[PATCH] ipv6: ioam: Replace 0-length array with flexible array

[Kees Cook <keescook@chromium.org>]

Zero-length arrays are deprecated[1]. Replace struct ioam6_trace_hdr's
"data" 0-length array with a flexible array. Detected with GCC 13,
using -fstrict-flex-arrays=3:

net/ipv6/ioam6_iptunnel.c: In function 'ioam6_build_state':
net/ipv6/ioam6_iptunnel.c:194:37: warning: array subscript <unknown> is outside array bounds of '__u8[0]' {aka 'unsigned char[]'} [-Warray-bounds=]
  194 |                 tuninfo->traceh.data[trace->remlen * 4] = IPV6_TLV_PADN;
      |                 ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
In file included from include/linux/ioam6.h:11,
                 from net/ipv6/ioam6_iptunnel.c:13:
include/uapi/linux/ioam6.h:130:17: note: while referencing 'data'
  130 |         __u8    data[0];
      |                 ^~~~

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Justin Iurman <justin.iurman@uliege.be>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/uapi/linux/ioam6.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/uapi/linux/ioam6.h b/include/uapi/linux/ioam6.h
index ac4de376f0ce..8f72b24fefb3 100644
--- a/include/uapi/linux/ioam6.h
+++ b/include/uapi/linux/ioam6.h
@@ -127,7 +127,7 @@ struct ioam6_trace_hdr {
 #endif
 
 #define IOAM6_TRACE_DATA_SIZE_MAX 244
-	__u8	data[0];
+	__u8	data[];
 } __attribute__((packed));
 
 #endif /* _UAPI_LINUX_IOAM6_H */
-- 
2.34.1