netdev - Re: [PATCH net-next 09/10] mptcp: preserve const qualifier in mptcp

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Date:   Fri, 17 Mar 2023 18:32:02 +0100
From:   Matthieu Baerts <matthieu.baerts@...sares.net>
To:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, David Ahern <dsahern@...nel.org>,
        Simon Horman <simon.horman@...igine.com>,
        Willem de Bruijn <willemb@...gle.com>,
        eric.dumazet@...il.com, MPTCP Upstream <mptcp@...ts.linux.dev>
Subject: Re: [PATCH net-next 09/10] mptcp: preserve const qualifier in
 mptcp_sk()

Hi Eric,

On 17/03/2023 16:55, Eric Dumazet wrote:
> We can change mptcp_sk() to propagate its argument const qualifier,
> thanks to container_of_const().
> 
> We need to change few things to avoid build errors:
> 
> mptcp_set_datafin_timeout() and mptcp_rtx_head() have to accept
> non-const sk pointers.
> 
> @msk local variable in mptcp_pending_tail() must be const.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Matthieu Baerts <matthieu.baerts@...sares.net>

Good idea!

Thank you for this patch and for having Cced me.

It looks good to me. I just have one question below if you don't mind.

(...)

> diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
> index 61fd8eabfca2028680e04558b4baca9f48bbaaaa..4ed8ffffb1ca473179217e640a23bc268742628d 100644
> --- a/net/mptcp/protocol.h
> +++ b/net/mptcp/protocol.h

(...)

> @@ -381,7 +378,7 @@ static inline struct mptcp_data_frag *mptcp_pending_tail(const struct sock *sk)
>  	return list_last_entry(&msk->rtx_queue, struct mptcp_data_frag, list);
>  }
>  
> -static inline struct mptcp_data_frag *mptcp_rtx_head(const struct sock *sk)
> +static inline struct mptcp_data_frag *mptcp_rtx_head(struct sock *sk)

It was not clear to me why you had to remove the "const" qualifier here
and not just have to add one when assigning the msk just below. But then
I looked at what was behind the list_first_entry_or_null() macro used in
this function and understood what was the issue.


My naive approach would be to modify this macro but I guess we don't
want to go down that road, right?

-------------------- 8< --------------------
diff --git a/include/linux/list.h b/include/linux/list.h
index f10344dbad4d..cd770766f451 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -550,7 +550,7 @@ static inline void list_splice_tail_init(struct
list_head *list,
  * Note that if the list is empty, it returns NULL.
  */
 #define list_first_entry_or_null(ptr, type, member) ({ \
-       struct list_head *head__ = (ptr); \
+       const struct list_head *head__ = (ptr); \
        struct list_head *pos__ = READ_ONCE(head__->next); \
        pos__ != head__ ? list_entry(pos__, type, member) : NULL; \
 })
-------------------- 8< --------------------


It looks safe to me to do that but I would not trust myself on a Friday
evening :)
(I'm sure I'm missing something, I'm sorry if it is completely wrong)

Anyway if we cannot modify list_first_entry_or_null() one way or
another, I'm fine with your modification!

Reviewed-by: Matthieu Baerts <matthieu.baerts@...sares.net>

Cheers,
Matt

>  {
>  	struct mptcp_sock *msk = mptcp_sk(sk);
>  

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