bpf: skmsg, replace comments with BUILD bug

Enforce comment on structure layout dependency with a BUILD_BUG_ON
to ensure the condition is maintained.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
index eb8f6cb84c10be11118cc7238065bb35e8895df7..dd57e6f408b180ee7a851041dc919d524f8ed64c 100644 (file)
--- a/include/linux/skmsg.h
+++ b/include/linux/skmsg.h
@@ -36,9 +36,7 @@ struct sk_msg_sg {
        struct scatterlist              data[MAX_MSG_FRAGS + 1];
 };
 
-/* UAPI in filter.c depends on struct sk_msg_sg being first element. If
- * this is moved filter.c also must be updated.
- */
+/* UAPI in filter.c depends on struct sk_msg_sg being first element. */
 struct sk_msg {
        struct sk_msg_sg                sg;
        void                            *data;

diff --git a/net/core/filter.c b/net/core/filter.c
index 6bd9f08f6162fcf7d1e039eefbc664f116856001..447dd1bad31fa953e222304e60af5519b2621798 100644 (file)
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -7425,6 +7425,9 @@ static u32 sk_msg_convert_ctx_access(enum bpf_access_type type,
        int off;
 #endif
 
+       /* convert ctx uses the fact sg element is first in struct */
+       BUILD_BUG_ON(offsetof(struct sk_msg, sg) != 0);
+
        switch (si->off) {
        case offsetof(struct sk_msg_md, data):
                *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_msg, data),