbacks(struct rq *rq)
+static inline struct balance_callback *splice_balance_callbacks(struct rq *rq)
 {
 	return __splice_balance_callbacks(rq, true);
 }
···
 	do_balance_callbacks(rq, __splice_balance_callbacks(rq, false));
 }
 
-static inline void balance_callbacks(struct rq *rq, struct callback_head *head)
+static inline void balance_callbacks(struct rq *rq, struct balance_callback *head)
 {
 	unsigned long flags;
 
···
 {
 }
 
-static inline struct callback_head *splice_balance_callbacks(struct rq *rq)
+static inline struct balance_callback *splice_balance_callbacks(struct rq *rq)
 {
 	return NULL;
 }
 
-static inline void balance_callbacks(struct rq *rq, struct callback_head *head)
+static inline void balance_callbacks(struct rq *rq, struct balance_callback *head)
 {
 }
 
···
 	preempt_enable();
 }
 
-static DEFINE_PER_CPU(struct callback_head, core_balance_head);
+static DEFINE_PER_CPU(struct balance_callback, core_balance_head);
 
 static void queue_core_balance(struct rq *rq)
 {
···
 	int oldpolicy = -1, policy = attr->sched_policy;
 	int retval, oldprio, newprio, queued, running;
 	const struct sched_class *prev_class;
-	struct callback_head *head;
+	struct balance_callback *head;
 	struct rq_flags rf;
 	int reset_on_fork;
 	int queue_flags = DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK;

kernel/sched/deadline.c

···
 	return rq->online && dl_task(prev);
 }
 
-static DEFINE_PER_CPU(struct callback_head, dl_push_head);
-static DEFINE_PER_CPU(struct callback_head, dl_pull_head);
+static DEFINE_PER_CPU(struct balance_callback, dl_push_head);
+static DEFINE_PER_CPU(struct balance_callback, dl_pull_head);
 
 static void push_dl_tasks(struct rq *);
 static void pull_dl_task(struct rq *);

kernel/sched/rt.c

···
 	return !plist_head_empty(&rq->rt.pushable_tasks);
 }
 
-static DEFINE_PER_CPU(struct callback_head, rt_push_head);
-static DEFINE_PER_CPU(struct callback_head, rt_pull_head);
+static DEFINE_PER_CPU(struct balance_callback, rt_push_head);
+static DEFINE_PER_CPU(struct balance_callback, rt_pull_head);
 
 static void push_rt_tasks(struct rq *);
 static void pull_rt_task(struct rq *);

kernel/sched/sched.h

···
 DECLARE_STATIC_KEY_FALSE(sched_uclamp_used);
 #endif /* CONFIG_UCLAMP_TASK */
 
+struct rq;
+struct balance_callback {
+	struct balance_callback *next;
+	void (*func)(struct rq *rq);
+};
+
 /*
  * This is the main, per-CPU runqueue data structure.
  *
···
 	unsigned long		cpu_capacity;
 	unsigned long		cpu_capacity_orig;
 
-	struct callback_head	*balance_callback;
+	struct balance_callback *balance_callback;
 
 	unsigned char		nohz_idle_balance;
 	unsigned char		idle_balance;
···
 #endif
 };
 
-extern struct callback_head balance_push_callback;
+extern struct balance_callback balance_push_callback;
 
 /*
  * Lockdep annotation that avoids accidental unlocks; it's like a
···
 
 static inline void
 queue_balance_callback(struct rq *rq,
-		       struct callback_head *head,
+		       struct balance_callback *head,
 		       void (*func)(struct rq *rq))
 {
 	lockdep_assert_rq_held(rq);
···
 	if (unlikely(head->next || rq->balance_callback == &balance_push_callback))
 		return;
 
-	head->func = (void (*)(struct callback_head *))func;
+	head->func = func;
 	head->next = rq->balance_callback;
 	rq->balance_callback = head;
 }