[libvirt] [PATCH] qemu: Stop recursive detection of image chains when an image is missing

[Peter Krempa]

On 11/21/12 15:14, Eric Blake wrote:
> On 11/21/2012 04:05 AM, Peter Krempa wrote:
>> Commit e0c469e58b93f852a72265919703cb6abd3779f8 that fixes the detection
>> of image chain wasn't complete. Iteration through the backing image
>> chain has to stop at the last existing image if some of the images are
>> missing otherwise the backing chain that is cached contains entries with
>> paths being set to NULL resulting to:
>>
>> error: Unable to allow access for disk path (null): Bad address
>>
>> Fortunately stat() is kind enough not to crash when it's presented with
>> a NULL argument.
>
> That's only true for stat() on Linux; other OSs are fully entitled to
> crash, so this definitely needs fixing :)
>
>> ---
>>   src/util/storage_file.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/src/util/storage_file.c b/src/util/storage_file.c
>> index 2249212..e7cbea7 100644
>> --- a/src/util/storage_file.c
>> +++ b/src/util/storage_file.c
>> @@ -729,6 +729,8 @@ virStorageFileGetMetadataFromBuf(int format,
>>                   if (meta->backingStore == NULL) {
>>                       /* the backing file is (currently) unavailable, treat this
>>                        * file as standalone */
>> +                    VIR_FREE(meta->backingStoreRaw);

Also I should explain the following thing:

This freeing of meta->backingStoreRaw actualy doesn't fix the bug. It 
just felt a right thing to do when the backing file is missing.

>> +                    meta->backingStoreIsFile = false;

This line is the actual fix. The code that recursively detects the image 
chain uses this value as a marker if another iteration of the recursion 
is needed (on the non-existing file).


>>                       backingFormat = VIR_STORAGE_FILE_NONE;
>
> Hmm, I'm wondering if this is the right place to fix it, or if we are
> throwing away information too early.  That is, are we sure it is not the
> later client of the chain at fault for not recognizing
> VIR_STORAGE_FILE_NONE coupled with non-NULL backingStoreRaw as the key
> for whether a backing file is missing?  I'm guessing
> domain_conf.c:virDomainDiskDefForeachPath would be the real function to
> fix here.

I will add the check also here, but I'd like to discuss the creating of 
the chain in the first place.

>
>
>
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