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 fib_info.

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

Cc: "David S. Miller" <da...@davemloft.net>
Cc: David Ahern <dsah...@kernel.org>
Cc: Eric Dumazet <eduma...@google.com>
Cc: Jakub Kicinski <k...@kernel.org>
Cc: Paolo Abeni <pab...@redhat.com>
Cc: net...@vger.kernel.org
Signed-off-by: Kees Cook <keesc...@chromium.org>
---
 include/net/ip_fib.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/ip_fib.h b/include/net/ip_fib.h
index f0c13864180e..84b0a82c9df4 100644
--- a/include/net/ip_fib.h
+++ b/include/net/ip_fib.h
@@ -156,7 +156,7 @@ struct fib_info {
        bool                    nh_updated;
        struct nexthop          *nh;
        struct rcu_head         rcu;
-       struct fib_nh           fib_nh[];
+       struct fib_nh           fib_nh[] __counted_by(fib_nhs);
 };
 
 
-- 
2.34.1


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