netdev - Re: [PATCH net v2 1/2] net: mrp: fix definitions of MRP test packets

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Date:   Thu, 21 Jan 2021 22:23:40 +0100
From:   Horatiu Vultur <horatiu.vultur@...rochip.com>
To:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>
CC:     <netdev@...r.kernel.org>, Andrew Lunn <andrew@...n.ch>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net v2 1/2] net: mrp: fix definitions of MRP test packets

The 01/21/2021 21:40, Rasmus Villemoes wrote:

It seems that is missing a Fixes tag, other than that it looks fine.

Reviewed-by: Horatiu Vultur <horatiu.vultur@...rochip.com>

> 
> 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.
> 
> Even though this makes the timestamp field in the struct unaligned, it
> actually makes it end up on a 32 bit boundary in the frame as mandated
> by the standard, since it is preceded by a two byte TLV header.
> 
> 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.
> 
> 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];
> --
> 2.23.0
> 

-- 
/Horatiu