Re: [Qemu-devel] [PATCH v2 7/7] block/qcow2-refcount: fix out-of-file L2 entries to be read-as-zero

[Max Reitz <mreitz@redhat.com>]

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On 09.10.18 01:14, Vladimir Sementsov-Ogievskiy wrote:
> 
> 
> On 10/09/2018 01:21 AM, Max Reitz wrote:
>> On 09.10.18 00:14, Vladimir Sementsov-Ogievskiy wrote:
>>>
>>>
>>> On 10/09/2018 01:08 AM, Max Reitz wrote:
>>>> On 09.10.18 00:02, Vladimir Sementsov-Ogievskiy wrote:
>>>>>
>>>>>
>>>>> On 10/08/2018 11:51 PM, Max Reitz wrote:
>>>>>> On 17.08.18 14:22, Vladimir Sementsov-Ogievskiy wrote:
>>>>>>> Rewrite corrupted L2 table entry, which reference space out of
>>>>>>> underlying file.
>>>>>>>
>>>>>>> Make this L2 table entry read-as-all-zeros without any allocation.
>>>>>>>
>>>>>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>>>>>> ---
>>>>>>>     block/qcow2-refcount.c | 32 ++++++++++++++++++++++++++++++++
>>>>>>>     1 file changed, 32 insertions(+)
>>>>>>>
>>>>>>> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
>>>>>>> index 3c004e5bfe..3de3768a3c 100644
>>>>>>> --- a/block/qcow2-refcount.c
>>>>>>> +++ b/block/qcow2-refcount.c
>>>>>>> @@ -1720,8 +1720,30 @@ static int check_refcounts_l2(BlockDriverState *bs, BdrvCheckResult *res,
>>>>>>>                 /* Mark cluster as used */
>>>>>>>                 csize = (((l2_entry >> s->csize_shift) & s->csize_mask) + 1) *
>>>>>>>                         BDRV_SECTOR_SIZE;
>>>>>>> +            if (csize > s->cluster_size) {
>>>>>>> +                ret = fix_l2_entry_to_zero(
>>>>>>> +                        bs, res, fix, l2_offset, i, active,
>>>>>>> +                        "compressed cluster larger than cluster: size 0x%"
>>>>>>> +                        PRIx64, csize);
>>>>>>> +                if (ret < 0) {
>>>>>>> +                    goto fail;
>>>>>>> +                }
>>>>>>> +                continue;
>>>>>>> +            }
>>>>>>> +
>>>>>>
>>>>>> This seems recoverable, isn't it?  Can we not try to just limit the
>>>>>> csize, or decompress the cluster with the given csize from the given
>>>>>> offset, disregarding the cluster limit?
>>>>>
>>>>> Hm, you want to assume that csize is corrupted but coffset may be
>>>>> correct? Unlikely, I think.
>>>>
>>>> Better to reconstruct probably garbage data than to definitely garbage
>>>> data (all zeroes) is what I think.
>>>>
>>>>> So, to carefully repair csize, we should decompress one cluster (or one
>>>>> cluster - 1 byte) of data, trying to get one cluster of decompressed
>>>>> data. If we succeed, we know csize, or we can safely set it to one cluster.
>>>>
>>>> Yes.
>>>>
>>>>> Or we can just set csize = 1 cluster, if it is larger. And leave
>>>>> problems to real execution which will lead to EIO in worst case.
>>>>
>>>> Or this, yes.
>>>>
>>>>>>>                 coffset = l2_entry & s->cluster_offset_mask &
>>>>>>>                           ~(BDRV_SECTOR_SIZE - 1);
>>>>>>> +            if (coffset >= bdrv_getlength(bs->file->bs)) {
>>>>>>> +                ret = fix_l2_entry_to_zero(
>>>>>>> +                        bs, res, fix, l2_offset, i, active,
>>>>>>> +                        "compressed cluster out of file: offset 0x%" PRIx64,
>>>>>>> +                        coffset);
>>>>>>> +                if (ret < 0) {
>>>>>>> +                    goto fail;
>>>>>>> +                }
>>>>>>> +                continue;
>>>>>>> +            }
>>>>>>> +
>>>>>>>                 ret = qcow2_inc_refcounts_imrt(bs, res,
>>>>>>>                                                refcount_table, refcount_table_size,
>>>>>>>                                                coffset, csize);
>>>>>>> @@ -1748,6 +1770,16 @@ static int check_refcounts_l2(BlockDriverState *bs, BdrvCheckResult *res,
>>>>>>>             {
>>>>>>>                 uint64_t offset = l2_entry & L2E_OFFSET_MASK;
>>>>>>>     
>>>>>>> +            if (offset >= bdrv_getlength(bs->file->bs)) {
>>>>>>> +                ret = fix_l2_entry_to_zero(
>>>>>>> +                        bs, res, fix, l2_offset, i, active,
>>>>>>> +                        "cluster out of file: offset 0x%" PRIx64, offset);
>>>>>>> +                if (ret < 0) {
>>>>>>> +                    goto fail;
>>>>>>> +                }
>>>>>>> +                continue;
>>>>>>> +            }
>>>>>>> +
>>>>>>
>>>>>> These other two look OK, but they have another issue:  If this is a v2
>>>>>> image, you cannot create zero clusters; so you'll have to unallocate the
>>>>>> cluster in that case.
>>>>>
>>>>>
>>>>> Oho, it's a problem. It may be unsafe to discard clusters, making
>>>>> backing image available through the holes. What discard do on v2?
>>>>> Zeroing or holes?
>>>>
>>>> Oh, right!  discard on v2 punches a hole.  So I see three ways:
>>>> (1) You can do the same and point to that bit of code, or
>>>> (2) You allocate a data cluster full of zeroes in case of v2, or
>>>> (3) You just error out.
>>>>
>>>> (3) doesn't seem like the worst option.
>>>
>>>> Amending the image to be v3 is
>>>> always possible and trivial.
>>>
>>> how to do it for corrupted image?
>>
>> Oh, yeah, you can't open a corrupted image, can you...  I suppose we
>> want a way to force-clear the flag anyway. :-)
> 
> am, which flag?

The corrupt flag in the image header.

Max


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