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Date:   Sun, 3 Jan 2021 14:29:50 +0100
From:   Horatiu Vultur <horatiu.vultur@microchip.com>
To:     Jakub Kicinski <kuba@kernel.org>
CC:     Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
        <netdev@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH net 1/2] net: mrp: fix definitions of MRP test packets
Message-ID: <20210103132950.khcn3kzmwrai5fxx@soft-dev3.localdomain>
References: <20201223144533.4145-1-rasmus.villemoes@prevas.dk>
 <20201223144533.4145-2-rasmus.villemoes@prevas.dk>
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The 12/28/2020 14:24, Jakub Kicinski wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> 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@prevas.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?

Or maybe we can just remove them, especially if they are not used by the
kernel.

-- 
/Horatiu