Subject: Re: [PATCH] ioc3: fix incorrect use of htons/ntohs

From: Lino Sanfilippo <[email-masked]>
Date: Tue, 02 Dec 2014 02:31:12 +0100

On 01.12.2014 05:09, Ben Hutchings wrote:
> On Sun, 2014-11-30 at 11:40 +0100, Lino Sanfilippo wrote:
>> The protocol type in the ip header struct is a single byte variable. So there
>> is no need to swap bytes depending on host endianness.
>> 
>> Signed-off-by: Lino Sanfilippo <[email-masked]>
>> ---
>> 
>> Please note that I could not test this, since I dont have access to the
>> concerning hardware.
>> 
>>  drivers/net/ethernet/sgi/ioc3-eth.c | 5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
>> 
>> diff --git a/drivers/net/ethernet/sgi/ioc3-eth.c b/drivers/net/ethernet/sgi/ioc3-eth.c
>> index 7a254da..0bb303d 100644
>> --- a/drivers/net/ethernet/sgi/ioc3-eth.c
>> +++ b/drivers/net/ethernet/sgi/ioc3-eth.c
>> @@ -540,8 +540,7 @@ static void ioc3_tcpudp_checksum(struct sk_buff *skb, uint32_t hwsum, int len)
>>  
>>  	/* Same as tx - compute csum of pseudo header  */
>>  	csum = hwsum +
>> -	       (ih->tot_len - (ih->ihl << 2)) +
>> -	       htons((uint16_t)ih->protocol) +
>> +	       (ih->tot_len - (ih->ihl << 2)) + ih->protocol +
>>  	       (ih->saddr >> 16) + (ih->saddr & 0xffff) +
>>  	       (ih->daddr >> 16) + (ih->daddr & 0xffff);
>>
> 
> The pseudo-header is specified as:
> 
>                      +--------+--------+--------+--------+
>                      |           Source Address          |
>                      +--------+--------+--------+--------+
>                      |         Destination Address       |
>                      +--------+--------+--------+--------+
>                      |  zero  |  PTCL  |    TCP Length   |
>                      +--------+--------+--------+--------+
> 
> The current code zero-extends the protocol number to produce the 5th
> 16-bit word of the pseudo-header, then uses htons() to put it in
> big-endian order, consistent with the other fields.  (Yes, it's doing
> addition on big-endian words; this works even on little-endian machines
> due to the way the checksum is specified.)
> 
> The driver should not be doing this at all, though.  It should set
> skb->csum = hwsum; skb->ip_summed = CHECKSUM_COMPLETE; and let the
> network stack adjust the hardware checksum.
> 
>> @@ -1417,7 +1416,7 @@ static int ioc3_start_xmit(struct sk_buff *skb, struct net_device *dev)
>>  	 */
>>  	if (skb->ip_summed == CHECKSUM_PARTIAL) {
>>  		const struct iphdr *ih = ip_hdr(skb);
>> -		const int proto = ntohs(ih->protocol);
>> +		const int proto = ih->protocol;
>>  		unsigned int csoff;
>>  		uint32_t csum, ehsum;
>>  		uint16_t *eh;
> 
> This should logically be __be16 proto = htons(ih->protocol), but the
> current version should work in practice.
> 
> However, the driver should really use skb->csum_start and
> skb->csum_offset to work out where the checksum belongs and which bytes
> to cancel out.
> 
> Ben.
> 

Hi Ben,

youre right, the use of htons/ntohs is correct in this case. So thank
you for the explanation and please ignore that patch.

Regards,
Lino