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

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Date:   Tue, 19 Jan 2021 16:42:14 +0100
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     Jakub Kicinski <kuba@...nel.org>
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 28/12/2020 23.24, Jakub Kicinski wrote:
> 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?
> 

Jakub, can we apply this patch to net, then later move the definitions
out of uapi (and deleting the unused ones in the process)?

Thanks,
Rasmus



-- 
Rasmus Villemoes
Software Developer
Prevas A/S
Hedeager 3
DK-8200 Aarhus N
+45 51210274
rasmus.villemoes@...vas.dk
www.prevas.dk