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

[Max Reitz <mreitz@redhat.com>]

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On 2017-12-08 14:51, Kevin Wolf wrote:
> Am 08.12.2017 um 14:39 hat Max Reitz geschrieben:
>> 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.
> 
> In order to answer this question, I'd first have to understand what
> GRAPH_MOD is even supposed to mean and which operations it needs to
> protect. There aren't currently any users of GRAPH_MOD.

That is exactly the reason why we could not come to a conclusion. :-)

Max


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