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

[Hannes Reinecke <hare@suse.de>]

On 10/28/2016 11:53 AM, Steffen Maier wrote:
> 
> 
> 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.
> 
I spoke to our compiler people, and they strongly believed this not to
be the case. Or, put it the other way round, if such a thing would
happen it would be a compiler issue.

Have you checked the compiler output?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@suse.de			               +49 911 74053 688
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