[PATCH bpf-next v2] bpf: Return an error pointer for skb metadata when CONFIG_NET=n

[PATCH bpf-next v2] bpf: Return an error pointer for skb metadata when CONFIG_NET=n

[Jakub Sitnicki]

Kernel Test Robot reported a compiler warning - a null pointer may be
passed to memmove in __bpf_dynptr_{read,write} when building without
networking support.

The warning is correct from a static analysis standpoint, but not actually
reachable. Without CONFIG_NET, creating dynptrs to skb metadata is
impossible since the constructor kfunc is missing.

Silence the false-postive diagnostic message by returning an error pointer
from bpf_skb_meta_pointer stub when CONFIG_NET=n.

Fixes: 6877cd392bae ("bpf: Enable read/write access to skb metadata through a dynptr")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202508212031.ir9b3B6Q-lkp@intel.com/
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
---
Changes in v2:
- Switch to an error pointer (Alexei)
- Link to v1: https://lore.kernel.org/r/20250827-dynptr-skb-meta-no-net-v1-1-42695c402b16@cloudflare.com
---
 include/linux/filter.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 9092d8ea95c8..4241a885975f 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1816,7 +1816,7 @@ static inline void bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off, voi
 
 static inline void *bpf_skb_meta_pointer(struct sk_buff *skb, u32 offset)
 {
-	return NULL;
+	return ERR_PTR(-EOPNOTSUPP);
 }
 #endif /* CONFIG_NET */