Re: [Qemu-devel] [PATCH 1/3] qcow2: Check bs->drv in copy_sectors()

[Paolo Bonzini <pbonzini@redhat.com>]

Il 11/03/2014 11:16, Kevin Wolf ha scritto:
> Am 11.03.2014 um 00:16 hat Laszlo Ersek geschrieben:
>> On 03/10/14 23:44, Max Reitz wrote:
>>> Before dereferencing bs->drv for a call to its member bdrv_co_readv(),
>>> copy_sectors() should check whether that pointer is indeed valid, since
>>> it may have been set to NULL by e.g. a concurrent write triggering the
>>> corruption prevention mechanism.
>>>
>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>>> ---
>>> To be precise, this still is a race condition. If bs->drv is set to NULL
>>> after the check and before the call to bdrv_co_readv(), QEMU will
>>> obviously still crash. However, in order to circumvent this behavior, we
>>> would probably have to re-lock s->lock, check bs->drv, take the function
>>> pointer to bdrv_co_readv() and then unlock s->lock before the function
>>> is called. I found this rather ugly and therefore this still has a very
>>> small chance of running into a race condition.
>>> Therefore, I'm asking for your opinion on this, whether we can really
>>> take this chance or should rather "do it right". In fact, if I were a
>>> reviewer, I'd probably reject this patch and request the solution with
>>> the function pointer (if there is no better solution), but I was afraid
>>> to send such an ugly patch.
>
> No, the code is fine. Remember that qcow2 is not threaded, we're talking
> about coroutines here. There is no way for the code to yield between
> your check and the protected place.
>
>>>  block/qcow2-cluster.c | 4 ++++
>>>  1 file changed, 4 insertions(+)
>>>
>>> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
>>> index 36c1bed..9499df9 100644
>>> --- a/block/qcow2-cluster.c
>>> +++ b/block/qcow2-cluster.c
>>> @@ -380,6 +380,10 @@ static int coroutine_fn copy_sectors(BlockDriverState *bs,
>>>
>>>      BLKDBG_EVENT(bs->file, BLKDBG_COW_READ);
>>>
>>> +    if (!bs->drv) {
>>> +        return -ENOMEDIUM;
>>> +    }
>>> +
>>>      /* Call .bdrv_co_readv() directly instead of using the public block-layer
>>>       * interface.  This avoids double I/O throttling and request tracking,
>>>       * which can lead to deadlock when block layer copy-on-read is enabled.
>>>
>>
>> I can't answer your question nor review this patch -- instead, I have a
>> question of my own: when you say "set to NULL by [...] the corruption
>> prevention mechanism", do you mean qcow2_pre_write_overlap_check():
>>
>>         bs->drv = NULL; /* make BDS unusable */
>
> Yes, this is the place.
>
>> If so: I thought that it was quite a bold move, but also that we'd find
>> the SIGSEGVs sooner or later... :)
>
> In fact, if you use the block layer API, most functions check for
> bs->drv and return -ENOMEDIUM if it is NULL. The problem here is that we
> directly dereference the pointer without going through block.c (there's
> a good reason for this, see the comment, but it still makes it somewhat
> special).

But why not call qcow2_co_readv directly?

Paolo