OvmfPkg: BDS: QemuBootOrder: don't leak unreferenced boot options

The Boot#### variables that have become unreferenced in the new BootOrder
variable won't ever be automatically reused for booting. They are
"unreachable" resources that take up room in the variable store. Make an
effort to remove them.

This should plug the leak which, given sufficient reboots, exhausts the
variable store with stale Boot#### variables and renders the VM
unbootable.

Reported-by: Michael Chang <mchang@suse.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15327 6f19259b-4bc3-4df7-8a09-765794883524

OvmfPkg/Library/PlatformBdsLib/QemuBootOrder.c

@@ -1120,6 +1120,47 @@ BootOrderComplete (
 }
 
 
+/**
+  Delete Boot#### variables that stand for such active boot options that have
+  been dropped (ie. have not been selected by either matching or "survival
+  policy").
+
+  @param[in]  ActiveOption  The array of active boot options to scan. Each
+                            entry not marked as appended will trigger the
+                            deletion of the matching Boot#### variable.
+
+  @param[in]  ActiveCount   Number of elements in ActiveOption.
+**/
+STATIC
+VOID
+PruneBootVariables (
+  IN  CONST ACTIVE_OPTION *ActiveOption,
+  IN  UINTN               ActiveCount
+  )
+{
+  UINTN Idx;
+
+  for (Idx = 0; Idx < ActiveCount; ++Idx) {
+    if (!ActiveOption[Idx].Appended) {
+      CHAR16 VariableName[9];
+
+      UnicodeSPrintAsciiFormat (VariableName, sizeof VariableName, "Boot%04x",
+        ActiveOption[Idx].BootOption->BootCurrent);
+
+      //
+      // "The space consumed by the deleted variable may not be available until
+      // the next power cycle", but that's good enough.
+      //
+      gRT->SetVariable (VariableName, &gEfiGlobalVariableGuid,
+             0,   // Attributes, 0 means deletion
+             0,   // DataSize, 0 means deletion
+             NULL // Data
+             );
+    }
+  }
+}
+
+
 /**
 
   Set the boot order based on configuration retrieved from QEMU.
@@ -1269,12 +1310,13 @@ SetBootOrderFromQemu (
                     BootOrder.Produced * sizeof (*BootOrder.Data),
                     BootOrder.Data
                     );
-    DEBUG ((
+    if (EFI_ERROR (Status)) {
-      DEBUG_INFO,
+      DEBUG ((DEBUG_ERROR, "%a: setting BootOrder: %r\n", __FUNCTION__, Status));
-      "%a: setting BootOrder: %a\n",
+      goto ErrorFreeActiveOption;
-      __FUNCTION__,
+    }
-      Status == EFI_SUCCESS ? "success" : "error"
+
-      ));
+    DEBUG ((DEBUG_INFO, "%a: setting BootOrder: success\n", __FUNCTION__));
+    PruneBootVariables (ActiveOption, ActiveCount);
   }
 
 ErrorFreeActiveOption: