Re: [PATCH v2 02/16] scsi: don't use fc_bsg_job::request and fc_bsg_job::reply directly

[Steffen Maier <maier@linux.vnet.ibm.com>]

On 10/13/2016 06:24 PM, Johannes Thumshirn wrote:
> On Thu, Oct 13, 2016 at 05:15:25PM +0200, Steffen Maier wrote:
>> I'm puzzled.
>>
>> $ git bisect start fc_bsg master

>>> 3087864ce3d7282f59021245d8a5f83ef1caef18 is the first bad commit
>>> commit 3087864ce3d7282f59021245d8a5f83ef1caef18
>>> Author: Johannes Thumshirn <jthumshirn@suse.de>
>>> Date:   Wed Oct 12 15:06:28 2016 +0200
>>>
>>>     scsi: don't use fc_bsg_job::request and fc_bsg_job::reply directly
>>>
>>>     Don't use fc_bsg_job::request and fc_bsg_job::reply directly, but use
>>>     helper variables bsg_request and bsg_reply. This will be helpfull  when
>>>     transitioning to bsg-lib.
>>>
>>>     Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
>>>
>>> :040000 040000 140c4b6829d5cfaec4079716e0795f63f8bc3bd2 0d9fe225615679550be91fbd9f84c09ab1e280fc M	drivers
>>
>> From there (on the reverse bisect path) I get the following Oops,
>> except for the full patch set having another stack trace as in my previous
>> mail (dying in zfcp code).
>>
>
> [...]
>
>>
>>> @@ -3937,6 +3944,7 @@ fc_bsg_request_handler(struct request_queue *q, struct Scsi_Host *shost,
>>>  	struct request *req;
>>>  	struct fc_bsg_job *job;
>>>  	enum fc_dispatch_result ret;
>>> +	struct fc_bsg_reply *bsg_reply;
>>>
>>>  	if (!get_device(dev))
>>>  		return;
>>> @@ -3973,8 +3981,9 @@ fc_bsg_request_handler(struct request_queue *q, struct Scsi_Host *shost,
>>>  		/* check if we have the msgcode value at least */
>>>  		if (job->request_len < sizeof(uint32_t)) {
>>>  			BUG_ON(job->reply_len < sizeof(uint32_t));
>>> -			job->reply->reply_payload_rcv_len = 0;
>>> -			job->reply->result = -ENOMSG;
>>> +			bsg_reply = job->reply;
>>> +			bsg_reply->reply_payload_rcv_len = 0;
>>> +			bsg_reply->result = -ENOMSG;

Compiler optimization re-ordered above two lines and the first pointer 
derefence is bsg_reply->result [field offset 0] where bsg_reply is NULL.
The assignment tries to write to memory at address NULL causing the 
kernel page fault.

Does your suggested change for [PATCH v3 02/16], shuffling the 
job->request_len checks, address above kernel page fault?

>>>  			job->reply_len = sizeof(uint32_t);
>>>  			fc_bsg_jobdone(job);
>>>  			spin_lock_irq(q->queue_lock);
>>>
>
> Ahm and what exactly can break here? It's just assigning variables. Now
> I'm puzzled too.


-- 
Mit freundlichen Grüßen / Kind regards
Steffen Maier

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