Re: [PATCH v2 3/5] bpf: add helper masks for ADJ_ROOM flags and encap validation

On Mar 26, 2026, at 5:49 PM, Martin KaFai Lau [off-list ref] wrote:

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On 3/26/26 10:02 AM, Hudson, Nick wrote:

quoted

quoted

quoted

   static int bpf_skb_net_grow(struct sk_buff *skb, u32 off, u32 len_diff,
      u64 flags)

@@ -3502,6 +3513,11 @@ static int bpf_skb_net_grow(struct sk_buff *skb, u32 off, u32 len_diff,
  unsigned int gso_type = SKB_GSO_DODGY;
  int ret;
 + if (unlikely(flags & ~(BPF_F_ADJ_ROOM_ENCAP_MASK |
+        BPF_F_ADJ_ROOM_NO_CSUM_RESET |
+        BPF_F_ADJ_ROOM_FIXED_GSO)))

Under which case this new check will be hit?

If a user supplies +ve len_diff and attempts to pass a DECAP flag.
The commit message had
    Add flag validation to bpf_skb_net_grow() to reject invalid encap
    flags early.

There is DECAP_MASK check in bpf_skb_adjust_room() and then !shrink is rejected. What am I missing?

Duh, right.

Do you prefer the do all the flag checking in bpf_skb_adjust_room or keep the encap/decap split?