Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct ip_sf_socklist.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Martin KaFai Lau <martin.lau@kernel.org>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Kees Cook <redacted>
---
 include/linux/igmp.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/igmp.h b/include/linux/igmp.h
index ebf4349a53af..5171231f70a8 100644
--- a/include/linux/igmp.h
+++ b/include/linux/igmp.h

@@ -39,7 +39,7 @@ struct ip_sf_socklist {
 	unsigned int		sl_max;
 	unsigned int		sl_count;
 	struct rcu_head		rcu;
-	__be32			sl_addr[];
+	__be32			sl_addr[] __counted_by(sl_max);
 };
 
 #define IP_SFBLOCK	10	/* allocate this many at once */

-- 
2.34.1