Hi
On Wed, Oct 28, 2026 at 12:08 PM Kostiantyn Kostiuk <[email protected]> wrote:
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> On Tue, Oct 27, 2026 at 6:06 PM <[email protected]> wrote:
>>
>> From: Marc-André Lureau <[email protected]>
>>
>> Since commit 374f4676788c, the provider's GetSnapshotProperties()
>> returns S_OK for any snapshot ID without filling in the properties. VSS
>> consults every registered provider when a requester looks up a snapshot,
>> so lookups of snapshots owned by other providers now succeed with an
>> all-zero VSS_SNAPSHOT_PROP. Backup software relying on it fails, e.g.
>> Veeam: "Original volume name is not specified for the shadow (shadow
>> volume id: '{00000000-0000-0000-0000-000000000000}')".
>>
>> Simply reverting is not an option: 374f4676788c switched to S_OK because
>> returning VSS_E_OBJECT_NOT_FOUND unconditionally broke qemu-ga's own
>> snapshot sets. The provider must answer for its own snapshots only. It
>> does not create real shadow copies, but it does take part in the
>> snapshot set during qemu-ga fsfreeze cycles. Remember the snapshot IDs
>> received in BeginPrepareSnapshot(), return their required properties,
>> and return VSS_E_OBJECT_NOT_FOUND for anything else. Fill every required
>> VSS_SNAPSHOT_PROP member except the exposed name and path, which are
>> optional. The snapshot device object is an empty string because the
>> host-side snapshot has no guest-visible device.
>>
>> Entries are dropped when VSS deletes the snapshots or the set, or when
>> the set is aborted (AbortSnapshots()).
>>
>> COM+ may hand out several CQGAVssProvider instances (and serve several
>> client processes), so this registry is module-global. It is per-process
>> by design and safe because the COM+ application is hosted by a single NT
>> service (see CreateServiceForApplication() in install.cpp): COM+ forbids
>> pooling and recycling for service-hosted applications, so exactly one
>> surrogate process ever hosts the provider.
>>
>> On the requester side, the snapshot set is created in the persistent
>> VSS_CTX_APP_ROLLBACK context, so VSS keeps it after the freeze/thaw
>> cycle and may keep querying providers about it. The provider registry
>> does not outlive the cycle (requester_thaw() stops the hosting service,
>> and the provider process with it), so later queries would get "not
>> found" for a set VSS still believes exists. Make requester_thaw() delete
>> the snapshot set explicitly at the end of the cycle, whatever its
>> outcome (including BackupComplete() failure and aborted snapshot sets).
>> A failure to delete is only logged, since the guest filesystems have
>> already been thawed.
>>
>> Cc: [email protected]
>> Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/4584
>> Buglink: https://redhat.atlassian.net/browse/RHEL-250662
>> Fixes: 374f4676788c ("qga: Fix VSS provider GetSnapshotProperties return
>> value")
>> Signed-off-by: Marc-André Lureau <[email protected]>
>> ---
>> v2:
>> - fill the required VSS_SNAPSHOT_PROP fields
>> ---
>> qga/vss-win32/install.cpp | 1 +
>> qga/vss-win32/provider.cpp | 200 +++++++++++++++++++++++++++++++++++-
>> qga/vss-win32/requester.cpp | 26 +++++
>> 3 files changed, 226 insertions(+), 1 deletion(-)
>>
>> diff --git a/qga/vss-win32/install.cpp b/qga/vss-win32/install.cpp
>> index 09d94edd4f53..3b58dc840520 100644
>> --- a/qga/vss-win32/install.cpp
>> +++ b/qga/vss-win32/install.cpp
>> @@ -331,6 +331,7 @@ STDAPI COMRegister(void)
>> chk(put_Value(pObj, L"Authentication", short(6)));
>> chk(put_Value(pObj, L"AuthenticationCapability", short(2)));
>> chk(put_Value(pObj, L"ImpersonationLevel", short(2)));
>> + /* NT-service-hosted apps have ConcurrentApps=1 enforced */
>> chk(pApps->SaveChanges(&n));
>>
>> /* The app should be deleted if something fails after SaveChanges */
>> diff --git a/qga/vss-win32/provider.cpp b/qga/vss-win32/provider.cpp
>> index 1c8eb236e57f..bbc30dd57d85 100644
>> --- a/qga/vss-win32/provider.cpp
>> +++ b/qga/vss-win32/provider.cpp
>> @@ -11,6 +11,11 @@
>> */
>>
>> #include "qemu/osdep.h"
>> +
>> +#include <vector>
>> +#include <algorithm>
>> +#include <string>
>> +
>> #include "vss-common.h"
>> #include "vss-debug.h"
>> #ifdef HAVE_VSS_SDK
>> @@ -138,6 +143,170 @@ STDMETHODIMP CQGAVSSEnumObject::Clone(IVssEnumObject
>> **ppenum)
>> return E_NOTIMPL;
>> }
>>
>> +static VSS_PWSZ qga_vss_strdup(const WCHAR *value)
>> +{
>> + size_t len = (wcslen(value) + 1) * sizeof(WCHAR);
>> + VSS_PWSZ str = (VSS_PWSZ)CoTaskMemAlloc(len);
>> +
>> + if (str) {
>> + memcpy(str, value, len);
>> + }
>> + return str;
>> +}
>> +
>> +static VSS_PWSZ qga_vss_strdup(const std::wstring &value)
>> +{
>> + return qga_vss_strdup(value.c_str());
>> +}
>> +
>> +static VSS_TIMESTAMP qga_vss_timestamp(void)
>> +{
>> + FILETIME ft;
>> + ULARGE_INTEGER value;
>> +
>> + GetSystemTimeAsFileTime(&ft);
>> + value.LowPart = ft.dwLowDateTime;
>> + value.HighPart = ft.dwHighDateTime;
>> + return value.QuadPart;
>> +}
>> +
>> +/* Registry of the snapshot IDs for this provider */
>> +class QGAVssSnapshotRegistry
>> +{
>> +public:
>> + bool Track(VSS_ID set_id, VSS_ID snapshot_id,
>> + VSS_PWSZ volume_name, LONG context);
>> + HRESULT GetProperties(VSS_ID snapshot_id, VSS_SNAPSHOT_PROP *prop)
>> const;
>> + LONG RemoveSet(VSS_ID set_id); /* returns how many were removed */
>> + bool Remove(VSS_ID snapshot_id);
>> +
>> +private:
>> + struct Entry {
>> + VSS_ID set_id;
>> + VSS_ID snapshot_id;
>> + std::wstring volume_name;
>> + LONG context;
>> + VSS_TIMESTAMP timestamp;
>> + };
>> +
>> + std::vector<Entry> m_entries;
>> + mutable SRWLOCK m_lock = SRWLOCK_INIT;
>> +};
>> +
>> +static QGAVssSnapshotRegistry qga_vss_registry;
>> +
>> +bool QGAVssSnapshotRegistry::Track(VSS_ID set_id, VSS_ID snapshot_id,
>> + VSS_PWSZ volume_name, LONG context)
>> +{
>> + bool ok;
>> +
>> + AcquireSRWLockExclusive(&m_lock);
>> + try {
>> + m_entries.push_back(Entry { set_id, snapshot_id, volume_name,
>> + context, qga_vss_timestamp() });
>> + ok = true;
>> + } catch (...) {
>> + ok = false;
>> + }
>> + ReleaseSRWLockExclusive(&m_lock);
>> + return ok;
>> +}
>> +
>> +HRESULT QGAVssSnapshotRegistry::GetProperties(VSS_ID snapshot_id,
>> + VSS_SNAPSHOT_PROP *prop) const
>> +{
>> + HRESULT hr = S_OK;
>> + VSS_PWSZ device = NULL, original = NULL;
>> + VSS_PWSZ originating = NULL, service = NULL;
>> +
>> + AcquireSRWLockShared(&m_lock);
>> + auto it = std::find_if(m_entries.begin(), m_entries.end(),
>> + [&](const Entry &e) {
>> + return IsEqualGUID(e.snapshot_id, snapshot_id);
>> + });
>> + if (it == m_entries.end()) {
>> + hr = VSS_E_OBJECT_NOT_FOUND;
>> + goto out;
>> + }
>> +
>> + {
>> + const Entry &e = *it;
>> + WCHAR machine[256] = L"";
>> + DWORD machine_size = ARRAYSIZE(machine);
>> +
>> + if (!GetComputerNameExW(ComputerNameDnsFullyQualified, machine,
>> + &machine_size)) {
>> + *machine = L'\0';
>> + }
>> +
>> + device = qga_vss_strdup(L"");
>> + original = qga_vss_strdup(e.volume_name);
>> + originating = qga_vss_strdup(machine);
>> + service = qga_vss_strdup(machine);
>> + if (!device || !original || !originating || !service) {
>> + hr = E_OUTOFMEMORY;
>> + goto out;
>> + }
>> +
>> + memset(prop, 0, sizeof(*prop));
>> + prop->m_SnapshotId = e.snapshot_id;
>> + prop->m_SnapshotSetId = e.set_id;
>> + prop->m_lSnapshotsCount = std::count_if(m_entries.begin(),
>> + m_entries.end(), [&](const Entry &o) {
>> + return IsEqualGUID(o.set_id, e.set_id);
>> + });
>> + prop->m_pwszSnapshotDeviceObject = device;
>> + prop->m_pwszOriginalVolumeName = original;
>> + prop->m_pwszOriginatingMachine = originating;
>> + prop->m_pwszServiceMachine = service;
>> + prop->m_ProviderId = g_gProviderId;
>> + prop->m_lSnapshotAttributes = e.context;
>> + prop->m_tsCreationTimestamp = e.timestamp;
>> + prop->m_eStatus = VSS_SS_CREATED;
>> +
>> + device = original = originating = service = NULL;
>> + }
>> +
>> +out:
>> + CoTaskMemFree(device);
>> + CoTaskMemFree(original);
>> + CoTaskMemFree(originating);
>> + CoTaskMemFree(service);
>> + ReleaseSRWLockShared(&m_lock);
>> + return hr;
>> +}
>> +
>> +LONG QGAVssSnapshotRegistry::RemoveSet(VSS_ID set_id)
>> +{
>> + LONG removed;
>> +
>> + AcquireSRWLockExclusive(&m_lock);
>> + auto it = std::remove_if(m_entries.begin(), m_entries.end(),
>> + [&](const Entry &e) {
>> + return IsEqualGUID(e.set_id, set_id);
>> + });
>> + removed = m_entries.end() - it;
>> + m_entries.erase(it, m_entries.end());
>> + ReleaseSRWLockExclusive(&m_lock);
>> + return removed;
>> +}
>> +
>> +bool QGAVssSnapshotRegistry::Remove(VSS_ID snapshot_id)
>> +{
>> + bool found;
>> +
>> + AcquireSRWLockExclusive(&m_lock);
>> + auto it = std::find_if(m_entries.begin(), m_entries.end(),
>> + [&](const Entry &e) {
>> + return IsEqualGUID(e.snapshot_id, snapshot_id);
>> + });
>> + found = it != m_entries.end();
>> + if (found) {
>> + m_entries.erase(it);
>> + }
>> + ReleaseSRWLockExclusive(&m_lock);
>> + return found;
>> +}
>>
>> /* QGAVssProvider */
>>
>> @@ -266,7 +435,7 @@ STDMETHODIMP CQGAVssProvider::SetContext(LONG lContext)
>> STDMETHODIMP CQGAVssProvider::GetSnapshotProperties(
>> VSS_ID SnapshotId, VSS_SNAPSHOT_PROP *pProp)
>> {
>> - return S_OK;
>> + return qga_vss_registry.GetProperties(SnapshotId, pProp);
>> }
>>
>> STDMETHODIMP CQGAVssProvider::Query(
>> @@ -286,6 +455,27 @@ STDMETHODIMP CQGAVssProvider::DeleteSnapshots(
>> VSS_ID SourceObjectId, VSS_OBJECT_TYPE eSourceObjectType,
>> BOOL bForceDelete, LONG *plDeletedSnapshots, VSS_ID
>> *pNondeletedSnapshotID)
>> {
>> + LONG deleted = 0;
>> +
>> + switch (eSourceObjectType) {
>> + case VSS_OBJECT_SNAPSHOT:
>> + if (qga_vss_registry.Remove(SourceObjectId)) {
>> + deleted = 1;
>> + }
>> + break;
>> + case VSS_OBJECT_SNAPSHOT_SET:
>> + deleted = qga_vss_registry.RemoveSet(SourceObjectId);
>> + break;
>> + default:
>> + break;
>> + }
>> +
>> + if (deleted > 0) {
>> + *plDeletedSnapshots = deleted;
>> + *pNondeletedSnapshotID = GUID_NULL;
>> + return S_OK;
>> + }
>> +
>> *plDeletedSnapshots = 0;
>> *pNondeletedSnapshotID = SourceObjectId;
>> return S_OK;
>
>
> return VSS_E_OBJECT_NOT_FOUND in this case looks more correct
Possibly, but I didn't want to change the existing behaviour here
without good reasons (it was fixed in 2014 by commit
d9e1f574cb6eac0a3a2f97b67d2e7a3ad9c1dc95).
>
>>
>> @@ -295,6 +485,13 @@ STDMETHODIMP CQGAVssProvider::BeginPrepareSnapshot(
>> VSS_ID SnapshotSetId, VSS_ID SnapshotId,
>> VSS_PWSZ pwszVolumeName, LONG lNewContext)
>> {
>> + if (!pwszVolumeName) {
>> + return E_INVALIDARG;
>> + }
>> + if (!qga_vss_registry.Track(SnapshotSetId, SnapshotId,
>> + pwszVolumeName, lNewContext)) {
>> + return E_OUTOFMEMORY;
>> + }
>> return S_OK;
>> }
>>
>> @@ -411,6 +608,7 @@ STDMETHODIMP
>> CQGAVssProvider::PostFinalCommitSnapshots(VSS_ID SnapshotSetId)
>>
>> STDMETHODIMP CQGAVssProvider::AbortSnapshots(VSS_ID SnapshotSetId)
>> {
>> + qga_vss_registry.RemoveSet(SnapshotSetId);
>> return S_OK;
>> }
>>
>> diff --git a/qga/vss-win32/requester.cpp b/qga/vss-win32/requester.cpp
>> index 1b0155c691ef..565eefcfb283 100644
>> --- a/qga/vss-win32/requester.cpp
>> +++ b/qga/vss-win32/requester.cpp
>> @@ -52,6 +52,7 @@ static t_VssFreeSnapshotProperties
>> pVssFreeSnapshotProperties;
>> static struct QGAVSSContext {
>> IVssBackupComponents *pVssbc; /* VSS requester interface */
>> IVssAsync *pAsyncSnapshot; /* async info of VSS snapshot operation
>> */
>> + VSS_ID idSnapshotSet; /* snapshot set created for this cycle */
>> HANDLE hEventFrozen; /* notify fs/writer freeze from provider
>> */
>> HANDLE hEventThaw; /* request provider to thaw */
>> HANDLE hEventTimeout; /* notify timeout in provider */
>> @@ -116,6 +117,7 @@ static void requester_cleanup(void)
>> vss_ctx.pVssbc->Release();
>> vss_ctx.pVssbc = NULL;
>> }
>> + vss_ctx.idSnapshotSet = GUID_NULL;
>> vss_ctx.cFrozenVols = 0;
>> qga_debug_end;
>> }
>> @@ -446,6 +448,7 @@ void requester_freeze(int *num_vols, void *mountpoints,
>> ErrorSet *errset)
>> err_set(errset, hr, "failed to start snapshot set");
>> goto out;
>> }
>> + vss_ctx.idSnapshotSet = guidSnapshotSet;
>>
>> if (mountpoints) {
>> PWCHAR volume_name_wchar;
>> @@ -680,6 +683,29 @@ void requester_thaw(int *num_vols, void *mountpints,
>> ErrorSet *errset)
>> err_set(errset, hr, "failed to do snapshot set");
>> }
>>
>> + /*
>> + * The snapshot set is created in VSS_CTX_APP_ROLLBACK context, so
>> + * it is persistent: if it was committed, VSS keeps it (and keeps
>> + * querying providers about it) until it is explicitly deleted. The
>> + * freeze/thaw cycle is over whatever the outcome above, so delete
>> + * it now. Failure is not fatal here, only log it.
>> + */
>> + {
>> + LONG lDeletedSnapshots = 0;
>> + VSS_ID idNonDeletedSnapshot;
>> + HRESULT hrDelete;
>> +
>> + hrDelete = vss_ctx.pVssbc->DeleteSnapshots(vss_ctx.idSnapshotSet,
>> + VSS_OBJECT_SNAPSHOT_SET,
>> + TRUE,
>> + &lDeletedSnapshots,
>> + &idNonDeletedSnapshot);
>> + if (FAILED(hrDelete)) {
>> + qga_debug("failed to delete snapshot set after thaw,"
>> + " hr=0x%08lx", hrDelete);
>> + }
>> + }
>> +
>> if (err_is_set(errset)) {
>> vss_ctx.pVssbc->AbortBackup();
>> }
>> --
>> 2.56.0.116.g6de20f6092dc
>>
