Re: [Qemu-devel] [PATCH 3/8] qcow2: Fix refcount blocks beyond image end

[Max Reitz <mreitz@redhat.com>]

On 14.08.2014 14:11, Benoît Canet wrote:
> The Wednesday 13 Aug 2014 à 23:01:45 (+0200), Max Reitz wrote :
>> If the qcow2 check function detects a refcount block located beyond the
>> image end, grow the image appropriately. This cannot break anything and
>> is the logical fix for such a case.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   block/qcow2-refcount.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++----
>>   1 file changed, 46 insertions(+), 4 deletions(-)
>>
>> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
>> index d1da8d5..a1d93e5 100644
>> --- a/block/qcow2-refcount.c
>> +++ b/block/qcow2-refcount.c
>> @@ -1504,7 +1504,8 @@ static int check_refblocks(BlockDriverState *bs, BdrvCheckResult *res,
>>                              int64_t *nb_clusters)
>>   {
>>       BDRVQcowState *s = bs->opaque;
>> -    int64_t i;
>> +    int64_t i, size;
>> +    int ret;
>>   
>>       for (i = 0; i < s->refcount_table_size; i++) {
>>           uint64_t offset, cluster;
>> @@ -1520,9 +1521,50 @@ static int check_refblocks(BlockDriverState *bs, BdrvCheckResult *res,
>>           }
>>   
>>           if (cluster >= *nb_clusters) {
>> -            fprintf(stderr, "ERROR refcount block %" PRId64
>> -                    " is outside image\n", i);
>> -            res->corruptions++;
>> +            fprintf(stderr, "%s refcount block %" PRId64 " is outside image\n",
>> +                    fix & BDRV_FIX_ERRORS ? "Repairing" : "ERROR", i);
>> +
>> +            if (fix & BDRV_FIX_ERRORS) {
>> +                int64_t old_nb_clusters = *nb_clusters;
>> +
>> +                ret = bdrv_truncate(bs->file, offset + s->cluster_size);
>> +                if (ret < 0) {
>> +                    goto resize_fail;
>> +                }
>> +                size = bdrv_getlength(bs->file);
>> +                if (size < 0) {
>> +                    ret = size;
>> +                    goto resize_fail;
>> +                }
>> +
>> +                *nb_clusters = size_to_clusters(s, size);
>> +                assert(*nb_clusters >= old_nb_clusters);
>> +
>> +                *refcount_table = g_try_realloc(*refcount_table,
>> +                        *nb_clusters * sizeof(uint16_t));
>> +                if (!*refcount_table) {
>> +                    res->check_errors++;
>> +                    return -ENOMEM;
> So you really want to make sure the code is not trying anything more
> by directly returning -ENOMEM and not doing goto resize_fail.
>
> This makes sense though.
>
>> +                }
>> +
>> +                memset(*refcount_table + old_nb_clusters, 0,
>> +                       (*nb_clusters - old_nb_clusters) * sizeof(uint16_t));
>> +
>> +                if (cluster >= *nb_clusters) {
>> +                    ret = -EINVAL;
>> +                    goto resize_fail;
>> +                }
>> +
>> +                res->corruptions_fixed++;
>> +                continue;
>> +
>> +resize_fail:
>> +                res->corruptions++;
>> +                fprintf(stderr, "ERROR could not resize image: %s\n",
>> +                        strerror(-ret));
> Isn't a "return ret;" missing here ?
> the code will fall in the continue statement without it.

And that it should. A corruption is reported to stderr, res->corruptions 
is incremented and that's it - just as it was without this patch. The 
only reason I see why we should completely abort here is because 
resizing the file should always work; if it doesn't, something may be 
completely wrong. But even that is no real reason to jump the shark; we 
can still continue with the check and if everything is indeed completely 
broken, we'll receive EIOs soon enough.

Perhaps I should add a *rebuild = true; here and in the else branch in 
the next patch, though.

Max

>> +            } else {
>> +                res->corruptions++;
>> +            }
>>               continue;
>>           }
>>   
>> -- 
>> 2.0.3
>>
>>