Hi On Tue, Oct 27, 2026 at 5:54 PM Kostiantyn Kostiuk <[email protected]> wrote: > > > > On Tue, Oct 27, 2026 at 3:31 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 >> (minimal) properties, and return VSS_E_OBJECT_NOT_FOUND for anything >> else. >> >> 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]> >> --- >> qga/vss-win32/install.cpp | 1 + >> qga/vss-win32/provider.cpp | 153 +++++++++++++++++++++++++++++++++++- >> qga/vss-win32/requester.cpp | 26 ++++++ >> 3 files changed, 179 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..ad2f3ee763e6 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 +143,123 @@ STDMETHODIMP CQGAVSSEnumObject::Clone(IVssEnumObject >> **ppenum) >> return E_NOTIMPL; >> } >> >> +/* 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; >> + }; >> + >> + 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 }); >> + 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; >> + >> + 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; >> + size_t len = (e.volume_name.size() + 1) * sizeof(WCHAR); >> + VSS_PWSZ volume = (VSS_PWSZ)CoTaskMemAlloc(len); > > > I'm afraid of this part. We allocate memory but don't call CoTaskMemFree. > This function is called from IVssBackupComponents::GetSnapshotProperties > From > https://learn.microsoft.com/en-us/windows/win32/api/vsbackup/nf-vsbackup-ivssbackupcomponents-getsnapshotproperties > "When the structure is no longer needed, the software provider is responsible > for freeing these strings by calling the VssFreeSnapshotProperties function." > > So we allocate memory but don't free it with VssFreeSnapshotProperties or > CoTaskMemFree.
I also wondered, but this is apparently what is expected: https://github.com/MicrosoftDocs/sdk-api/blob/docs/sdk-api-src/content/vss/ns-vss-vss_object_prop.md A requester obtains VSS_OBJECT_PROP structures by using IVssEnumObject::Next to iterate over the list of objects returned by a call to IVssBackupComponents::Query. As its members are filled by a COM interface, prior to deleting the property structures VSS_SNAPSHOT_PROP and VSS_PROVIDER_PROP, the memory they contain must be released by calling CoTaskMemFree for every string and byte array value contained in each structure. In the case of VSS_SNAPSHOT_PROP, this can be done manually, or the utility function VssFreeSnapshotProperties can be used. > >> + >> + if (!volume) { >> + hr = E_OUTOFMEMORY; >> + goto out; >> + } >> + memcpy(volume, e.volume_name.c_str(), len); >> + >> + /* not a real snapshot: no device object, exposure or timestamp */ >> + 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_pwszOriginalVolumeName = volume; >> + prop->m_ProviderId = g_gProviderId; >> + prop->m_lSnapshotAttributes = e.context; >> + prop->m_eStatus = VSS_SS_CREATED; >> + } >> + >> +out: >> + 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 +388,7 @@ 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 +408,27 @@ 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; >> @@ -295,6 +438,13 @@ 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 +561,7 @@ 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 >>
