Re: [PATCH bpf-next 03/13] bpf: Add new variant of skb dynptr for the metadata area

On Mon, Jun 30, 2025 at 09:27 AM -07, Stanislav Fomichev wrote:

On 06/30, Jakub Sitnicki wrote:

quoted

Add a new flag for the bpf_dynptr_from_skb helper to let users to create
dynptrs to skb metadata area. Access paths are stubbed out. Implemented by
the following changes.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
---
 include/uapi/linux/bpf.h |  9 ++++++++
 net/core/filter.c        | 60 +++++++++++++++++++++++++++++++++++++++++-------
 2 files changed, 61 insertions(+), 8 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 719ba230032f..ab5730d2fb29 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -7591,4 +7591,13 @@ enum bpf_kfunc_flags {
 	BPF_F_PAD_ZEROS = (1ULL << 0),
 };
 
+/**
+ * enum bpf_dynptr_from_skb_flags - Flags for bpf_dynptr_from_skb()
+ *
+ * @BPF_DYNPTR_F_SKB_METADATA: Create dynptr to the SKB metadata area
+ */
+enum bpf_dynptr_from_skb_flags {
+	BPF_DYNPTR_F_SKB_METADATA = (1ULL << 0),
+};
+
 #endif /* _UAPI__LINUX_BPF_H__ */

diff --git a/net/core/filter.c b/net/core/filter.c
index 1fee51b72220..3c2948517838 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c

@@ -11967,12 +11967,27 @@ bpf_sk_base_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 	return func;
 }
 
+enum skb_dynptr_offset {
+	SKB_DYNPTR_METADATA	= -1,

nit: any reason not do make it 1? The offset is u32, so that -1 reads a bit
intentional and I don't get the intention :-)

Since we're abusing the "offset" field to serve as an enum tag, I
figured seeing 0xffffffff in a memory dump will be an clear indication
that this is not an offset.

Also, metadata comes before payload, like -1 does before 0...

JK of course. No preference here. Went with my gut.