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

[Vladimir Sementsov-Ogievskiy]

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 <address@hidden>
>>>> ---
>>>>    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?

> Maybe point the user to that option.
> 
> Max
>