Subject: Re: [PATCH v7] Add payload to be 32-bit aligned to fix dropped packets

From: Eric Dumazet <[email-masked]>
Date: Thu, 18 Nov 2021 09:14:00 -0800

On 11/18/21 8:03 AM, Kumar Thangavel wrote:
> Update NC-SI command handler (both standard and OEM) to take into
> account of payload paddings in allocating skb (in case of payload
> size is not 32-bit aligned).
> 
> The checksum field follows payload field, without taking payload
> padding into account can cause checksum being truncated, leading to
> dropped packets.
> 

Patch title should start with a prefix, identifying which layer/driver is
involved.

Probably in this case

net/ncsi: Add payload to be 32-bit aligned to fix dropped packets

> Fixes: fb4ee67529ff ("net/ncsi: Add NCSI OEM command support")
> Signed-off-by: Kumar Thangavel <[email-masked]>
> Acked-by: Samuel Mendoza-Jonas <[email-masked]>
> Reviewed-by: Paul Menzel <[email-masked]>
> 
> ---
>   v7:
>    - Updated padding_bytes as const static int variable
> 
>   v6:
>    - Updated type of padding_bytes variable
>    - Updated type of payload
>    - Seperated variable declarations and code
> 
>   v5:
>    - Added Fixes tag
>    - Added const variable for padding_bytes
> 
>   v4:
>    - Used existing macro for max function
> 
>   v3:
>    - Added Macro for MAX
>    - Fixed the missed semicolon
> 
>   v2:
>    - Added NC-SI spec version and section
>    - Removed blank line
>    - corrected spellings
> 
>   v1:
>    - Initial draft
> 
> ---
> ---
>  net/ncsi/ncsi-cmd.c | 24 ++++++++++++++++--------
>  1 file changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/net/ncsi/ncsi-cmd.c b/net/ncsi/ncsi-cmd.c
> index ba9ae482141b..9a6f10f4833e 100644
> --- a/net/ncsi/ncsi-cmd.c
> +++ b/net/ncsi/ncsi-cmd.c
> @@ -18,6 +18,8 @@
>  #include "internal.h"
>  #include "ncsi-pkt.h"
>  
> +const static int padding_bytes = 26;

It would be nice to have some kind of reference, to explain
what is this magic value.

Moving the comment [1] here might make sense.

> +
>  u32 ncsi_calculate_checksum(unsigned char *data, int len)
>  {
>  	u32 checksum = 0;
> @@ -213,12 +215,17 @@ static int ncsi_cmd_handler_oem(struct sk_buff *skb,
>  {
>  	struct ncsi_cmd_oem_pkt *cmd;
>  	unsigned int len;
> +	int payload;

[1] This comment could be moved before @padding_bytes

> +	/* NC-SI spec DSP_0222_1.2.0, section 8.2.2.2
> +	 * requires payload to be padded with 0 to
> +	 * 32-bit boundary before the checksum field.
> +	 * Ensure the padding bytes are accounted for in
> +	 * skb allocation
> +	 */
>  
> +	payload = ALIGN(nca->payload, 4);
>  	len = sizeof(struct ncsi_cmd_pkt_hdr) + 4;
> -	if (nca->payload < 26)
> -		len += 26;
> -	else
> -		len += nca->payload;
> +	len += max(payload, padding_bytes);
>  
>  	cmd = skb_put_zero(skb, len);
>  	memcpy(&cmd->mfr_id, nca->data, nca->payload);
> @@ -272,6 +279,7 @@ static struct ncsi_request *ncsi_alloc_command(struct ncsi_cmd_arg *nca)
>  	struct net_device *dev = nd->dev;
>  	int hlen = LL_RESERVED_SPACE(dev);
>  	int tlen = dev->needed_tailroom;
> +	int payload;
>  	int len = hlen + tlen;
>  	struct sk_buff *skb;
>  	struct ncsi_request *nr;
> @@ -281,14 +289,14 @@ static struct ncsi_request *ncsi_alloc_command(struct ncsi_cmd_arg *nca)
>  		return NULL;
>  
>  	/* NCSI command packet has 16-bytes header, payload, 4 bytes checksum.
> +	 * Payload needs padding so that the checksum field following payload is
> +	 * aligned to 32-bit boundary.
>  	 * The packet needs padding if its payload is less than 26 bytes to
>  	 * meet 64 bytes minimal ethernet frame length.
>  	 */
>  	len += sizeof(struct ncsi_cmd_pkt_hdr) + 4;
> -	if (nca->payload < 26)
> -		len += 26;
> -	else
> -		len += nca->payload;
> +	payload = ALIGN(nca->payload, 4);
> +	len += max(payload, padding_bytes);
>  
>  	/* Allocate skb */
>  	skb = alloc_skb(len, GFP_ATOMIC);
>