[linux.git] / kernel / bpf / devmap.c

/* Copyright (c) 2017 Covalent IO, Inc. http://covalent.io
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of version 2 of the GNU General Public
 * License as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * General Public License for more details.
 */

/* Devmaps primary use is as a backend map for XDP BPF helper call
 * bpf_redirect_map(). Because XDP is mostly concerned with performance we
 * spent some effort to ensure the datapath with redirect maps does not use
 * any locking. This is a quick note on the details.
 *
 * We have three possible paths to get into the devmap control plane bpf
 * syscalls, bpf programs, and driver side xmit/flush operations. A bpf syscall
 * will invoke an update, delete, or lookup operation. To ensure updates and
 * deletes appear atomic from the datapath side xchg() is used to modify the
 * netdev_map array. Then because the datapath does a lookup into the netdev_map
 * array (read-only) from an RCU critical section we use call_rcu() to wait for
 * an rcu grace period before free'ing the old data structures. This ensures the
 * datapath always has a valid copy. However, the datapath does a "flush"
 * operation that pushes any pending packets in the driver outside the RCU
 * critical section. Each bpf_dtab_netdev tracks these pending operations using
 * an atomic per-cpu bitmap. The bpf_dtab_netdev object will not be destroyed
 * until all bits are cleared indicating outstanding flush operations have
 * completed.
 *
 * BPF syscalls may race with BPF program calls on any of the update, delete
 * or lookup operations. As noted above the xchg() operation also keep the
 * netdev_map consistent in this case. From the devmap side BPF programs
 * calling into these operations are the same as multiple user space threads
 * making system calls.
 *
 * Finally, any of the above may race with a netdev_unregister notifier. The
 * unregister notifier must search for net devices in the map structure that
 * contain a reference to the net device and remove them. This is a two step
 * process (a) dereference the bpf_dtab_netdev object in netdev_map and (b)
 * check to see if the ifindex is the same as the net_device being removed.
 * When removing the dev a cmpxchg() is used to ensure the correct dev is
 * removed, in the case of a concurrent update or delete operation it is
 * possible that the initially referenced dev is no longer in the map. As the
 * notifier hook walks the map we know that new dev references can not be
 * added by the user because core infrastructure ensures dev_get_by_index()
 * calls will fail at this point.
 */
#include <linux/bpf.h>
#include <linux/filter.h>

#define DEV_CREATE_FLAG_MASK \
        (BPF_F_NUMA_NODE | BPF_F_RDONLY | BPF_F_WRONLY)

struct bpf_dtab_netdev {
        struct net_device *dev;
        struct bpf_dtab *dtab;
        unsigned int bit;
        struct rcu_head rcu;
};

struct bpf_dtab {
        struct bpf_map map;
        struct bpf_dtab_netdev **netdev_map;
        unsigned long __percpu *flush_needed;
        struct list_head list;
};

static DEFINE_SPINLOCK(dev_map_lock);
static LIST_HEAD(dev_map_list);

static u64 dev_map_bitmap_size(const union bpf_attr *attr)
{
        return BITS_TO_LONGS((u64) attr->max_entries) * sizeof(unsigned long);
}

static struct bpf_map *dev_map_alloc(union bpf_attr *attr)
{
        struct bpf_dtab *dtab;
        int err = -EINVAL;
        u64 cost;

        if (!capable(CAP_NET_ADMIN))
                return ERR_PTR(-EPERM);

        /* check sanity of attributes */
        if (attr->max_entries == 0 || attr->key_size != 4 ||
            attr->value_size != 4 || attr->map_flags & ~DEV_CREATE_FLAG_MASK)
                return ERR_PTR(-EINVAL);

        dtab = kzalloc(sizeof(*dtab), GFP_USER);
        if (!dtab)
                return ERR_PTR(-ENOMEM);

        bpf_map_init_from_attr(&dtab->map, attr);

        /* make sure page count doesn't overflow */
        cost = (u64) dtab->map.max_entries * sizeof(struct bpf_dtab_netdev *);
        cost += dev_map_bitmap_size(attr) * num_possible_cpus();
        if (cost >= U32_MAX - PAGE_SIZE)
                goto free_dtab;

        dtab->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(dtab->map.pages);
        if (err)
                goto free_dtab;

        err = -ENOMEM;

        /* A per cpu bitfield with a bit per possible net device */
        dtab->flush_needed = __alloc_percpu_gfp(dev_map_bitmap_size(attr),
                                                __alignof__(unsigned long),
                                                GFP_KERNEL | __GFP_NOWARN);
        if (!dtab->flush_needed)
                goto free_dtab;

        dtab->netdev_map = bpf_map_area_alloc(dtab->map.max_entries *
                                              sizeof(struct bpf_dtab_netdev *),
                                              dtab->map.numa_node);
        if (!dtab->netdev_map)
                goto free_dtab;

        spin_lock(&dev_map_lock);
        list_add_tail_rcu(&dtab->list, &dev_map_list);
        spin_unlock(&dev_map_lock);

        return &dtab->map;
free_dtab:
        free_percpu(dtab->flush_needed);
        kfree(dtab);
        return ERR_PTR(err);
}

static void dev_map_free(struct bpf_map *map)
{
        struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
        int i, cpu;

        /* At this point bpf_prog->aux->refcnt == 0 and this map->refcnt == 0,
         * so the 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 reads against netdev_map. It does __not__ ensure pending
         * flush operations (if any) are complete.
         */

        spin_lock(&dev_map_lock);
        list_del_rcu(&dtab->list);
        spin_unlock(&dev_map_lock);

        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(dtab->flush_needed, cpu);

                while (!bitmap_empty(bitmap, dtab->map.max_entries))
                        cond_resched();
        }

        for (i = 0; i < dtab->map.max_entries; i++) {
                struct bpf_dtab_netdev *dev;

                dev = dtab->netdev_map[i];
                if (!dev)
                        continue;

                dev_put(dev->dev);
                kfree(dev);
        }

        free_percpu(dtab->flush_needed);
        bpf_map_area_free(dtab->netdev_map);
        kfree(dtab);
}

static int dev_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
{
        struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
        u32 index = key ? *(u32 *)key : U32_MAX;
        u32 *next = next_key;

        if (index >= dtab->map.max_entries) {
                *next = 0;
                return 0;
        }

        if (index == dtab->map.max_entries - 1)
                return -ENOENT;
        *next = index + 1;
        return 0;
}

void __dev_map_insert_ctx(struct bpf_map *map, u32 bit)
{
        struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
        unsigned long *bitmap = this_cpu_ptr(dtab->flush_needed);

        __set_bit(bit, bitmap);
}

/* __dev_map_flush is called from xdp_do_flush_map() which _must_ be signaled
 * from the driver before returning from its napi->poll() routine. The poll()
 * routine is called either from busy_poll context or net_rx_action signaled
 * from NET_RX_SOFTIRQ. Either way the poll routine must complete before the
 * net device can be torn down. On devmap tear down we ensure the ctx bitmap
 * is zeroed before completing to ensure all flush operations have completed.
 */
void __dev_map_flush(struct bpf_map *map)
{
        struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
        unsigned long *bitmap = this_cpu_ptr(dtab->flush_needed);
        u32 bit;

        for_each_set_bit(bit, bitmap, map->max_entries) {
                struct bpf_dtab_netdev *dev = READ_ONCE(dtab->netdev_map[bit]);
                struct net_device *netdev;

                /* This is possible if the dev entry is removed by user space
                 * between xdp redirect and flush op.
                 */
                if (unlikely(!dev))
                        continue;

                __clear_bit(bit, bitmap);
                netdev = dev->dev;
                if (likely(netdev->netdev_ops->ndo_xdp_flush))
                        netdev->netdev_ops->ndo_xdp_flush(netdev);
        }
}

/* rcu_read_lock (from syscall and BPF contexts) ensures that if a delete and/or
 * update happens in parallel here a dev_put wont happen until after reading the
 * ifindex.
 */
struct net_device  *__dev_map_lookup_elem(struct bpf_map *map, u32 key)
{
        struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
        struct bpf_dtab_netdev *dev;

        if (key >= map->max_entries)
                return NULL;

        dev = READ_ONCE(dtab->netdev_map[key]);
        return dev ? dev->dev : NULL;
}

static void *dev_map_lookup_elem(struct bpf_map *map, void *key)
{
        struct net_device *dev = __dev_map_lookup_elem(map, *(u32 *)key);

        return dev ? &dev->ifindex : NULL;
}

static void dev_map_flush_old(struct bpf_dtab_netdev *dev)
{
        if (dev->dev->netdev_ops->ndo_xdp_flush) {
                struct net_device *fl = dev->dev;
                unsigned long *bitmap;
                int cpu;

                for_each_online_cpu(cpu) {
                        bitmap = per_cpu_ptr(dev->dtab->flush_needed, cpu);
                        __clear_bit(dev->bit, bitmap);

                        fl->netdev_ops->ndo_xdp_flush(dev->dev);
                }
        }
}

static void __dev_map_entry_free(struct rcu_head *rcu)
{
        struct bpf_dtab_netdev *dev;

        dev = container_of(rcu, struct bpf_dtab_netdev, rcu);
        dev_map_flush_old(dev);
        dev_put(dev->dev);
        kfree(dev);
}

static int dev_map_delete_elem(struct bpf_map *map, void *key)
{
        struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
        struct bpf_dtab_netdev *old_dev;
        int k = *(u32 *)key;

        if (k >= map->max_entries)
                return -EINVAL;

        /* Use call_rcu() here to ensure any 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. However, for pending flush operations the
         * dev and ctx are stored in another per cpu map. And additionally,
         * the driver tear down ensures all soft irqs are complete before
         * removing the net device in the case of dev_put equals zero.
         */
        old_dev = xchg(&dtab->netdev_map[k], NULL);
        if (old_dev)
                call_rcu(&old_dev->rcu, __dev_map_entry_free);
        return 0;
}

static int dev_map_update_elem(struct bpf_map *map, void *key, void *value,
                                u64 map_flags)
{
        struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
        struct net *net = current->nsproxy->net_ns;
        struct bpf_dtab_netdev *dev, *old_dev;
        u32 i = *(u32 *)key;
        u32 ifindex = *(u32 *)value;

        if (unlikely(map_flags > BPF_EXIST))
                return -EINVAL;
        if (unlikely(i >= dtab->map.max_entries))
                return -E2BIG;
        if (unlikely(map_flags == BPF_NOEXIST))
                return -EEXIST;

        if (!ifindex) {
                dev = NULL;
        } else {
                dev = kmalloc_node(sizeof(*dev), GFP_ATOMIC | __GFP_NOWARN,
                                   map->numa_node);
                if (!dev)
                        return -ENOMEM;

                dev->dev = dev_get_by_index(net, ifindex);
                if (!dev->dev) {
                        kfree(dev);
                        return -EINVAL;
                }

                dev->bit = i;
                dev->dtab = dtab;
        }

        /* Use call_rcu() here to ensure rcu critical sections have completed
         * Remembering the driver side flush operation will happen before the
         * net device is removed.
         */
        old_dev = xchg(&dtab->netdev_map[i], dev);
        if (old_dev)
                call_rcu(&old_dev->rcu, __dev_map_entry_free);

        return 0;
}

const struct bpf_map_ops dev_map_ops = {
        .map_alloc = dev_map_alloc,
        .map_free = dev_map_free,
        .map_get_next_key = dev_map_get_next_key,
        .map_lookup_elem = dev_map_lookup_elem,
        .map_update_elem = dev_map_update_elem,
        .map_delete_elem = dev_map_delete_elem,
};

static int dev_map_notification(struct notifier_block *notifier,
                                ulong event, void *ptr)
{
        struct net_device *netdev = netdev_notifier_info_to_dev(ptr);
        struct bpf_dtab *dtab;
        int i;

        switch (event) {
        case NETDEV_UNREGISTER:
                /* This rcu_read_lock/unlock pair is needed because
                 * dev_map_list is an RCU list AND to ensure a delete
                 * operation does not free a netdev_map entry while we
                 * are comparing it against the netdev being unregistered.
                 */
                rcu_read_lock();
                list_for_each_entry_rcu(dtab, &dev_map_list, list) {
                        for (i = 0; i < dtab->map.max_entries; i++) {
                                struct bpf_dtab_netdev *dev, *odev;

                                dev = READ_ONCE(dtab->netdev_map[i]);
                                if (!dev ||
                                    dev->dev->ifindex != netdev->ifindex)
                                        continue;
                                odev = cmpxchg(&dtab->netdev_map[i], dev, NULL);
                                if (dev == odev)
                                        call_rcu(&dev->rcu,
                                                 __dev_map_entry_free);
                        }
                }
                rcu_read_unlock();
                break;
        default:
                break;
        }
        return NOTIFY_OK;
}

static struct notifier_block dev_map_notifier = {
        .notifier_call = dev_map_notification,
};

static int __init dev_map_init(void)
{
        register_netdevice_notifier(&dev_map_notifier);
        return 0;
}

subsys_initcall(dev_map_init);