Subject: Re: [PATCH v2] net: sock: move ->sk_shutdown out of bitfields.

From: Andrey Ryabinin <[email-masked]>
Date: Wed, 18 May 2016 16:14:36 +0300

On 05/18/2016 04:03 PM, Eric Dumazet wrote:
> On Wed, 2016-05-18 at 15:03 +0300, Andrey Ryabinin wrote:
>> ->sk_shutdown bits share one bitfield with some other bits in sock struct,
>> such as ->sk_no_check_[r,t]x, ->sk_userlocks ...
>> sock_setsockopt() may write to these bits, while holding the socket lock.
>>
>> In case of AF_UNIX sockets, we change ->sk_shutdown bits while holding only
>> unix_state_lock(). So concurrent setsockopt() and shutdown() may lead
>> to corrupting these bits.
>>
>> Fix this by moving ->sk_shutdown bits out of bitfield into a separate byte.
>> This will not change the 'struct sock' size since ->sk_shutdown moved into
>> previously unused 16-bit hole.
>>
>> Signed-off-by: Andrey Ryabinin <[email-masked]>
>> Suggested-by: Hannes Frederic Sowa <[email-masked]>
>> ---
>>  include/net/sock.h | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/net/sock.h b/include/net/sock.h
>> index c9c8b19..04dc131 100644
>> --- a/include/net/sock.h
>> +++ b/include/net/sock.h
>> @@ -383,8 +383,7 @@ struct sock {
>>  	int			sk_sndbuf;
>>  	struct sk_buff_head	sk_write_queue;
>>  	kmemcheck_bitfield_begin(flags);
>> -	unsigned int		sk_shutdown  : 2,
> 
> 
> Please replace by a padding, so that sk_protocol is sill a byte,
> not 8 bits spaning 2 bytes in memory.

I think, it would be better to have something like this:

	u16 sk_type;
	u8 sk_protocol;
	kmemcheck_bitfield_begin(flags);
	u8			sk_no_check_tx : 1,
				sk_no_check_rx : 1,
				sk_userlocks : 4,
	kmemcheck_bitfield_end(flags);