netdev - Re: [PATCH] tcp: fix tcp_fastopen unaligned access complaints on sparc

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Message-ID: <cd152a9f-4855-a552-2b45-bd619436a628@oracle.com>
Date:   Thu, 12 Jan 2017 13:15:41 -0700
From:   Rob Gardner <rob.gardner@...cle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Shannon Nelson <shannon.nelson@...cle.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tcp: fix tcp_fastopen unaligned access complaints on
 sparc

On 01/12/2017 01:13 PM, Eric Dumazet wrote:
> On Thu, 2017-01-12 at 11:59 -0800, Shannon Nelson wrote:
>> Fix up a data alignment issue on sparc by swapping the order
>> of the cookie byte array field with the length field in
>> struct tcp_fastopen_cookie
>>
>> This addresses log complaints like these:
>>      log_unaligned: 113 callbacks suppressed
>>      Kernel unaligned access at TPC[976490] tcp_try_fastopen+0x2d0/0x360
>>      Kernel unaligned access at TPC[9764ac] tcp_try_fastopen+0x2ec/0x360
>>      Kernel unaligned access at TPC[9764c8] tcp_try_fastopen+0x308/0x360
>>      Kernel unaligned access at TPC[9764e4] tcp_try_fastopen+0x324/0x360
>>      Kernel unaligned access at TPC[976490] tcp_try_fastopen+0x2d0/0x360
>>
>> Signed-off-by: Shannon Nelson <shannon.nelson@...cle.com>
>> ---
>>   include/linux/tcp.h |    2 +-
>>   1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
>> index fc5848d..95cda75 100644
>> --- a/include/linux/tcp.h
>> +++ b/include/linux/tcp.h
>> @@ -62,8 +62,8 @@ static inline unsigned int tcp_optlen(const struct sk_buff *skb)
>>   
>>   /* TCP Fast Open Cookie as stored in memory */
>>   struct tcp_fastopen_cookie {
>> -	s8	len;
>>   	u8	val[TCP_FASTOPEN_COOKIE_MAX];
>> +	s8	len;
>>   	bool	exp;	/* In RFC6994 experimental option format */
>>   };
>>   
> Strange... Do you have an explanation of why this patch would be
> needed ? A compiler issue ?
>
>
> s8 and u8 are bytes after all.
>
>


I suspect that someplace, somebody is casting val to an int * or 
something like that.