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
