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

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Message-ID: <7c448f02-4031-0a90-97e2-0cc663b0cff9@gmail.com>
Date:   Mon, 19 Jun 2023 11:25:49 +0100
From:   Edward Cree <ecree.xilinx@...il.com>
To:     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:     Arnd Bergmann <arnd@...db.de>, netdev@...r.kernel.org,
        linux-net-drivers@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] sfc: selftest: fix struct packing

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];
>