netdev - Re: [PATCH net 1/2] net: mrp: fix definitions of MRP test packets

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Date:   Mon, 28 Dec 2020 14:24:11 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc:     netdev@...r.kernel.org,
        Horatiu Vultur <horatiu.vultur@...rochip.com>,
        linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net 1/2] net: mrp: fix definitions of MRP test packets

On Wed, 23 Dec 2020 15:45:32 +0100 Rasmus Villemoes wrote:
> Wireshark says that the MRP test packets cannot be decoded - and the
> reason for that is that there's a two-byte hole filled with garbage
> between the "transitions" and "timestamp" members.
> 
> So Wireshark decodes the two garbage bytes and the top two bytes of
> the timestamp written by the kernel as the timestamp value (which thus
> fluctuates wildly), and interprets the lower two bytes of the
> timestamp as a new (type, length) pair, which is of course broken.
> 
> While my copy of the MRP standard is still under way [*], I cannot
> imagine the standard specifying a two-byte hole here, and whoever
> wrote the Wireshark decoding code seems to agree with that.
> 
> The struct definitions live under include/uapi/, but they are not
> really part of any kernel<->userspace API/ABI, so fixing the
> definitions by adding the packed attribute should not cause any
> compatibility issues.
> 
> The remaining on-the-wire packet formats likely also don't contain
> holes, but pahole and manual inspection says the current definitions
> suffice. So adding the packed attribute to those is not strictly
> needed, but might be done for good measure.
> 
> [*] I will never understand how something hidden behind a +1000$
> paywall can be called a standard.
> 
> Signed-off-by: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
> ---
>  include/uapi/linux/mrp_bridge.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/mrp_bridge.h b/include/uapi/linux/mrp_bridge.h
> index 6aeb13ef0b1e..d1d0cf65916d 100644
> --- a/include/uapi/linux/mrp_bridge.h
> +++ b/include/uapi/linux/mrp_bridge.h
> @@ -96,7 +96,7 @@ struct br_mrp_ring_test_hdr {
>  	__be16 state;
>  	__be16 transitions;
>  	__be32 timestamp;
> -};
> +} __attribute__((__packed__));
>  
>  struct br_mrp_ring_topo_hdr {
>  	__be16 prio;
> @@ -141,7 +141,7 @@ struct br_mrp_in_test_hdr {
>  	__be16 state;
>  	__be16 transitions;
>  	__be32 timestamp;
> -};
> +} __attribute__((__packed__));
>  
>  struct br_mrp_in_topo_hdr {
>  	__u8 sa[ETH_ALEN];

Can we use this opportunity to move the definitions of these structures
out of the uAPI to a normal kernel header?