Date:	Fri, 12 Feb 2010 14:20:23 -0500
From:	Jeff Mahoney <jeffm@...e.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>
Subject: Re: [PATCH] delayacct: align to 8 byte boundary on 64-bit systems

On 02/12/2010 01:19 PM, Andrew Morton wrote:
> On Fri, 12 Feb 2010 11:48:27 -0500
> Jeff Mahoney <jeffm@...e.com> wrote:
> 
>>  prepare_reply sets up an skb for the response. If I understand it correctly,
>>  the payload contains:
>>
>>  +--------------------------------+
>>  | genlmsghdr - 4 bytes           |
>>  +--------------------------------+
>>  | NLA header - 4 bytes           | /* Aggregate header */
>>  +-+------------------------------+
>>  | | NLA header - 4 bytes         | /* PID header */
>>  | +------------------------------+
>>  | | pid/tgid   - 4 bytes         |
> 
> So we put another four zero bytes in here and add four to the "PID header".
> 
>>  | +------------------------------+
>>  | | NLA header - 4 bytes         | /* stats header */
>>  | + -----------------------------+ <- oops. aligned on 4 byte boundary
>>  | | struct taskstats - 328 bytes |
>>  +-+------------------------------+
>>
>>  The start of the taskstats struct must be 8 byte aligned on IA64 (and other
>>  systems with 8 byte alignment rules for 64-bit types) or runtime alignment
>>  warnings will be issued.
>>
>>  This patch pads the pid/tgid field out to sizeof(long), which forces
>>  the alignment of taskstats. The getdelays userspace code is ok with this
>>  since it assumes 32-bit pid/tgid and then honors that header's length field.
>>
>>  An array is used to avoid exposing kernel memory contents to userspace in the
>>  response.
>>
>> Signed-off-by: Jeff Mahoney <jeffm@...e.com>
>> ---
>>  kernel/taskstats.c |    8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> --- a/kernel/taskstats.c
>> +++ b/kernel/taskstats.c
>> @@ -362,6 +362,12 @@ static struct taskstats *mk_reply(struct
>>  	struct nlattr *na, *ret;
>>  	int aggr;
>>  
>> +	/* If we don't pad, we end up with alignment on a 4 byte boundary.
>> +	 * This causes lots of runtime warnings on systems requiring 8 byte
>> +	 * alignment */
>> +	u32 pids[2] = { pid, 0 };
>> +	int pid_size = ALIGN(sizeof(pid), sizeof(long));
>> +
>>  	aggr = (type == TASKSTATS_TYPE_PID)
>>  			? TASKSTATS_TYPE_AGGR_PID
>>  			: TASKSTATS_TYPE_AGGR_TGID;
>> @@ -369,7 +375,7 @@ static struct taskstats *mk_reply(struct
>>  	na = nla_nest_start(skb, aggr);
>>  	if (!na)
>>  		goto err;
>> -	if (nla_put(skb, type, sizeof(pid), &pid) < 0)
>> +	if (nla_put(skb, type, pid_size, pids) < 0)
>>  		goto err;
>>  	ret = nla_reserve(skb, TASKSTATS_TYPE_STATS, sizeof(struct taskstats));
>>  	if (!ret)
> 
> So any code which assumes that the pid/tgid field is four bytes long
> will break.  Code which takes that length from the netlink message
> header will work OK.
> 
> 32-bit architectures are unaltered.
> 
> Seems safe enough.  We'd be safer still if we didn't do this on 64-bit
> architectures which don't need it.  ie: x86_64.  But if we do that we
> add a risk that people will develop shoddy code which works on x86_64
> and doesn't work on ia64.

Is there a way to do that without needlessly complicating things? I
didn't see any existing infrastructure to do that.

Another option was to put an empty attribute in with a garbage type,
which would add a 4 byte header - but even the getdelays code included
with the kernel can't deal with that.

It's ugly all the way around.

-Jeff

-- 
Jeff Mahoney
SUSE Labs
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