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 v4:
- dropped __bpf_timer_compute_key()
- use a spin_lock instead of a semaphore
- ensure bpf_timer_cancel_and_free is not complaining about
non sleepable context and use cancel_work() instead of
cancel_work_sync()
- return -EINVAL if a delay is given to bpf_timer_start() with
BPF_F_TIMER_SLEEPABLE
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 | 86 ++++++++++++++++++++++++++++++++++++++++++++++--
2 files changed, 88 insertions(+), 2 deletions(-)
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 3c42b9f1bada..b90def29d796 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -7461,10 +7461,14 @@ 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 a89587859571..38de73a9df83 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1094,14 +1094,20 @@ 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 rcu_head rcu;
+ spinlock_t sleepable_lock;
};
/* the actual struct hidden inside uapi struct bpf_timer */
@@ -1114,6 +1120,49 @@ struct bpf_timer_kern {
struct bpf_spin_lock lock;
} __attribute__((aligned(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;
+ bpf_callback_t callback_fn;
+ void *value = t->value;
+ unsigned long flags;
+ void *key;
+ u32 idx;
+
+ BTF_TYPE_EMIT(struct bpf_timer);
+
+ spin_lock_irqsave(&t->sleepable_lock, flags);
+
+ callback_fn = READ_ONCE(t->callback_fn);
+ if (!callback_fn) {
+ spin_unlock_irqrestore(&t->sleepable_lock, flags);
+ return;
+ }
+
+ 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);
+ }
+
+ /* prevent the callback to be freed by bpf_timer_cancel() while running
+ * so we can release the sleepable lock
+ */
+ bpf_prog_inc(t->prog);
+
+ spin_unlock_irqrestore(&t->sleepable_lock, flags);
+
+ 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)
@@ -1192,6 +1241,8 @@ 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);
+ spin_lock_init(&t->sleepable_lock);
t->timer.function = bpf_timer_cb;
WRITE_ONCE(timer->timer, t);
/* Guarantee the order between timer->timer and map->usercnt. So
@@ -1237,6 +1288,7 @@ BPF_CALL_3(bpf_timer_set_callback, struct bpf_timer_kern *, timer, void *, callb
ret = -EINVAL;
goto out;
}
+ spin_lock(&t->sleepable_lock);
if (!atomic64_read(&t->map->usercnt)) {
/* maps with timers must be either held by user space
* or pinned in bpffs. Otherwise timer might still be
@@ -1263,6 +1315,8 @@ BPF_CALL_3(bpf_timer_set_callback, struct bpf_timer_kern *, timer, void *, callb
}
rcu_assign_pointer(t->callback_fn, callback_fn);
out:
+ if (t)
+ spin_unlock(&t->sleepable_lock);
__bpf_spin_unlock_irqrestore(&timer->lock);
return ret;
}
@@ -1283,8 +1337,12 @@ 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;
+
+ if ((flags & BPF_F_TIMER_SLEEPABLE) && nsecs)
+ return -EINVAL;
+
__bpf_spin_lock_irqsave(&timer->lock);
t = timer->timer;
if (!t || !t->prog) {
@@ -1300,7 +1358,10 @@ 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;
@@ -1348,13 +1409,22 @@ BPF_CALL_1(bpf_timer_cancel, struct bpf_timer_kern *, timer)
ret = -EDEADLK;
goto out;
}
+ spin_lock(&t->sleepable_lock);
drop_prog_refcnt(t);
+ spin_unlock(&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);
+
rcu_read_unlock();
return ret;
}
@@ -1383,11 +1453,13 @@ void bpf_timer_cancel_and_free(void *val)
t = timer->timer;
if (!t)
goto out;
+ spin_lock(&t->sleepable_lock);
drop_prog_refcnt(t);
/* The subsequent bpf_timer_start/cancel() helpers won't be able to use
* this timer, since it won't be initialized.
*/
WRITE_ONCE(timer->timer, NULL);
+ spin_unlock(&t->sleepable_lock);
out:
__bpf_spin_unlock_irqrestore(&timer->lock);
if (!t)
@@ -1410,6 +1482,16 @@ void bpf_timer_cancel_and_free(void *val)
*/
if (this_cpu_read(hrtimer_running) != t)
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. Same reason as above, it's fine to
+ * free 't': the subprog callback will never access it anymore
+ * and can not reschedule itself since timer->timer = NULL was
+ * already done.
+ */
+ cancel_work(&t->work);
+
kfree_rcu(t, rcu);
}
--
2.44.0
