linux-kernel - Re: [PATCH 3/3] sfc: selftest: fix struct packing

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Message-Id: <7047eaa9-30b6-47bd-a878-7508449c9e20@app.fastmail.com>
Date:   Mon, 19 Jun 2023 16:55:27 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Edward Cree" <ecree.xilinx@...il.com>,
        "Arnd Bergmann" <arnd@...nel.org>,
        "Martin Habets" <habetsm.xilinx@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        "Eric Dumazet" <edumazet@...gle.com>,
        "Jakub Kicinski" <kuba@...nel.org>,
        "Paolo Abeni" <pabeni@...hat.com>
Cc:     Netdev <netdev@...r.kernel.org>, linux-net-drivers@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] sfc: selftest: fix struct packing

On Mon, Jun 19, 2023, at 12:25, Edward Cree wrote:
> On 19/06/2023 10:12, Arnd Bergmann wrote:
>> From: Arnd Bergmann <arnd@...db.de>
>> 
>> Three of the sfc drivers define a packed loopback_payload structure with an
>> ethernet header followed by an IP header. However, the kernel definition
>> of iphdr specifies that this is 4-byte aligned, causing a W=1 warning:
>> 
>> net/ethernet/sfc/siena/selftest.c:46:15: error: field ip within 'struct efx_loopback_payload' is less aligned than 'struct iphdr' and is usually due to 'struct efx_loopback_payload' being packed, which can lead to unaligned accesses [-Werror,-Wunaligned-access]
>>         struct iphdr ip;
>> 
>> As the iphdr packing is not easily changed without breaking other code,
>> change the three structures to use a local definition instead.
>> 
>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>
> Duplicating the definition isn't the prettiest thing in the world; it'd
>  do for a quick fix if needed but I assume W=1 warnings aren't blocking
>  anyone, so maybe defer this one for now and I'll follow up soon with a
>  rewrite that fixes this more cleanly?  My idea is to drop the __packed
>  from the containing struct, make efx_begin_loopback() copy the layers
>  separately, and efx_loopback_rx_packet() similarly do something less
>  direct than casting the packet data to the struct.
>
> But I don't insist on it; if you want this fix in immediately then I'm
>  okay with that too.

It's not urgent, if you can come up with a nicer solution, that is
probably better than applying my patch now. I have a patch [1] that
addresses -Wunaligned-access for all x86 and arm randconfig builds,
and this currently affects 23 drivers. Most of the changes don't
have changelog texts yet, and some need a more detailed analysis to
come up with a correct patch. I'd probably aim for linux-6.6 or
later to get them all done, at which point we could move the warning
from W=1 to the default set.

     Arnd

[1] https://pastebin.com/g6D9NTS4