netdev - [PATCH bpf-next 3/8] bpf: skmsg, replace comments with BUILD bug

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Date:   Thu, 20 Dec 2018 11:35:31 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     ast@...nel.org, daniel@...earbox.net
Cc:     netdev@...r.kernel.org, john.fastabend@...il.com
Subject: [PATCH bpf-next 3/8] 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@...earbox.net>
Signed-off-by: John Fastabend <john.fastabend@...il.com>
---
 include/linux/skmsg.h | 4 +---
 net/core/filter.c     | 3 +++
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
index eb8f6cb..dd57e6f4 100644
--- 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 6bd9f08..447dd1b 100644
--- 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),
-- 
2.7.4