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SubjectRe: [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@arndb.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@arndb.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

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