bpf: add devmap, a map for storing net device references

Device map (devmap) is a BPF map, primarily useful for networking
applications, that uses a key to lookup a reference to a netdevice.

The map provides a clean way for BPF programs to build virtual port
to physical port maps. Additionally, it provides a scoping function
for the redirect action itself allowing multiple optimizations. Future
patches will leverage the map to provide batching at the XDP layer.

Another optimization/feature, that is not yet implemented, would be
to support multiple netdevices per key to support efficient multicast
and broadcast support.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 275 insertions, 0 deletions

diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
index e1e5e658f2db..48e92705be59 100644
--- a/kernel/bpf/Makefile
+++ b/kernel/bpf/Makefile
@@ -2,6 +2,9 @@ obj-y := core.o
 
 obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o
 obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o
+ifeq ($(CONFIG_NET),y)
+obj-$(CONFIG_BPF_SYSCALL) += devmap.o
+endif
 ifeq ($(CONFIG_PERF_EVENTS),y)
 obj-$(CONFIG_BPF_SYSCALL) += stackmap.o
 endif
diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
new file mode 100644
index 000000000000..1a878356bd37
--- /dev/null
+++ b/kernel/bpf/devmap.c
@@ -0,0 +1,264 @@
+/* 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.
+ */
+#include <linux/bpf.h>
+#include <linux/jhash.h>
+#include <linux/filter.h>
+#include <linux/rculist_nulls.h>
+#include "percpu_freelist.h"
+#include "bpf_lru_list.h"
+#include "map_in_map.h"
+
+struct bpf_dtab_netdev {
+        struct net_device *dev;
+        int key;
+        struct rcu_head rcu;
+        struct bpf_dtab *dtab;
+};
+
+struct bpf_dtab {
+        struct bpf_map map;
+        struct bpf_dtab_netdev **netdev_map;
+};
+
+static struct bpf_map *dev_map_alloc(union bpf_attr *attr)
+{
+        struct bpf_dtab *dtab;
+        u64 cost;
+        int err;
+
+        /* check sanity of attributes */
+        if (attr->max_entries == 0 || attr->key_size != 4 ||
+            attr->value_size != 4 || attr->map_flags)
+                return ERR_PTR(-EINVAL);
+
+        /* if value_size is bigger, the user space won't be able to
+         * access the elements.
+         */
+        if (attr->value_size > KMALLOC_MAX_SIZE)
+                return ERR_PTR(-E2BIG);
+
+        dtab = kzalloc(sizeof(*dtab), GFP_USER);
+        if (!dtab)
+                return ERR_PTR(-ENOMEM);
+
+        /* mandatory map attributes */
+        dtab->map.map_type = attr->map_type;
+        dtab->map.key_size = attr->key_size;
+        dtab->map.value_size = attr->value_size;
+        dtab->map.max_entries = attr->max_entries;
+        dtab->map.map_flags = attr->map_flags;
+
+        err = -ENOMEM;
+
+        /* make sure page count doesn't overflow */
+        cost = (u64) dtab->map.max_entries * sizeof(struct bpf_dtab_netdev *);
+        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;
+
+        dtab->netdev_map = bpf_map_area_alloc(dtab->map.max_entries *
+                                              sizeof(struct bpf_dtab_netdev *));
+        if (!dtab->netdev_map)
+                goto free_dtab;
+
+        return &dtab->map;
+
+free_dtab:
+        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;
+
+        /* 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.
+         */
+        synchronize_rcu();
+
+        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);
+        }
+
+        /* At this point bpf program is detached and all pending operations
+         * _must_ be complete
+         */
+        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 = (u32 *)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;
+}
+
+/* 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.
+ */
+static void *dev_map_lookup_elem(struct bpf_map *map, void *key)
+{
+        struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
+        struct bpf_dtab_netdev *dev;
+        u32 i = *(u32 *)key;
+
+        if (i >= map->max_entries)
+                return NULL;
+
+        dev = READ_ONCE(dtab->netdev_map[i]);
+        return dev ? &dev->dev->ifindex : NULL;
+}
+
+static void __dev_map_entry_free(struct rcu_head *rcu)
+{
+        struct bpf_dtab_netdev *old_dev;
+
+        old_dev = container_of(rcu, struct bpf_dtab_netdev, rcu);
+        dev_put(old_dev->dev);
+        kfree(old_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 synchronize_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(sizeof(*dev), GFP_ATOMIC | __GFP_NOWARN);
+                if (!dev)
+                        return -ENOMEM;
+
+                dev->dev = dev_get_by_index(net, ifindex);
+                if (!dev->dev) {
+                        kfree(dev);
+                        return -EINVAL;
+                }
+
+                dev->key = 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,
+};
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6a86723c5b64..4016774d5cca 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1276,6 +1276,14 @@ static int check_map_func_compatibility(struct bpf_map *map, int func_id)
                     func_id != BPF_FUNC_current_task_under_cgroup)
                         goto error;
                 break;
+        /* devmap returns a pointer to a live net_device ifindex that we cannot
+         * allow to be modified from bpf side. So do not allow lookup elements
+         * for now.
+         */
+        case BPF_MAP_TYPE_DEVMAP:
+                if (func_id == BPF_FUNC_map_lookup_elem)
+                        goto error;
+                break;
         case BPF_MAP_TYPE_ARRAY_OF_MAPS:
         case BPF_MAP_TYPE_HASH_OF_MAPS:
                 if (func_id != BPF_FUNC_map_lookup_elem)