[PATCH RFC bpf-next v3 04/16] bpf/helpers: introduce sleepable bpf_timers

[Benjamin Tissoires]

They are implemented as a workqueue, which means that there are no
guarantees of timing nor ordering.

Signed-off-by: Benjamin Tissoires <bentiss@xxxxxxxxxx>

---

changes in v3:
- extracted the implementation in bpf_timer only, without
  bpf_timer_set_sleepable_cb()
- rely on schedule_work() only, from bpf_timer_start()
- add semaphore to ensure bpf_timer_work_cb() is accessing
  consistent data

changes in v2 (compared to the one attaches to v1 0/9):
- make use of a kfunc
- add a (non-used) BPF_F_TIMER_SLEEPABLE
- the callback is *not* called, it makes the kernel crashes
---
 include/uapi/linux/bpf.h |  4 +++
 kernel/bpf/helpers.c     | 92 ++++++++++++++++++++++++++++++++++++++++--------
 2 files changed, 82 insertions(+), 14 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index d96708380e52..1fc7ecbd9d33 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -7421,10 +7421,14 @@ struct bpf_core_relo {
  *     - BPF_F_TIMER_ABS: Timeout passed is absolute time, by default it is
  *       relative to current time.
  *     - BPF_F_TIMER_CPU_PIN: Timer will be pinned to the CPU of the caller.
+ *     - BPF_F_TIMER_SLEEPABLE: Timer will run in a sleepable context, with
+ *       no guarantees of ordering nor timing (consider this as being just
+ *       offloaded immediately).
  */
 enum {
 	BPF_F_TIMER_ABS = (1ULL << 0),
 	BPF_F_TIMER_CPU_PIN = (1ULL << 1),
+	BPF_F_TIMER_SLEEPABLE = (1ULL << 2),
 };
 
 /* BPF numbers iterator state */
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 93edf730d288..f9add0abe40a 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -23,6 +23,7 @@
 #include <linux/btf_ids.h>
 #include <linux/bpf_mem_alloc.h>
 #include <linux/kasan.h>
+#include <linux/semaphore.h>
 
 #include "../../lib/kstrtox.h"
 
@@ -1094,13 +1095,19 @@ const struct bpf_func_proto bpf_snprintf_proto = {
  * bpf_timer_cancel() cancels the timer and decrements prog's refcnt.
  * Inner maps can contain bpf timers as well. ops->map_release_uref is
  * freeing the timers when inner map is replaced or deleted by user space.
+ *
+ * sleepable_lock protects only the setup of the workqueue, not the callback
+ * itself. This is done to ensure we don't run concurrently a free of the
+ * callback or the associated program.
  */
 struct bpf_hrtimer {
 	struct hrtimer timer;
+	struct work_struct work;
 	struct bpf_map *map;
 	struct bpf_prog *prog;
 	void __rcu *callback_fn;
 	void *value;
+	struct semaphore sleepable_lock;
 };
 
 /* the actual struct hidden inside uapi struct bpf_timer */
@@ -1113,6 +1120,55 @@ struct bpf_timer_kern {
 	struct bpf_spin_lock lock;
 } __attribute__((aligned(8)));
 
+static u32 __bpf_timer_compute_key(struct bpf_hrtimer *timer)
+{
+	struct bpf_map *map = timer->map;
+	void *value = timer->value;
+
+	if (map->map_type == BPF_MAP_TYPE_ARRAY) {
+		struct bpf_array *array = container_of(map, struct bpf_array, map);
+
+		/* compute the key */
+		return ((char *)value - array->value) / array->elem_size;
+	}
+
+	/* hash or lru */
+	return *(u32 *)(value - round_up(map->key_size, 8));
+}
+
+static void bpf_timer_work_cb(struct work_struct *work)
+{
+	struct bpf_hrtimer *t = container_of(work, struct bpf_hrtimer, work);
+	struct bpf_map *map = t->map;
+	void *value = t->value;
+	bpf_callback_t callback_fn;
+	u32 key;
+
+	BTF_TYPE_EMIT(struct bpf_timer);
+
+	down(&t->sleepable_lock);
+
+	callback_fn = READ_ONCE(t->callback_fn);
+	if (!callback_fn) {
+		up(&t->sleepable_lock);
+		return;
+	}
+
+	key = __bpf_timer_compute_key(t);
+
+	/* prevent the callback to be freed by bpf_timer_cancel() while running
+	 * so we can release the semaphore
+	 */
+	bpf_prog_inc(t->prog);
+
+	up(&t->sleepable_lock);
+
+	callback_fn((u64)(long)map, (u64)(long)&key, (u64)(long)value, 0, 0);
+	/* The verifier checked that return value is zero. */
+
+	bpf_prog_put(t->prog);
+}
+
 static DEFINE_PER_CPU(struct bpf_hrtimer *, hrtimer_running);
 
 static enum hrtimer_restart bpf_timer_cb(struct hrtimer *hrtimer)
@@ -1121,8 +1177,7 @@ static enum hrtimer_restart bpf_timer_cb(struct hrtimer *hrtimer)
 	struct bpf_map *map = t->map;
 	void *value = t->value;
 	bpf_callback_t callback_fn;
-	void *key;
-	u32 idx;
+	u32 key;
 
 	BTF_TYPE_EMIT(struct bpf_timer);
 	callback_fn = rcu_dereference_check(t->callback_fn, rcu_read_lock_bh_held());
@@ -1136,17 +1191,9 @@ static enum hrtimer_restart bpf_timer_cb(struct hrtimer *hrtimer)
 	 * bpf_map_delete_elem() on the same timer.
 	 */
 	this_cpu_write(hrtimer_running, t);
-	if (map->map_type == BPF_MAP_TYPE_ARRAY) {
-		struct bpf_array *array = container_of(map, struct bpf_array, map);
-
-		/* compute the key */
-		idx = ((char *)value - array->value) / array->elem_size;
-		key = &idx;
-	} else { /* hash or lru */
-		key = value - round_up(map->key_size, 8);
-	}
+	key = __bpf_timer_compute_key(t);
 
-	callback_fn((u64)(long)map, (u64)(long)key, (u64)(long)value, 0, 0);
+	callback_fn((u64)(long)map, (u64)(long)&key, (u64)(long)value, 0, 0);
 	/* The verifier checked that return value is zero. */
 
 	this_cpu_write(hrtimer_running, NULL);
@@ -1191,6 +1238,8 @@ BPF_CALL_3(bpf_timer_init, struct bpf_timer_kern *, timer, struct bpf_map *, map
 	t->prog = NULL;
 	rcu_assign_pointer(t->callback_fn, NULL);
 	hrtimer_init(&t->timer, clockid, HRTIMER_MODE_REL_SOFT);
+	INIT_WORK(&t->work, bpf_timer_work_cb);
+	sema_init(&t->sleepable_lock, 1);
 	t->timer.function = bpf_timer_cb;
 	WRITE_ONCE(timer->timer, t);
 	/* Guarantee the order between timer->timer and map->usercnt. So
@@ -1245,6 +1294,7 @@ BPF_CALL_3(bpf_timer_set_callback, struct bpf_timer_kern *, timer, void *, callb
 		ret = -EPERM;
 		goto out;
 	}
+	down(&t->sleepable_lock);
 	prev = t->prog;
 	if (prev != prog) {
 		/* Bump prog refcnt once. Every bpf_timer_set_callback()
@@ -1261,6 +1311,7 @@ BPF_CALL_3(bpf_timer_set_callback, struct bpf_timer_kern *, timer, void *, callb
 		t->prog = prog;
 	}
 	rcu_assign_pointer(t->callback_fn, callback_fn);
+	up(&t->sleepable_lock);
 out:
 	__bpf_spin_unlock_irqrestore(&timer->lock);
 	return ret;
@@ -1282,7 +1333,7 @@ BPF_CALL_3(bpf_timer_start, struct bpf_timer_kern *, timer, u64, nsecs, u64, fla
 
 	if (in_nmi())
 		return -EOPNOTSUPP;
-	if (flags & ~(BPF_F_TIMER_ABS | BPF_F_TIMER_CPU_PIN))
+	if (flags & ~(BPF_F_TIMER_ABS | BPF_F_TIMER_CPU_PIN | BPF_F_TIMER_SLEEPABLE))
 		return -EINVAL;
 	__bpf_spin_lock_irqsave(&timer->lock);
 	t = timer->timer;
@@ -1299,7 +1350,10 @@ BPF_CALL_3(bpf_timer_start, struct bpf_timer_kern *, timer, u64, nsecs, u64, fla
 	if (flags & BPF_F_TIMER_CPU_PIN)
 		mode |= HRTIMER_MODE_PINNED;
 
-	hrtimer_start(&t->timer, ns_to_ktime(nsecs), mode);
+	if (flags & BPF_F_TIMER_SLEEPABLE)
+		schedule_work(&t->work);
+	else
+		hrtimer_start(&t->timer, ns_to_ktime(nsecs), mode);
 out:
 	__bpf_spin_unlock_irqrestore(&timer->lock);
 	return ret;
@@ -1346,13 +1400,21 @@ BPF_CALL_1(bpf_timer_cancel, struct bpf_timer_kern *, timer)
 		ret = -EDEADLK;
 		goto out;
 	}
+	down(&t->sleepable_lock);
 	drop_prog_refcnt(t);
+	up(&t->sleepable_lock);
 out:
 	__bpf_spin_unlock_irqrestore(&timer->lock);
 	/* Cancel the timer and wait for associated callback to finish
 	 * if it was running.
 	 */
 	ret = ret ?: hrtimer_cancel(&t->timer);
+
+	/* also cancel the sleepable work, but *do not* wait for
+	 * it to finish if it was running as we might not be in a
+	 * sleepable context
+	 */
+	ret = ret ?: cancel_work(&t->work);
 	return ret;
 }
 
@@ -1407,6 +1469,8 @@ void bpf_timer_cancel_and_free(void *val)
 	 */
 	if (this_cpu_read(hrtimer_running) != t)
 		hrtimer_cancel(&t->timer);
+
+	cancel_work_sync(&t->work);
 	kfree(t);
 }
 

-- 
2.43.0