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

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Message-Id: <e3e9f246-5bf7-4868-aedb-0e194f52de5b@app.fastmail.com>
Date:   Mon, 19 Jun 2023 15:04:10 +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.
>
>> ---
>>  drivers/net/ethernet/sfc/falcon/selftest.c | 21 ++++++++++++++++++++-
>>  drivers/net/ethernet/sfc/selftest.c        | 21 ++++++++++++++++++++-
>>  drivers/net/ethernet/sfc/siena/selftest.c  | 21 ++++++++++++++++++++-
>>  3 files changed, 60 insertions(+), 3 deletions(-)
>> 
>> diff --git a/drivers/net/ethernet/sfc/falcon/selftest.c b/drivers/net/ethernet/sfc/falcon/selftest.c
>> index 6a454ac6f8763..fb7fcd27a33a5 100644
>> --- a/drivers/net/ethernet/sfc/falcon/selftest.c
>> +++ b/drivers/net/ethernet/sfc/falcon/selftest.c
>> @@ -40,7 +40,26 @@
>>   */
>>  struct ef4_loopback_payload {
>>  	struct ethhdr header;
>> -	struct iphdr ip;
>> +	struct {
>> +#if defined(__LITTLE_ENDIAN_BITFIELD)
>> +		__u8	ihl:4,
>> +			version:4;
>> +#elif defined (__BIG_ENDIAN_BITFIELD)
>> +		__u8	version:4,
>> +			ihl:4;
>> +#else
>> +#error	"Please fix <asm/byteorder.h>"
>> +#endif
>> +		__u8	tos;
>> +		__be16	tot_len;
>> +		__be16	id;
>> +		__be16	frag_off;
>> +		__u8	ttl;
>> +		__u8	protocol;
>> +		__sum16	check;
>> +		__be32	saddr;
>> +		__be32	daddr;
>> +	} __packed ip; /* unaligned struct iphdr */
>>  	struct udphdr udp;
>>  	__be16 iteration;
>>  	char msg[64];
>> diff --git a/drivers/net/ethernet/sfc/selftest.c b/drivers/net/ethernet/sfc/selftest.c
>> index 3c5227afd4977..440a57953779c 100644
>> --- a/drivers/net/ethernet/sfc/selftest.c
>> +++ b/drivers/net/ethernet/sfc/selftest.c
>> @@ -43,7 +43,26 @@
>>   */
>>  struct efx_loopback_payload {
>>  	struct ethhdr header;
>> -	struct iphdr ip;
>> +	struct {
>> +#if defined(__LITTLE_ENDIAN_BITFIELD)
>> +		__u8	ihl:4,
>> +			version:4;
>> +#elif defined (__BIG_ENDIAN_BITFIELD)
>> +		__u8	version:4,
>> +			ihl:4;
>> +#else
>> +#error	"Please fix <asm/byteorder.h>"
>> +#endif
>> +		__u8	tos;
>> +		__be16	tot_len;
>> +		__be16	id;
>> +		__be16	frag_off;
>> +		__u8	ttl;
>> +		__u8	protocol;
>> +		__sum16	check;
>> +		__be32	saddr;
>> +		__be32	daddr;
>> +	} __packed ip; /* unaligned struct iphdr */
>>  	struct udphdr udp;
>>  	__be16 iteration;
>>  	char msg[64];
>> diff --git a/drivers/net/ethernet/sfc/siena/selftest.c b/drivers/net/ethernet/sfc/siena/selftest.c
>> index 07715a3d6beab..b8a8b0495f661 100644
>> --- a/drivers/net/ethernet/sfc/siena/selftest.c
>> +++ b/drivers/net/ethernet/sfc/siena/selftest.c
>> @@ -43,7 +43,26 @@
>>   */
>>  struct efx_loopback_payload {
>>  	struct ethhdr header;
>> -	struct iphdr ip;
>> +	struct {
>> +#if defined(__LITTLE_ENDIAN_BITFIELD)
>> +		__u8	ihl:4,
>> +			version:4;
>> +#elif defined (__BIG_ENDIAN_BITFIELD)
>> +		__u8	version:4,
>> +			ihl:4;
>> +#else
>> +#error	"Please fix <asm/byteorder.h>"
>> +#endif
>> +		__u8	tos;
>> +		__be16	tot_len;
>> +		__be16	id;
>> +		__be16	frag_off;
>> +		__u8	ttl;
>> +		__u8	protocol;
>> +		__sum16	check;
>> +		__be32	saddr;
>> +		__be32	daddr;
>> +	} __packed ip; /* unaligned struct iphdr */
>>  	struct udphdr udp;
>>  	__be16 iteration;
>>  	char msg[64];
>>