Re: [PATCH bpf-next 11/18] bpf: wq: add bpf_wq_init

On Tue, Apr 16, 2024 at 04:08:24PM +0200, Benjamin Tissoires wrote:

We need to teach the verifier about the second argument which is declared as void * but which is of type KF_ARG_PTR_TO_MAP. We could have dropped this extra case if we declared the second argument as struct bpf_map *, but that means users will have to do extra casting to have their program compile.

We also need to duplicate the timer code for the checking if the map argument is matching the provided workqueue.

Signed-off-by: Benjamin Tissoires bentiss@kernel.org

---

Note that the timer code when matching for the map is checking for constant map pointers. I wonder if this needs to be enforced too (being constant?)

---

include/uapi/linux/bpf.h | 9 ++++ kernel/bpf/helpers.c | 114 ++++++++++++++++++++++++++++++++++++++++++++++- kernel/bpf/verifier.c | 6 +++ 3 files changed, 127 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h index e4ae83550fb3..519f6019d158 100644 --- a/include/uapi/linux/bpf.h +++ b/include/uapi/linux/bpf.h @@ -7502,4 +7502,13 @@ struct bpf_iter_num { __u64 __opaque[1]; } __attribute__((aligned(8))); +/*

• • Flags to control bpf_wq_init() and bpf_wq_set_callback() behaviour.

• • - BPF_F_WQ_SLEEPABLE: the callback needs to run in
    

• •   a sleepable context
    

• */

+enum {

• BPF_F_WQ_SLEEPABLE = (1ULL << 0),

+};

Just started looking at the patch set. The first reaction that this flag is odd. Why add it? wq provides sleepable ctx. Why would the program ask to be non-sleepable in wq? If it needs to callback to run in rcu cs, it can use bpf_rcu_read_lock() kfunc as the first call in such callback and it will be equivalent to not-passing this BPF_F_WQ_SLEEPABLE flag. It seem it can be dropped and complexity reduced. The verifier complications in later patches due to this flag too... I just don't see the point.

#endif /* _UAPI__LINUX_BPF_H__ */ diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index 9fd12d480b8b..9ac1b8bb3a01 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -1109,11 +1109,18 @@ struct bpf_hrtimer { struct hrtimer timer; }; -/* the actual struct hidden inside uapi struct bpf_timer */ +struct bpf_work {

• struct bpf_async_cb cb;
• struct work_struct work;
• struct work_struct delete_work;

+};

+/* the actual struct hidden inside uapi struct bpf_timer and bpf_wq */ struct bpf_async_kern { union { struct bpf_async_cb *cb; struct bpf_hrtimer *timer;

• struct bpf_work *work;
  

  }; /* bpf_spin_lock is used here instead of spinlock_t to make
  • sure that it always fits into space reserved by struct bpf_timer

@@ -1124,6 +1131,7 @@ struct bpf_async_kern { enum bpf_async_type { BPF_ASYNC_TYPE_TIMER = 0,

• BPF_ASYNC_TYPE_WQ,

}; static DEFINE_PER_CPU(struct bpf_hrtimer *, hrtimer_running); @@ -1167,11 +1175,75 @@ static enum hrtimer_restart bpf_timer_cb(struct hrtimer *hrtimer) return HRTIMER_NORESTART; } +static void bpf_wq_work(struct work_struct *work) +{

• struct bpf_work *w = container_of(work, struct bpf_work, work);
• struct bpf_tramp_run_ctx __maybe_unused run_ctx;
• struct bpf_prog *prog = w->cb.prog;
• unsigned int flags = w->cb.flags;
• struct bpf_map *map = w->cb.map;
• bpf_callback_t callback_fn;
• void *value = w->cb.value;
• void *key;
• u32 idx;
• BTF_TYPE_EMIT(struct bpf_wq);
• callback_fn = READ_ONCE(w->cb.callback_fn);
• if (!callback_fn || !prog)

• 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);
  

• }
• run_ctx.bpf_cookie = 0;
• if (flags & BPF_F_WQ_SLEEPABLE) {

• if (!__bpf_prog_enter_sleepable_recur(prog, &run_ctx)) {
  

• 	/* recursion detected */
  

• 	__bpf_prog_exit_sleepable_recur(prog, 0, &run_ctx);
  

• 	return;
  

• }
  

• } else {

• if (!__bpf_prog_enter_recur(prog, &run_ctx)) {
  

• 	/* recursion detected */
  

• 	__bpf_prog_exit_recur(prog, 0, &run_ctx);
  

• 	return;
  

• }
  

• }
• callback_fn((u64)(long)map, (u64)(long)key, (u64)(long)value, 0, 0);
• /* The verifier checked that return value is zero. */
• if (flags & BPF_F_WQ_SLEEPABLE)

• __bpf_prog_exit_sleepable_recur(prog, 0 /* bpf_prog_run does runtime stats */,
  

• 				&run_ctx);
  

• else

• __bpf_prog_exit_recur(prog, 0, &run_ctx);
  

+}

+static void bpf_wq_delete_work(struct work_struct *work) +{

• struct bpf_work *w = container_of(work, struct bpf_work, delete_work);
• cancel_work_sync(&w->work);
• kfree_rcu(w, cb.rcu);

+}

static int __bpf_async_init(struct bpf_async_kern *async, struct bpf_map *map, u64 flags, enum bpf_async_type type) { struct bpf_async_cb *cb; struct bpf_hrtimer *t;

• struct bpf_work *w; clockid_t clockid; size_t size; int ret = 0;

@@ -1183,6 +1255,9 @@ static int __bpf_async_init(struct bpf_async_kern *async, struct bpf_map *map, u case BPF_ASYNC_TYPE_TIMER: size = sizeof(struct bpf_hrtimer); break;

• case BPF_ASYNC_TYPE_WQ:

• size = sizeof(struct bpf_work);
  

• break;
  

  default: return -EINVAL; }

@@ -1201,13 +1276,22 @@ static int __bpf_async_init(struct bpf_async_kern *async, struct bpf_map *map, u goto out; }

• if (type == BPF_ASYNC_TYPE_TIMER) {

• switch (type) {
• case BPF_ASYNC_TYPE_TIMER: clockid = flags & (MAX_CLOCKS - 1); t = (struct bpf_hrtimer *)cb;

hrtimer_init(&t->timer, clockid, HRTIMER_MODE_REL_SOFT); t->timer.function = bpf_timer_cb; cb->value = (void *)async - map->record->timer_off;

• break;
  

• case BPF_ASYNC_TYPE_WQ:

• w = (struct bpf_work *)cb;
  

• INIT_WORK(&w->work, bpf_wq_work);
  

• INIT_WORK(&w->delete_work, bpf_wq_delete_work);
  

• cb->value = (void *)async - map->record->wq_off;
  

• break;
  

  } cb->map = map; cb->prog = NULL;

@@ -1473,7 +1557,19 @@ void bpf_timer_cancel_and_free(void *val) */ void bpf_wq_cancel_and_free(void *val) {

• struct bpf_work *work;
• BTF_TYPE_EMIT(struct bpf_wq);
• work = (struct bpf_work *)__bpf_async_cancel_and_free(val);
• if (!work)

• return;
  

• /* Trigger cancel of the sleepable work, but *do not* wait for

• * it to finish if it was running as we might not be in a
  

• * sleepable context.
  

• * kfree will be called once the work has finished.
  

• */
  

• schedule_work(&work->delete_work);

} BPF_CALL_2(bpf_kptr_xchg, void *, map_value, void *, ptr) @@ -2612,6 +2708,19 @@ __bpf_kfunc void bpf_throw(u64 cookie) WARN(1, "A call to BPF exception callback should never return\n"); } +__bpf_kfunc int bpf_wq_init(struct bpf_wq *wq, void *map, unsigned int flags) +{

• struct bpf_async_kern *async = (struct bpf_async_kern *)wq;
• BUILD_BUG_ON(sizeof(struct bpf_async_kern) > sizeof(struct bpf_wq));
• BUILD_BUG_ON(__alignof__(struct bpf_async_kern) != __alignof__(struct bpf_wq));
• if (flags & ~BPF_F_WQ_SLEEPABLE)

• return -EINVAL;
  

• return __bpf_async_init(async, map, flags, BPF_ASYNC_TYPE_WQ);

+}

__bpf_kfunc_end_defs(); BTF_KFUNCS_START(generic_btf_ids) @@ -2689,6 +2798,7 @@ BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly) BTF_ID_FLAGS(func, bpf_dynptr_size) BTF_ID_FLAGS(func, bpf_dynptr_clone) BTF_ID_FLAGS(func, bpf_modify_return_test_tp) +BTF_ID_FLAGS(func, bpf_wq_init) BTF_KFUNCS_END(common_btf_ids) static const struct btf_kfunc_id_set common_kfunc_set = { diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 112faf2cd7e9..5e8c1e65fe8c 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -11038,6 +11038,7 @@ enum special_kfunc_type { KF_bpf_percpu_obj_drop_impl, KF_bpf_throw, KF_bpf_iter_css_task_new,

• KF_bpf_wq_init,

}; BTF_SET_START(special_kfunc_set) @@ -11064,6 +11065,7 @@ BTF_ID(func, bpf_throw) #ifdef CONFIG_CGROUPS BTF_ID(func, bpf_iter_css_task_new) #endif +BTF_ID(func, bpf_wq_init) BTF_SET_END(special_kfunc_set) BTF_ID_LIST(special_kfunc_list) @@ -11094,6 +11096,7 @@ BTF_ID(func, bpf_iter_css_task_new) #else BTF_ID_UNUSED #endif +BTF_ID(func, bpf_wq_init) static bool is_kfunc_ret_null(struct bpf_kfunc_call_arg_meta *meta) { @@ -11171,6 +11174,9 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env, if (is_kfunc_arg_wq(meta->btf, &args[argno])) return KF_ARG_PTR_TO_WORKQUEUE;

• if (meta->func_id == special_kfunc_list[KF_bpf_wq_init] && argno == 1)

• return KF_ARG_PTR_TO_MAP;
  

Hmm. This function has this bit: if (is_kfunc_arg_map(meta->btf, &args[argno])) return KF_ARG_PTR_TO_MAP;

Just do: +__bpf_kfunc int bpf_wq_init(struct bpf_wq *wq, void *map__map, ...

It was specifically added for bpf_arena_alloc_pages to pass pointer to a map. In case of arena map type it's used as: struct { __uint(type, BPF_MAP_TYPE_ARENA); ... } arena SEC(".maps");

page = bpf_arena_alloc_pages(&arena, ...) libbpf and the verifier do the right thing. I think it should work here as well.