Re: [PATCH] bpf: use RCU-safe iteration in dev_map_redirect_multi() SKB path

On 3/19/26 6:37 PM, Martin KaFai Lau wrote:

On 3/18/26 9:24 AM, David Carlier wrote:

quoted

The DEVMAP_HASH branch in dev_map_redirect_multi() uses
hlist_for_each_entry_safe() to iterate hash buckets, but this function
runs under RCU protection (called from xdp_do_generic_redirect_map()
in softirq context). Concurrent writers (__dev_map_hash_update_elem,
dev_map_hash_delete_elem) modify the list using RCU primitives
(hlist_add_head_rcu, hlist_del_rcu).

hlist_for_each_entry_safe() performs plain pointer dereferences without
rcu_dereference(), missing the acquire barrier needed to pair with
writers' rcu_assign_pointer(). On weakly-ordered architectures (ARM64,
POWER), a reader can observe a partially-constructed node. It also
defeats CONFIG_PROVE_RCU lockdep validation and KCSAN data-race
detection.

Replace with hlist_for_each_entry_rcu(), matching the XDP-frame path
(dev_map_enqueue_multi) which already uses the correct macro for the
same hash iteration.

quoted

Fixes: e624d4ed4aa8 ("xdp: Extend xdp_redirect_map with broadcast 
support")
Signed-off-by: David Carlier <redacted>
---
  kernel/bpf/devmap.c | 3 +--
  1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index 3d619d01088e..c8d256405c29 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c

@@ -747,7 +747,6 @@ int dev_map_redirect_multi(struct net_device *dev, 

struct sk_buff *skb,
      struct bpf_dtab_netdev *dst, *last_dst = NULL;
      int excluded_devices[1+MAX_NEST_DEV];
      struct hlist_head *head;
-    struct hlist_node *next;
      int num_excluded = 0;
      unsigned int i;
      int err;

@@ -787,7 +786,7 @@ int dev_map_redirect_multi(struct net_device *dev, 

struct sk_buff *skb,
      } else { /* BPF_MAP_TYPE_DEVMAP_HASH */
          for (i = 0; i < dtab->n_buckets; i++) {
              head = dev_map_index_hash(dtab, i);
-            hlist_for_each_entry_safe(dst, next, head, index_hlist) {
+            hlist_for_each_entry_rcu(dst, head, index_hlist, 
lockdep_is_held(&dtab->index_lock)) {he 

Where is the dtab->index_lock acquired? dev_map_enqueue_multi() has been 
incorrect also. Take a look at the rcu_read_lock_bh_held() usage in the 
rcu_dereference_check() a few lines above.

pw-bot: cr

Please cc the bpf list and tag the target tree in the subject. imo, bpf- 
next instead of bpf should be fine for this.

cc bpf list.

quoted

                  if (is_ifindex_excluded(excluded_devices, num_excluded,
                              dst->dev->ifindex))
                      continue;