We don't currently allow lookups into a devmap from eBPF, because the map
lookup returns a pointer directly to the dev->ifindex, which shouldn't be
modifiable from eBPF.
However, being able to do lookups in devmaps is useful to know (e.g.)
whether forwarding to a specific interface is enabled. Currently, programs
work around this by keeping a shadow map of another type which indicates
whether a map index is valid.
To allow lookups, simply copy the ifindex into a scratch variable and
return a pointer to this. If an eBPF program does modify it, this doesn't
matter since it will be overridden on the next lookup anyway. While this
does add a write to every lookup, the overhead of this is negligible
because the cache line is hot when both the write and the subsequent read
happens.
Signed-off-by: Toke Høiland-Jørgensen <t...@redhat.com>
---
kernel/bpf/devmap.c | 8 +++++++-
kernel/bpf/verifier.c | 7 ++-----
2 files changed, 9 insertions(+), 6 deletions(-)
diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index 5ae7cce5ef16..830650300ea4 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -65,6 +65,7 @@ struct xdp_bulk_queue {
struct bpf_dtab_netdev {
struct net_device *dev; /* must be first member, due to tracepoint */
struct bpf_dtab *dtab;
+ int ifindex_scratch;
unsigned int bit;
struct xdp_bulk_queue __percpu *bulkq;
struct rcu_head rcu;
@@ -375,7 +376,12 @@ static void *dev_map_lookup_elem(struct bpf_map *map, void
*key)
struct bpf_dtab_netdev *obj = __dev_map_lookup_elem(map, *(u32 *)key);
struct net_device *dev = obj ? obj->dev : NULL;
- return dev ? &dev->ifindex : NULL;
+ if (dev) {
+ obj->ifindex_scratch = dev->ifindex;
+ return &obj->ifindex_scratch;
+ }
+
+ return NULL;
}
static void dev_map_flush_old(struct bpf_dtab_netdev *dev)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5c2cb5bd84ce..7128a9821481 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2893,12 +2893,9 @@ static int check_map_func_compatibility(struct
bpf_verifier_env *env,
if (func_id != BPF_FUNC_get_local_storage)
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_redirect_map)
+ if (func_id != BPF_FUNC_redirect_map &&
+ func_id != BPF_FUNC_map_lookup_elem)
goto error;
break;
/* Restrict bpf side of cpumap and xskmap, open when use-cases
