[PATCH net-next] selftests/net/ipsec: Fix variable size type not at the end of struct

[Ankit Khushwaha <ankitkhushwaha.linux@gmail.com>]

The "struct alg" object contains a union of 3 xfrm structures:

	union {
		struct xfrm_algo;
		struct xfrm_algo_aead;
		struct xfrm_algo_auth;
	}

All of them end with a flexible array member used to store key material,
but the flexible array appears at *different offsets* in each struct.
bcz of this, union itself is of variable-sized & Placing it above
char buf[...] triggers:

ipsec.c:835:5: warning: field 'u' with variable sized type 'union
(unnamed union at ipsec.c:831:3)' not at the end of a struct or class
is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
  835 |                 } u;
      |                   ^

one fix is to use "TRAILING_OVERLAP()" which works with one flexible
array member only.

But In "struct alg" flexible array member exists in all union members,
but not at the same offset, so TRAILING_OVERLAP cannot be applied.

so the fix is to explicitly overlay the key buffer at the correct offset
for the largest union member (xfrm_algo_auth). This ensures that the
flexible-array region and the fixed buffer line up.

No functional change.

Signed-off-by: Ankit Khushwaha <ankitkhushwaha.linux@gmail.com>
---
 tools/testing/selftests/net/ipsec.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/net/ipsec.c b/tools/testing/selftests/net/ipsec.c
index 0ccf484b1d9d..f4afef51b930 100644
--- a/tools/testing/selftests/net/ipsec.c
+++ b/tools/testing/selftests/net/ipsec.c
@@ -43,6 +43,10 @@

 #define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))

+#ifndef offsetof
+#define offsetof(TYPE, MEMBER)	__builtin_offsetof(TYPE, MEMBER)
+#endif
+
 #define IPV4_STR_SZ	16	/* xxx.xxx.xxx.xxx is longest + \0 */
 #define MAX_PAYLOAD	2048
 #define XFRM_ALGO_KEY_BUF_SIZE	512
@@ -827,13 +831,16 @@ static int xfrm_fill_key(char *name, char *buf,
 static int xfrm_state_pack_algo(struct nlmsghdr *nh, size_t req_sz,
 		struct xfrm_desc *desc)
 {
-	struct {
+	union {
 		union {
 			struct xfrm_algo	alg;
 			struct xfrm_algo_aead	aead;
 			struct xfrm_algo_auth	auth;
 		} u;
-		char buf[XFRM_ALGO_KEY_BUF_SIZE];
+		struct {
+			unsigned char __offset_to_FAM[offsetof(struct xfrm_algo_auth, alg_key)];
+			char buf[XFRM_ALGO_KEY_BUF_SIZE];
+		};
 	} alg = {};
 	size_t alen, elen, clen, aelen;
 	unsigned short type;
--
2.52.0