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bpf: add devmap, a map for storing net device references
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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>
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jrfastab authored and davem330 committed Jul 17, 2017
1 parent 5acaee0 commit 546ac1f
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Showing 6 changed files with 294 additions and 0 deletions.
3 changes: 3 additions & 0 deletions include/linux/bpf_types.h
Original file line number Diff line number Diff line change
Expand Up @@ -35,3 +35,6 @@ BPF_MAP_TYPE(BPF_MAP_TYPE_STACK_TRACE, stack_map_ops)
#endif
BPF_MAP_TYPE(BPF_MAP_TYPE_ARRAY_OF_MAPS, array_of_maps_map_ops)
BPF_MAP_TYPE(BPF_MAP_TYPE_HASH_OF_MAPS, htab_of_maps_map_ops)
#ifdef CONFIG_NET
BPF_MAP_TYPE(BPF_MAP_TYPE_DEVMAP, dev_map_ops)
#endif
1 change: 1 addition & 0 deletions include/uapi/linux/bpf.h
Original file line number Diff line number Diff line change
Expand Up @@ -104,6 +104,7 @@ enum bpf_map_type {
BPF_MAP_TYPE_LPM_TRIE,
BPF_MAP_TYPE_ARRAY_OF_MAPS,
BPF_MAP_TYPE_HASH_OF_MAPS,
BPF_MAP_TYPE_DEVMAP,
};

enum bpf_prog_type {
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3 changes: 3 additions & 0 deletions kernel/bpf/Makefile
Original file line number Diff line number Diff line change
Expand Up @@ -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
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264 changes: 264 additions & 0 deletions kernel/bpf/devmap.c
Original file line number Diff line number Diff line change
@@ -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,
};
8 changes: 8 additions & 0 deletions kernel/bpf/verifier.c
Original file line number Diff line number Diff line change
Expand Up @@ -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)
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15 changes: 15 additions & 0 deletions tools/testing/selftests/bpf/test_maps.c
Original file line number Diff line number Diff line change
Expand Up @@ -438,6 +438,21 @@ static void test_arraymap_percpu_many_keys(void)
close(fd);
}

static void test_devmap(int task, void *data)
{
int next_key, fd;
__u32 key, value;

fd = bpf_create_map(BPF_MAP_TYPE_DEVMAP, sizeof(key), sizeof(value),
2, 0);
if (fd < 0) {
printf("Failed to create arraymap '%s'!\n", strerror(errno));
exit(1);
}

close(fd);
}

#define MAP_SIZE (32 * 1024)

static void test_map_large(void)
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