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

On Mar 24, 2026, at 6:12 PM, Martin KaFai Lau [off-list ref] wrote:

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On 3/18/26 6:42 AM, Nick Hudson wrote:

quoted

Introduce helper masks for bpf_skb_adjust_room() flags to simplify
validation logic:
- BPF_F_ADJ_ROOM_DECAP_L4_MASK
- BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK
- BPF_F_ADJ_ROOM_ENCAP_MASK
- BPF_F_ADJ_ROOM_DECAP_MASK
Add flag validation to bpf_skb_net_grow() to reject invalid encap
flags early. Refactor existing validation checks in bpf_skb_net_shrink()
and bpf_skb_adjust_room() to use the new masks (no behavior change).
Co-developed-by: Max Tottenham <redacted>
Signed-off-by: Max Tottenham <redacted>
Co-developed-by: Anna Glasgall <redacted>
Signed-off-by: Anna Glasgall <redacted>
Signed-off-by: Nick Hudson <redacted>
---
 net/core/filter.c | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 0d5d5a17acb2..7c2871b40fe4 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c

@@ -3483,14 +3483,25 @@ static u32 bpf_skb_net_base_len(const struct sk_buff *skb)
 #define BPF_F_ADJ_ROOM_DECAP_L3_MASK (BPF_F_ADJ_ROOM_DECAP_L3_IPV4 | \
   BPF_F_ADJ_ROOM_DECAP_L3_IPV6)
 -#define BPF_F_ADJ_ROOM_MASK (BPF_F_ADJ_ROOM_FIXED_GSO | \
-  BPF_F_ADJ_ROOM_ENCAP_L3_MASK | \
+#define BPF_F_ADJ_ROOM_DECAP_L4_MASK (BPF_F_ADJ_ROOM_DECAP_L4_UDP | \
+  BPF_F_ADJ_ROOM_DECAP_L4_GRE)
+
+#define BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK (BPF_F_ADJ_ROOM_DECAP_IPXIP4 | \
+  BPF_F_ADJ_ROOM_DECAP_IPXIP6)
+
+#define BPF_F_ADJ_ROOM_ENCAP_MASK (BPF_F_ADJ_ROOM_ENCAP_L3_MASK | \
   BPF_F_ADJ_ROOM_ENCAP_L4_GRE | \
   BPF_F_ADJ_ROOM_ENCAP_L4_UDP | \
   BPF_F_ADJ_ROOM_ENCAP_L2_ETH | \
   BPF_F_ADJ_ROOM_ENCAP_L2( \
-   BPF_ADJ_ROOM_ENCAP_L2_MASK) | \
-  BPF_F_ADJ_ROOM_DECAP_L3_MASK)
+   BPF_ADJ_ROOM_ENCAP_L2_MASK))
+
+#define BPF_F_ADJ_ROOM_DECAP_MASK (BPF_F_ADJ_ROOM_DECAP_L3_MASK)
+
+#define BPF_F_ADJ_ROOM_MASK (BPF_F_ADJ_ROOM_FIXED_GSO | \
+  BPF_F_ADJ_ROOM_ENCAP_MASK | \
+  BPF_F_ADJ_ROOM_DECAP_MASK | \
+  BPF_F_ADJ_ROOM_NO_CSUM_RESET)

The patch does two things: refactoring of existing macros (BPF_F_ADJ_ROOM_ENCAP_MASK, BPF_F_ADJ_ROOM_DECAP_MASK) and new additions (BPF_F_ADJ_ROOM_DECAP_L4_MASK, BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK) that depend on the new flags from the UAPI changes in patch 2.

The refactoring does not depend on the new UAPI flags and could be a separate patch placed earlier in the series. That way a reviewer can verify it is a no-op without the new flag additions getting in
the way. The (BPF_F_ADJ_ROOM_DECAP_L4_MASK, BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK) can be introduced together in patch 4 when it is first used.

OK, will split further.

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.

quoted

+ return -EINVAL;
+
  if (skb_is_gso(skb) && !skb_is_gso_tcp(skb)) {
  /* udp gso_size delineates datagrams, only allow if fixed */
  if (!(skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4) ||

@@ -3611,8 +3627,8 @@ static int bpf_skb_net_shrink(struct sk_buff *skb, u32 off, u32 len_diff,
 {
  int ret;
 - if (unlikely(flags & ~(BPF_F_ADJ_ROOM_FIXED_GSO |
-        BPF_F_ADJ_ROOM_DECAP_L3_MASK |
+ if (unlikely(flags & ~(BPF_F_ADJ_ROOM_DECAP_MASK |
+        BPF_F_ADJ_ROOM_FIXED_GSO |
         BPF_F_ADJ_ROOM_NO_CSUM_RESET)))
  return -EINVAL;
 @@ -3708,8 +3724,7 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
  u32 off;
  int ret;
 - if (unlikely(flags & ~(BPF_F_ADJ_ROOM_MASK |
-        BPF_F_ADJ_ROOM_NO_CSUM_RESET)))
+ if (unlikely(flags & ~BPF_F_ADJ_ROOM_MASK))
  return -EINVAL;
  if (unlikely(len_diff_abs > 0xfffU))
  return -EFAULT;