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 <https://learn.microsoft.com/en-us/windows/desktop/api/vsbackup/nf-vsbackup-vssfreesnapshotproperties> function." So we allocate memory but don't free it with VssFreeSnapshotProperties or CoTaskMemFree. + > + 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 > >
