[Qemu-devel] [PATCH 2/3] qga: vss-win32: Fix interference with snapshot creation by other VSS requesters

[Tomoki Sekiyama <tomoki.sekiyama@hds.com>]

When a VSS requester such as vshadow.exe or diskshadow.exe requests to
create disk snapshots, Windows may choose qemu-ga VSS provider if it is
only provider registered on the system. However, because it provides only a
function to freeze the filesystem, the snapshotting fails.

This patch adds a check into CQGAVssProvider::IsVolumeSupported() to reject
the request from other VSS requesters, so that the other provider is chosen.

The check of requester is done by confirming event channels between
qemu-ga's requester and provider established. To ensure that the events are
initialized when CQGAVssProvider::IsVolumeSupported() is called, it moves
the initialization earlier.

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
---
 qga/vss-win32/provider.cpp  |   11 ++++++++-
 qga/vss-win32/requester.cpp |   52 ++++++++++++++++++++++---------------------
 2 files changed, 36 insertions(+), 27 deletions(-)

diff --git a/qga/vss-win32/provider.cpp b/qga/vss-win32/provider.cpp
index c3030d8..b233646 100644
--- a/qga/vss-win32/provider.cpp
+++ b/qga/vss-win32/provider.cpp
@@ -291,8 +291,17 @@ STDMETHODIMP CQGAVssProvider::BeginPrepareSnapshot(
 STDMETHODIMP CQGAVssProvider::IsVolumeSupported(
     VSS_PWSZ pwszVolumeName, BOOL *pbSupportedByThisProvider)
 {
-    *pbSupportedByThisProvider = TRUE;
+    HANDLE hEventFrozen;
+
+    /* Check if a requester is qemu-ga by whether an event is created */
+    hEventFrozen = OpenEvent(EVENT_ALL_ACCESS, FALSE, EVENT_NAME_FROZEN);
+    if (!hEventFrozen) {
+        *pbSupportedByThisProvider = FALSE;
+        return S_OK;
+    }
+    CloseHandle(hEventFrozen);
 
+    *pbSupportedByThisProvider = TRUE;
     return S_OK;
 }
 
diff --git a/qga/vss-win32/requester.cpp b/qga/vss-win32/requester.cpp
index 0a55447..922e74d 100644
--- a/qga/vss-win32/requester.cpp
+++ b/qga/vss-win32/requester.cpp
@@ -252,6 +252,32 @@ void requester_freeze(int *num_vols, ErrorSet *errset)
 
     CoInitialize(NULL);
 
+    /* Allow unrestricted access to events */
+    InitializeSecurityDescriptor(&sd, SECURITY_DESCRIPTOR_REVISION);
+    SetSecurityDescriptorDacl(&sd, TRUE, NULL, FALSE);
+    sa.nLength = sizeof(sa);
+    sa.lpSecurityDescriptor = &sd;
+    sa.bInheritHandle = FALSE;
+
+    vss_ctx.hEventFrozen = CreateEvent(&sa, TRUE, FALSE, EVENT_NAME_FROZEN);
+    if (!vss_ctx.hEventFrozen) {
+        err_set(errset, GetLastError(), "failed to create event %s",
+                EVENT_NAME_FROZEN);
+        goto out;
+    }
+    vss_ctx.hEventThaw = CreateEvent(&sa, TRUE, FALSE, EVENT_NAME_THAW);
+    if (!vss_ctx.hEventThaw) {
+        err_set(errset, GetLastError(), "failed to create event %s",
+                EVENT_NAME_THAW);
+        goto out;
+    }
+    vss_ctx.hEventTimeout = CreateEvent(&sa, TRUE, FALSE, EVENT_NAME_TIMEOUT);
+    if (!vss_ctx.hEventTimeout) {
+        err_set(errset, GetLastError(), "failed to create event %s",
+                EVENT_NAME_TIMEOUT);
+        goto out;
+    }
+
     assert(pCreateVssBackupComponents != NULL);
     hr = pCreateVssBackupComponents(&vss_ctx.pVssbc);
     if (FAILED(hr)) {
@@ -362,32 +388,6 @@ void requester_freeze(int *num_vols, ErrorSet *errset)
         goto out;
     }
 
-    /* Allow unrestricted access to events */
-    InitializeSecurityDescriptor(&sd, SECURITY_DESCRIPTOR_REVISION);
-    SetSecurityDescriptorDacl(&sd, TRUE, NULL, FALSE);
-    sa.nLength = sizeof(sa);
-    sa.lpSecurityDescriptor = &sd;
-    sa.bInheritHandle = FALSE;
-
-    vss_ctx.hEventFrozen = CreateEvent(&sa, TRUE, FALSE, EVENT_NAME_FROZEN);
-    if (!vss_ctx.hEventFrozen) {
-        err_set(errset, GetLastError(), "failed to create event %s",
-                EVENT_NAME_FROZEN);
-        goto out;
-    }
-    vss_ctx.hEventThaw = CreateEvent(&sa, TRUE, FALSE, EVENT_NAME_THAW);
-    if (!vss_ctx.hEventThaw) {
-        err_set(errset, GetLastError(), "failed to create event %s",
-                EVENT_NAME_THAW);
-        goto out;
-    }
-    vss_ctx.hEventTimeout = CreateEvent(&sa, TRUE, FALSE, EVENT_NAME_TIMEOUT);
-    if (!vss_ctx.hEventTimeout) {
-        err_set(errset, GetLastError(), "failed to create event %s",
-                EVENT_NAME_TIMEOUT);
-        goto out;
-    }
-
     /*
      * Start VSS quiescing operations.
      * CQGAVssProvider::CommitSnapshots will kick vss_ctx.hEventFrozen