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



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);
+                    meta->backingStoreIsFile = false;
                      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

But in the other case, we are gathering incomplete information. If we don't fix this here, the backing chain will contain the entry representing the missing image and the previous part of the chain will be a raw image.

later client of the chain at fault for not recognizing
VIR_STORAGE_FILE_NONE coupled with non-NULL backingStoreRaw as the key

Hm we could use this to report a broken chain before qemu dies with a not very helpful message.

for whether a backing file is missing?  I'm guessing
domain_conf.c:virDomainDiskDefForeachPath would be the real function to

That was the second place I was thinking of fixing this but the approach depends on if we want to check if the chain is OK before qemu does (In that case I think it's OK to have that bogus entry of the missing file in the chain but we should report that as a broken chain and not as an raw image at the end of the chain that actually isn't the end of the chain) or we leave the chain enumeration on qemu (and we will not need to store the indicator of the broken chain as we don't care in that moment).

fix here.


Which road are we going to take?

Peter


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