The size of the imsf_slist_flex member is determined by imsf_numsrc, so
annotate it as such.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: netdev@vger.kernel.org
Cc: linux-hardening@vger.kernel.org
Acked-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/uapi/linux/in.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/in.h b/include/uapi/linux/in.h
index e682ab628dfa..2f0c4d40bc7b 100644
--- a/include/uapi/linux/in.h
+++ b/include/uapi/linux/in.h
@@ -199,7 +199,8 @@ struct ip_msfilter {
 	__u32		imsf_numsrc;
 	union {
 		__be32		imsf_slist[1];
-		__DECLARE_FLEX_ARRAY(__be32, imsf_slist_flex);
+		__DECLARE_FLEX_ARRAY(__be32, imsf_slist_flex,
+				     __counted_by(imsf_numsrc));
 	};
 };
 
-- 
2.34.1