From: Gang Yan <yangang@kylinos.cn>

The 'overhead' in struct mptcp_data_frag can safely use u8, as it
represents 'alignment + sizeof(mptcp_data_frag)'. With a maximum
alignment of 7('ALIGN(1, sizeof(long)) - 1'), the overhead is at most
47, well below U8_MAX and validated with BUILD_BUG_ON().

This patch also adds a field named 'unused' for further extensions.

Signed-off-by: Gang Yan <yangang@kylinos.cn>
---
 net/mptcp/protocol.c | 4 ++++
 net/mptcp/protocol.h | 3 ++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 46fedfa05a54..01690a84ea6d 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -4624,6 +4624,10 @@ void __init mptcp_proto_init(void)
 	inet_register_protosw(&mptcp_protosw);
 
 	BUILD_BUG_ON(sizeof(struct mptcp_skb_cb) > sizeof_field(struct sk_buff, cb));
+	/* ensure 'overhead' (alignment + sizeof(struct mptcp_data_frag)) fits in u8.
+	 * 'ALIGN(1, sizeof(long)) - 1' represents the maximum of alignment.
+	 */
+	BUILD_BUG_ON(ALIGN(1, sizeof(long)) - 1 + sizeof(struct mptcp_data_frag) > U8_MAX);
 }
 
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index f5d4d7d030f2..afdead91a4d7 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -264,7 +264,8 @@ struct mptcp_data_frag {
 	u64 data_seq;
 	u16 data_len;
 	u16 offset;
-	u16 overhead;
+	u8 overhead;
+	u8 __unused;
 	u16 already_sent;
 	struct page *page;
 };
-- 
2.43.0

On Tue, 31 Mar 2026 17:08:08 +0800, Gang Yan <gang.yan@linux.dev> wrote:
> The 'overhead' in struct mptcp_data_frag can safely use u8, as it
> represents 'alignment + sizeof(mptcp_data_frag)'. With a maximum
> alignment of 7('ALIGN(1, sizeof(long)) - 1'), the overhead is at most
> 47, well below U8_MAX and validated with BUILD_BUG_ON().
> 
> This patch also adds a field named 'unused' for further extensions.
> 

Thank you, that's clearer like that in a dedicated patch.

We can probably add Paolo's suggested-by while at it.

Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Suggested-by: Paolo Abeni <pabeni@redhat.com>

-- 
Matthieu Baerts (NGI0) <matttbe@kernel.org>