Re: [Qemu-devel] About [PULL 20/25] block: Guard against NULL bs->drv

[Max Reitz]

On 2017-12-06 10:12, Kevin Wolf wrote:
> Am 06.12.2017 um 08:28 hat Kangjie Xi geschrieben:
>> Hi,
>>
>> I encountered a qemu-nbd segfault, finally I found it was caused by
>> NULL bs-drv,  which is located in block/io.c function bdrv_co_flush
>> line 2377:
>>
>> https://git.qemu.org/?p=qemu.git;a=blob;f=block/io.c;h=4fdf93a0144fa4761a14b8cc6b2a9a6b6e5d5bec;hb=d470ad42acfc73c45d3e8ed5311a491160b4c100#l2377
>>
>> It is before the patch at line 2402, so the patch needs to be updated
>> to fix NULL bs-drv at line 2337.
>>
>> https://lists.gnu.org/archive/html/qemu-devel/2017-11/msg03425.html
> 
> Can you please post a full backtrace? Do you see any error message
> on stderr before the process crashes?
> 
> I don't see at the moment how this can happen, except the case that Max
> mentioned where bs->drv = NULL is set when an image corruption is
> detected - this involves an error message, though.
> 
> We check bdrv_is_inserted() as the first thing, which includes a NULL
> check for bs->drv. So it must have been non-NULL at the start of the
> function and then become NULL. I suppose this can theoretically happen
> in qemu_co_queue_wait() if another flush request detects image
> corruption.
> 
> Max: I think bs->drv = NULL in the middle of a request was a stupid
> idea. In fact, it's already a stupid idea to have any BDS with
> bs->drv = NULL. Maybe it would be better to schedule a BH that replaces
> the qcow2 node with a dummy node (null-co?) and properly closes the
> qcow2 one.

Yes, that is an idea John had, too.  It sounded good to me (we'd just
need to add a new flag to null-co so it would respond with -ENOMEDIUM to
all requests or something)...  The only issue I had is how that would
work together with the GRAPH_MOD op blocker.

Max

> 
> Kevin
> 
>>> @@ -2373,6 +2399,12 @@ int coroutine_fn bdrv_co_flush(BlockDriverState *bs)
>>>      }
>>>
>>>      BLKDBG_EVENT(bs->file, BLKDBG_FLUSH_TO_DISK);
>>> +    if (!bs->drv) {
>>> +        /* bs->drv->bdrv_co_flush() might have ejected the BDS
>>> +         * (even in case of apparent success) */
>>> +        ret = -ENOMEDIUM;
>>> +        goto out;
>>> +    }
>>>      if (bs->drv->bdrv_co_flush_to_disk) {
>>>          ret = bs->drv->bdrv_co_flush_to_disk(bs);
>>>      } else if (bs->drv->bdrv_aio_flush) {
>>
>> I have tested the latest qemu-2.11.0-rc2 and I am sure the qemu-nbd
>> segfault is caused by NULL bs-drv in block/io.c line 2337.
>>
>> kernel: qemu-nbd[18768]: segfault at f8 ip 000055a24f7536a7 sp
>> 00007f59b1137a40 error 4 in qemu-nbd[55a24f6d1000+188000]
>>
>> However I have no methods to reproduce the segfault manually, the
>> qemu-nbd segfaut just occurs in my server cluster every week.
>>
>> Thanks
>> -Kangjie

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