Subject: Re: [net-next V4 PATCH 1/5] bpf: introduce new bpf cpu map type BPF_MAP_TYPE_CPUMAP

From: Daniel Borkmann <[email-masked]>
Date: Thu, 05 Oct 2017 11:40:15 +0200

On 10/04/2017 02:03 PM, Jesper Dangaard Brouer wrote:
[...]
> +#define CPU_MAP_BULK_SIZE 8  /* 8 == one cacheline on 64-bit archs */
> +struct xdp_bulk_queue {
> +	void *q[CPU_MAP_BULK_SIZE];
> +	unsigned int count;
> +};
> +
> +/* Struct for every remote "destination" CPU in map */
> +struct bpf_cpu_map_entry {
> +	u32 cpu;    /* kthread CPU and map index */
> +	int map_id; /* Back reference to map */

map_id is not used here if I read it correctly? We should
then remove it.

> +	u32 qsize;  /* Redundant queue size for map lookup */
> +
> +	/* XDP can run multiple RX-ring queues, need __percpu enqueue store */
> +	struct xdp_bulk_queue __percpu *bulkq;
> +
> +	/* Queue with potential multi-producers, and single-consumer kthread */
> +	struct ptr_ring *queue;
> +	struct task_struct *kthread;
> +	struct work_struct kthread_stop_wq;
> +
> +	atomic_t refcnt; /* Control when this struct can be free'ed */
> +	struct rcu_head rcu;
> +};
> +
> +struct bpf_cpu_map {
> +	struct bpf_map map;
> +	/* Below members specific for map type */
> +	struct bpf_cpu_map_entry **cpu_map;
> +	unsigned long __percpu *flush_needed;
> +};
> +
> +static int bq_flush_to_queue(struct bpf_cpu_map_entry *rcpu,
> +			     struct xdp_bulk_queue *bq);

Could we avoid forward declaration?

> +static u64 cpu_map_bitmap_size(const union bpf_attr *attr)
> +{
> +	return BITS_TO_LONGS(attr->max_entries) * sizeof(unsigned long);
> +}
> +
> +static struct bpf_map *cpu_map_alloc(union bpf_attr *attr)
> +{
> +	struct bpf_cpu_map *cmap;
> +	u64 cost;
> +	int err;
> +
> +	/* check sanity of attributes */
> +	if (attr->max_entries == 0 || attr->key_size != 4 ||
> +	    attr->value_size != 4 || attr->map_flags & ~BPF_F_NUMA_NODE)
> +		return ERR_PTR(-EINVAL);
> +
> +	cmap = kzalloc(sizeof(*cmap), GFP_USER);
> +	if (!cmap)
> +		return ERR_PTR(-ENOMEM);
> +
> +	/* mandatory map attributes */
> +	cmap->map.map_type = attr->map_type;
> +	cmap->map.key_size = attr->key_size;
> +	cmap->map.value_size = attr->value_size;
> +	cmap->map.max_entries = attr->max_entries;
> +	cmap->map.map_flags = attr->map_flags;
> +	cmap->map.numa_node = bpf_map_attr_numa_node(attr);
> +
> +	/* Pre-limit array size based on NR_CPUS, not final CPU check */
> +	if (cmap->map.max_entries > NR_CPUS)
> +		return ERR_PTR(-E2BIG);
> +
> +	/* make sure page count doesn't overflow */
> +	cost = (u64) cmap->map.max_entries * sizeof(struct bpf_cpu_map_entry *);
> +	cost += cpu_map_bitmap_size(attr) * num_possible_cpus();
> +	if (cost >= U32_MAX - PAGE_SIZE)
> +		goto free_cmap;
> +	cmap->map.pages = round_up(cost, PAGE_SIZE) >> PAGE_SHIFT;
> +
> +	/* if map size is larger than memlock limit, reject it early */
> +	err = bpf_map_precharge_memlock(cmap->map.pages);
> +	if (err)
> +		goto free_cmap;

Given this is almost the same as devmap and touches user land reporting,
we should probably go and do the same as in 582db7e0c4c2 ("bpf: devmap:
pass on return value of bpf_map_precharge_memlock").

> +	/* A per cpu bitfield with a bit per possible CPU in map  */
> +	cmap->flush_needed = __alloc_percpu(cpu_map_bitmap_size(attr),
> +					    __alignof__(unsigned long));
> +	if (!cmap->flush_needed)
> +		goto free_cmap;
> +
> +	/* Alloc array for possible remote "destination" CPUs */
> +	cmap->cpu_map = bpf_map_area_alloc(cmap->map.max_entries *
> +					   sizeof(struct bpf_cpu_map_entry *),
> +					   cmap->map.numa_node);
> +	if (!cmap->cpu_map)
> +		goto free_cmap;
> +
> +	return &cmap->map;
> +free_cmap:
> +	free_percpu(cmap->flush_needed);
> +	kfree(cmap);
> +	return ERR_PTR(-ENOMEM);
> +}
> +
> +void __cpu_map_queue_destructor(void *ptr)
> +{
> +	/* For now, just catch this as an error */
> +	if (!ptr)
> +		return;
> +	pr_err("ERROR: %s() cpu_map queue was not empty\n", __func__);

Can you elaborate on this "for now" condition? Is this a race
when kthread doesn't consume queue on thread exit, or should it
be impossible to trigger (should it then be replaced with a
'if (WARN_ON_ONCE(ptr)) page_frag_free(ptr)' and a more elaborate
comment)?

> +	page_frag_free(ptr);
> +}
> +
> +static void put_cpu_map_entry(struct bpf_cpu_map_entry *rcpu)
> +{
> +	if (atomic_dec_and_test(&rcpu->refcnt)) {
> +		/* The queue should be empty at this point */
> +		ptr_ring_cleanup(rcpu->queue, __cpu_map_queue_destructor);
> +		kfree(rcpu->queue);
> +		kfree(rcpu);
> +	}
> +}
> +
> +static void get_cpu_map_entry(struct bpf_cpu_map_entry *rcpu)
> +{
> +	atomic_inc(&rcpu->refcnt);
> +}
> +
> +/* called from workqueue, to workaround syscall using preempt_disable */
> +static void cpu_map_kthread_stop(struct work_struct *work)
> +{
> +	struct bpf_cpu_map_entry *rcpu;
> +
> +	rcpu = container_of(work, struct bpf_cpu_map_entry, kthread_stop_wq);
> +	synchronize_rcu(); /* wait for flush in __cpu_map_entry_free() */
> +	kthread_stop(rcpu->kthread); /* calls put_cpu_map_entry */
> +}
> +
> +static int cpu_map_kthread_run(void *data)
> +{
> +	struct bpf_cpu_map_entry *rcpu = data;
> +
> +	set_current_state(TASK_INTERRUPTIBLE);
> +	while (!kthread_should_stop()) {
> +		struct xdp_pkt *xdp_pkt;
> +
> +		schedule();
> +		/* Do work */
> +		while ((xdp_pkt = ptr_ring_consume(rcpu->queue))) {
> +			/* For now just "refcnt-free" */
> +			page_frag_free(xdp_pkt);
> +		}
> +		__set_current_state(TASK_INTERRUPTIBLE);
> +	}
> +	put_cpu_map_entry(rcpu);
> +
> +	__set_current_state(TASK_RUNNING);
> +	return 0;
> +}
> +
> +struct bpf_cpu_map_entry *__cpu_map_entry_alloc(u32 qsize, u32 cpu, int map_id)
> +{
> +	gfp_t gfp = GFP_ATOMIC|__GFP_NOWARN;
> +	struct bpf_cpu_map_entry *rcpu;
> +	int numa, err;
> +
> +	/* Have map->numa_node, but choose node of redirect target CPU */
> +	numa = cpu_to_node(cpu);
> +
> +	rcpu = kzalloc_node(sizeof(*rcpu), gfp, numa);
> +	if (!rcpu)
> +		return NULL;
> +
> +	/* Alloc percpu bulkq */
> +	rcpu->bulkq = __alloc_percpu_gfp(sizeof(*rcpu->bulkq),
> +					 sizeof(void *), gfp);
> +	if (!rcpu->bulkq)
> +		goto fail;
> +
> +	/* Alloc queue */
> +	rcpu->queue = kzalloc_node(sizeof(*rcpu->queue), gfp, numa);
> +	if (!rcpu->queue)
> +		goto fail;
> +
> +	err = ptr_ring_init(rcpu->queue, qsize, gfp);
> +	if (err)
> +		goto fail;
> +	rcpu->qsize = qsize;
> +
> +	/* Setup kthread */
> +	rcpu->kthread = kthread_create_on_node(cpu_map_kthread_run, rcpu, numa,
> +					       "cpumap/%d/map:%d", cpu, map_id);
> +	if (IS_ERR(rcpu->kthread))
> +		goto fail;

What about ptr_ring_cleanup() when we fail here?

> +
> +	/* Make sure kthread runs on a single CPU */
> +	kthread_bind(rcpu->kthread, cpu);
> +	wake_up_process(rcpu->kthread);
> +
> +	get_cpu_map_entry(rcpu); /* 1-refcnt for being in cmap->cpu_map[] */
> +	get_cpu_map_entry(rcpu); /* 1-refcnt for kthread */
> +
> +	return rcpu;
> +
> +fail:   /* Hint: free API detect NULL values */
> +	free_percpu(rcpu->bulkq);
> +	kfree(rcpu->queue);
> +	kfree(rcpu);
> +	return NULL;
> +}
> +
> +void __cpu_map_entry_free(struct rcu_head *rcu)
> +{
> +	struct bpf_cpu_map_entry *rcpu;
> +	int cpu;
> +
> +	/* This cpu_map_entry have been disconnected from map and one
> +	 * RCU graze-period have elapsed.  Thus, XDP cannot queue any
> +	 * new packets and cannot change/set flush_needed that can
> +	 * find this entry.
> +	 */
> +	rcpu = container_of(rcu, struct bpf_cpu_map_entry, rcu);
> +
> +	/* Flush remaining packets in percpu bulkq */
> +	for_each_online_cpu(cpu) {
> +		struct xdp_bulk_queue *bq = per_cpu_ptr(rcpu->bulkq, cpu);
> +
> +		/* No concurrent bq_enqueue can run at this point */
> +		bq_flush_to_queue(rcpu, bq);
> +	}
> +	free_percpu(rcpu->bulkq);
> +	/* Cannot kthread_stop() here, last put free rcpu resources */
> +	put_cpu_map_entry(rcpu);
> +}
> +
> +/* After xchg pointer to bpf_cpu_map_entry, use the call_rcu() to
> + * ensure any driver rcu critical sections have completed, but this
> + * does not guarantee a flush has happened yet. Because driver side
> + * rcu_read_lock/unlock only protects the running XDP program.  The
> + * atomic xchg and NULL-ptr check in __cpu_map_flush() makes sure a
> + * pending flush op doesn't fail.
> + *
> + * The bpf_cpu_map_entry is still used by the kthread, and there can
> + * still be pending packets (in queue and percpu bulkq).  A refcnt
> + * makes sure to last user (kthread_stop vs. call_rcu) free memory
> + * resources.
> + *
> + * The rcu callback __cpu_map_entry_free flush remaining packets in
> + * percpu bulkq to queue.  Due to caller map_delete_elem() disable
> + * preemption, cannot call kthread_stop() to make sure queue is empty.
> + * Instead a work_queue is started for stopping kthread,
> + * cpu_map_kthread_stop, which waits for an RCU graze period before
> + * stopping kthread, emptying the queue.
> + */
> +void __cpu_map_entry_replace(struct bpf_cpu_map *cmap,
> +			     u32 key_cpu, struct bpf_cpu_map_entry *rcpu)
> +{
> +	struct bpf_cpu_map_entry *old_rcpu;
> +
> +	old_rcpu = xchg(&cmap->cpu_map[key_cpu], rcpu);
> +	if (old_rcpu) {
> +		call_rcu(&old_rcpu->rcu, __cpu_map_entry_free);
> +		INIT_WORK(&old_rcpu->kthread_stop_wq, cpu_map_kthread_stop);
> +		schedule_work(&old_rcpu->kthread_stop_wq);
> +	}
> +}
> +
> +int cpu_map_delete_elem(struct bpf_map *map, void *key)
> +{
> +	struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
> +	u32 key_cpu = *(u32 *)key;
> +
> +	if (key_cpu >= map->max_entries)
> +		return -EINVAL;
> +
> +	/* notice caller map_delete_elem() use preempt_disable() */
> +	__cpu_map_entry_replace(cmap, key_cpu, NULL);
> +	return 0;
> +}
> +
> +int cpu_map_update_elem(struct bpf_map *map, void *key, void *value,
> +				u64 map_flags)
> +{
> +	struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
> +	struct bpf_cpu_map_entry *rcpu;
> +
> +	/* Array index key correspond to CPU number */
> +	u32 key_cpu = *(u32 *)key;
> +	/* Value is the queue size */
> +	u32 qsize = *(u32 *)value;
> +
> +	/* Make sure CPU is a valid possible cpu */
> +	if (!cpu_possible(key_cpu))
> +		return -ENODEV;
> +
> +	if (unlikely(map_flags > BPF_EXIST))
> +		return -EINVAL;
> +	if (unlikely(key_cpu >= cmap->map.max_entries))
> +		return -E2BIG;
> +	if (unlikely(map_flags == BPF_NOEXIST))
> +		return -EEXIST;
> +	if (unlikely(qsize > 16384)) /* sanity limit on qsize */
> +		return -EOVERFLOW;
> +
> +	if (qsize == 0) {
> +		rcpu = NULL; /* Same as deleting */
> +	} else {
> +		/* Updating qsize cause re-allocation of bpf_cpu_map_entry */
> +		rcpu = __cpu_map_entry_alloc(qsize, key_cpu, map->id);
> +		if (!rcpu)
> +			return -ENOMEM;
> +	}
> +	rcu_read_lock();
> +	__cpu_map_entry_replace(cmap, key_cpu, rcpu);
> +	rcu_read_unlock();
> +	return 0;

You need to update verifier such that this function cannot be called
out of an BPF program, otherwise it would be possible under full RCU
read context, which is explicitly avoided here and also it would otherwise
be allowed for other maps of different type as well, which needs to
be avoided.

> +}
> +
> +void cpu_map_free(struct bpf_map *map)
> +{
> +	struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
> +	int cpu;
> +	u32 i;
> +
> +	/* At this point bpf_prog->aux->refcnt == 0 and this map->refcnt == 0,
> +	 * so the bpf programs (can be more than one that used this map) were
> +	 * disconnected from events. Wait for outstanding critical sections in
> +	 * these programs to complete. The rcu critical section only guarantees
> +	 * no further "XDP/bpf-side" reads against bpf_cpu_map->cpu_map.
> +	 * It does __not__ ensure pending flush operations (if any) are
> +	 * complete.
> +	 */
> +	synchronize_rcu();
> +
> +	/* To ensure all pending flush operations have completed wait for flush
> +	 * bitmap to indicate all flush_needed bits to be zero on _all_ cpus.
> +	 * Because the above synchronize_rcu() ensures the map is disconnected
> +	 * from the program we can assume no new bits will be set.
> +	 */
> +	for_each_online_cpu(cpu) {
> +		unsigned long *bitmap = per_cpu_ptr(cmap->flush_needed, cpu);
> +
> +		while (!bitmap_empty(bitmap, cmap->map.max_entries))
> +			cond_resched();
> +	}
> +
> +	/* For cpu_map the remote CPUs can still be using the entries
> +	 * (struct bpf_cpu_map_entry).
> +	 */
> +	for (i = 0; i < cmap->map.max_entries; i++) {
> +		struct bpf_cpu_map_entry *rcpu;
> +
> +		rcpu = READ_ONCE(cmap->cpu_map[i]);
> +		if (!rcpu)
> +			continue;
> +
> +		/* bq flush and cleanup happens after RCU graze-period */
> +		__cpu_map_entry_replace(cmap, i, NULL); /* call_rcu */
> +	}
> +	free_percpu(cmap->flush_needed);
> +	bpf_map_area_free(cmap->cpu_map);
> +	kfree(cmap);
> +}
> +
> +struct bpf_cpu_map_entry *__cpu_map_lookup_elem(struct bpf_map *map, u32 key)
> +{
> +	struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
> +	struct bpf_cpu_map_entry *rcpu;
> +
> +	if (key >= map->max_entries)
> +		return NULL;
> +
> +	rcpu = READ_ONCE(cmap->cpu_map[key]);
> +	return rcpu;
> +}
> +
> +static void *cpu_map_lookup_elem(struct bpf_map *map, void *key)
> +{
> +	struct bpf_cpu_map_entry *rcpu =
> +		__cpu_map_lookup_elem(map, *(u32 *)key);
> +
> +	return rcpu ? &rcpu->qsize : NULL;

The qsize doesn't seem used anywhere else besides here, but you
probably should update verifier such that this cannot be called
out of the BPF program, which could then mangle qsize value.

> +}
> +
> +static int cpu_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
> +{
> +	struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
> +	u32 index = key ? *(u32 *)key : U32_MAX;
> +	u32 *next = next_key;
> +
> +	if (index >= cmap->map.max_entries) {
> +		*next = 0;
> +		return 0;
> +	}
> +
> +	if (index == cmap->map.max_entries - 1)
> +		return -ENOENT;
> +	*next = index + 1;
> +	return 0;
> +}
> +
> +const struct bpf_map_ops cpu_map_ops = {
> +	.map_alloc		= cpu_map_alloc,
> +	.map_free		= cpu_map_free,
> +	.map_delete_elem	= cpu_map_delete_elem,
> +	.map_update_elem	= cpu_map_update_elem,
> +	.map_lookup_elem	= cpu_map_lookup_elem,
> +	.map_get_next_key	= cpu_map_get_next_key,
> +};
> +
> +static int bq_flush_to_queue(struct bpf_cpu_map_entry *rcpu,
> +			     struct xdp_bulk_queue *bq)
> +{
> +	struct ptr_ring *q;
> +	int i;
> +
> +	if (unlikely(!bq->count))
> +		return 0;
> +
> +	q = rcpu->queue;
> +	spin_lock(&q->producer_lock);
> +
> +	for (i = 0; i < bq->count; i++) {
> +		void *xdp_pkt = bq->q[i];
> +		int err;
> +
> +		err = __ptr_ring_produce(q, xdp_pkt);
> +		if (err) {
> +			/* Free xdp_pkt */
> +			page_frag_free(xdp_pkt);
> +		}
> +	}
> +	bq->count = 0;
> +	spin_unlock(&q->producer_lock);
> +
> +	return 0;
> +}
> +
> +/* Notice: Will change in later patch */
> +struct xdp_pkt {
> +	void *data;
> +	u16 len;
> +	u16 headroom;
> +};
> +
> +/* Runs under RCU-read-side, plus in softirq under NAPI protection.
> + * Thus, safe percpu variable access.
> + */
> +static int bq_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_pkt *xdp_pkt)
> +{
> +	struct xdp_bulk_queue *bq = this_cpu_ptr(rcpu->bulkq);
> +
> +	if (unlikely(bq->count == CPU_MAP_BULK_SIZE))
> +		bq_flush_to_queue(rcpu, bq);
> +
> +	/* Notice, xdp_buff/page MUST be queued here, long enough for
> +	 * driver to code invoking us to finished, due to driver
> +	 * (e.g. ixgbe) recycle tricks based on page-refcnt.
> +	 *
> +	 * Thus, incoming xdp_pkt is always queued here (else we race
> +	 * with another CPU on page-refcnt and remaining driver code).
> +	 * Queue time is very short, as driver will invoke flush
> +	 * operation, when completing napi->poll call.
> +	 */
> +	bq->q[bq->count++] = xdp_pkt;
> +	return 0;
> +}
> +
> +int cpu_map_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_buff *xdp)
> +{
> +	struct xdp_pkt *xdp_pkt;
> +	int headroom;
> +
> +	/* Convert xdp_buff to xdp_pkt */
> +	headroom = xdp->data - xdp->data_hard_start;
> +	if (headroom < sizeof(*xdp_pkt))
> +		return -EOVERFLOW;
> +	xdp_pkt = xdp->data_hard_start;
> +	xdp_pkt->data = xdp->data;
> +	xdp_pkt->len  = xdp->data_end - xdp->data;
> +	xdp_pkt->headroom = headroom;
> +	/* For now this is just used as a void pointer to data_hard_start */
> +
> +	bq_enqueue(rcpu, xdp_pkt);
> +	return 0;
> +}
> +
> +void __cpu_map_insert_ctx(struct bpf_map *map, u32 bit)
> +{
> +	struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
> +	unsigned long *bitmap = this_cpu_ptr(cmap->flush_needed);
> +
> +	__set_bit(bit, bitmap);
> +}
> +
> +void __cpu_map_flush(struct bpf_map *map)
> +{
> +	struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
> +	unsigned long *bitmap = this_cpu_ptr(cmap->flush_needed);
> +	u32 bit;
> +
> +	/* The napi->poll softirq makes sure __cpu_map_insert_ctx()
> +	 * and __cpu_map_flush() happen on same CPU. Thus, the percpu
> +	 * bitmap indicate which percpu bulkq have packets.
> +	 */
> +	for_each_set_bit(bit, bitmap, map->max_entries) {
> +		struct bpf_cpu_map_entry *rcpu = READ_ONCE(cmap->cpu_map[bit]);
> +		struct xdp_bulk_queue *bq;
> +
> +		/* This is possible if entry is removed by user space
> +		 * between xdp redirect and flush op.
> +		 */
> +		if (unlikely(!rcpu))
> +			continue;
> +
> +		__clear_bit(bit, bitmap);
> +
> +		/* Flush all frames in bulkq to real queue */
> +		bq = this_cpu_ptr(rcpu->bulkq);
> +		bq_flush_to_queue(rcpu, bq);
> +
> +		/* If already running, costs spin_lock_irqsave + smb_mb */
> +		wake_up_process(rcpu->kthread);
> +	}
> +}
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index b927da66f653..641bdb0df020 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -593,6 +593,12 @@ static int map_update_elem(union bpf_attr *attr)
>   	if (copy_from_user(value, uvalue, value_size) != 0)
>   		goto free_value;
>
> +	/* Need to create a kthread, thus must support schedule */
> +	if (map->map_type == BPF_MAP_TYPE_CPUMAP) {
> +		err = map->ops->map_update_elem(map, key, value, attr->flags);
> +		goto out;
> +	}
> +
>   	/* must increment bpf_prog_active to avoid kprobe+bpf triggering from
>   	 * inside bpf map update or delete otherwise deadlocks are possible
>   	 */
> @@ -623,7 +629,7 @@ static int map_update_elem(union bpf_attr *attr)
>   	}
>   	__this_cpu_dec(bpf_prog_active);
>   	preempt_enable();
> -
> +out:
>   	if (!err)
>   		trace_bpf_map_update_elem(map, ufd, key, value);
>   free_value:
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 6d2137b4cf38..03f8e2827a95 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -111,6 +111,7 @@ enum bpf_map_type {
>   	BPF_MAP_TYPE_HASH_OF_MAPS,
>   	BPF_MAP_TYPE_DEVMAP,
>   	BPF_MAP_TYPE_SOCKMAP,
> +	BPF_MAP_TYPE_CPUMAP,
>   };
>
>   enum bpf_prog_type {
>