Re: [PATCH net-next 1/8] inet: preserve const qualifier in inet_sk()

[Eric Dumazet <edumazet@google.com>]

On Wed, Mar 15, 2023 at 2:28 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Wed, 15 Mar 2023 15:42:38 +0000 Eric Dumazet wrote:
> > -static inline struct inet_sock *inet_sk(const struct sock *sk)
> > -{
> > -     return (struct inet_sock *)sk;
> > -}
> > +#define inet_sk(sk) \
> > +       _Generic(sk,                                                          \
> > +                const struct sock * : ((const struct inet_sock *)(sk)),      \
> > +                struct sock * : ((struct inet_sock *)(sk))           \
> > +       )
>
> Could we possibly use container_of_const() or define a new common
> macro for this downcast? I'm worried it will spread and it's a bit
> verbose.

I did see container_of_const() but the default: case was not appealing to me.

Maybe something like this?

diff --git a/include/linux/container_of.h b/include/linux/container_of.h
index 713890c867bea78804defe1a015e3c362f40f85d..9a24d8db1f4c46166c07589bb084eda9b9ede8ba
100644
--- a/include/linux/container_of.h
+++ b/include/linux/container_of.h
@@ -35,4 +35,10 @@
                default: ((type *)container_of(ptr, type, member))      \
        )

+#define promote_to_type(ptr, oldtype, newtype)                 \
+       _Generic(ptr,                                           \
+                const oldtype *: ((const newtype *)(ptr)),     \
+                oldtype *: ((newtype *)(ptr))                  \
+       )
+
 #endif /* _LINUX_CONTAINER_OF_H */
diff --git a/include/net/inet_sock.h b/include/net/inet_sock.h
index 6eb8235be67f8b8265cd86782aed2b489e8850ee..96f41b3c1a3e4b6ad1c6d2058ca46686be282c03
100644
--- a/include/net/inet_sock.h
+++ b/include/net/inet_sock.h
@@ -305,11 +305,7 @@ static inline struct sock *skb_to_full_sk(const
struct sk_buff *skb)
        return sk_to_full_sk(skb->sk);
 }

-#define inet_sk(sk) \
-       _Generic(sk,                                                    \
-                const struct sock * : ((const struct inet_sock
*)(sk)),        \
-                struct sock * : ((struct inet_sock *)(sk))             \
-       )
+#define inet_sk(sk) promote_to_type(sk, struct sock, struct inet_sock)

 static inline void __inet_sk_copy_descendant(struct sock *sk_to,
                                             const struct sock *sk_from,