Index: clang/lib/AST/ASTContext.cpp
===================================================================
--- clang/lib/AST/ASTContext.cpp
+++ clang/lib/AST/ASTContext.cpp
@@ -72,6 +72,7 @@
 #include "llvm/ADT/PointerUnion.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringRef.h"
@@ -81,6 +82,7 @@
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
+#include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cassert>
@@ -826,13 +828,27 @@
   llvm_unreachable("getAddressSpaceMapMangling() doesn't cover anything.");
 }
 
+static std::vector<std::string>
+getRealPaths(llvm::vfs::FileSystem &VFS, llvm::ArrayRef<std::string> Paths) {
+  std::vector<std::string> Result;
+  llvm::SmallString<128> Buffer;
+  for (const auto &File : Paths) {
+    VFS.getRealPath(File, Buffer);
+    Result.push_back(Buffer.str());
+  }
+  return Result;
+}
+
 ASTContext::ASTContext(LangOptions &LOpts, SourceManager &SM,
                        IdentifierTable &idents, SelectorTable &sels,
                        Builtin::Context &builtins)
     : FunctionProtoTypes(this_()), TemplateSpecializationTypes(this_()),
       DependentTemplateSpecializationTypes(this_()),
       SubstTemplateTemplateParmPacks(this_()), SourceMgr(SM), LangOpts(LOpts),
-      SanitizerBL(new SanitizerBlacklist(LangOpts.SanitizerBlacklistFiles, SM)),
+      SanitizerBL(new SanitizerBlacklist(
+          getRealPaths(SM.getFileManager().getVirtualFileSystem(),
+                       LangOpts.SanitizerBlacklistFiles),
+          SM)),
       XRayFilter(new XRayFunctionFilter(LangOpts.XRayAlwaysInstrumentFiles,
                                         LangOpts.XRayNeverInstrumentFiles,
                                         LangOpts.XRayAttrListFiles, SM)),
Index: clang/lib/Driver/SanitizerArgs.cpp
===================================================================
--- clang/lib/Driver/SanitizerArgs.cpp
+++ clang/lib/Driver/SanitizerArgs.cpp
@@ -12,12 +12,13 @@
 #include "clang/Driver/DriverDiagnostic.h"
 #include "clang/Driver/Options.h"
 #include "clang/Driver/ToolChain.h"
+#include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringSwitch.h"
-#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/SpecialCaseList.h"
 #include "llvm/Support/TargetParser.h"
+#include "llvm/Support/VirtualFileSystem.h"
 #include <memory>
 
 using namespace clang;
@@ -119,7 +120,8 @@
 static std::string toString(const clang::SanitizerSet &Sanitizers);
 
 static void addDefaultBlacklists(const Driver &D, SanitizerMask Kinds,
-                                 std::vector<std::string> &BlacklistFiles) {
+                                 std::vector<std::string> &BlacklistFiles,
+                                 llvm::vfs::FileSystem &VFS) {
   struct Blacklist {
     const char *File;
     SanitizerMask Mask;
@@ -141,7 +143,7 @@
 
     clang::SmallString<64> Path(D.ResourceDir);
     llvm::sys::path::append(Path, "share", BL.File);
-    if (llvm::sys::fs::exists(Path))
+    if (VFS.exists(Path))
       BlacklistFiles.push_back(Path.str());
     else if (BL.Mask == SanitizerKind::CFI)
       // If cfi_blacklist.txt cannot be found in the resource dir, driver
@@ -557,14 +559,20 @@
 
   // Setup blacklist files.
   // Add default blacklist from resource directory.
-  addDefaultBlacklists(D, Kinds, BlacklistFiles);
+  auto &VFS = TC.getVFS();
+  addDefaultBlacklists(D, Kinds, BlacklistFiles, VFS);
+
+  std::vector<std::string> RealBlackListFiles = BlacklistFiles;
   // Parse -f(no-)sanitize-blacklist options.
   for (const auto *Arg : Args) {
     if (Arg->getOption().matches(options::OPT_fsanitize_blacklist)) {
       Arg->claim();
       std::string BLPath = Arg->getValue();
-      if (llvm::sys::fs::exists(BLPath)) {
+      if (VFS.exists(BLPath)) {
         BlacklistFiles.push_back(BLPath);
+        llvm::SmallString<128> RealBLPath;
+        VFS.getRealPath(BLPath, RealBLPath);
+        RealBlackListFiles.push_back(RealBLPath.str());
         ExtraDeps.push_back(BLPath);
       } else {
         D.Diag(clang::diag::err_drv_no_such_file) << BLPath;
@@ -579,7 +587,7 @@
   {
     std::string BLError;
     std::unique_ptr<llvm::SpecialCaseList> SCL(
-        llvm::SpecialCaseList::create(BlacklistFiles, BLError));
+        llvm::SpecialCaseList::create(RealBlackListFiles, BLError));
     if (!SCL.get())
       D.Diag(clang::diag::err_drv_malformed_sanitizer_blacklist) << BLError;
   }
Index: clang/test/Driver/Inputs/actual_file.txt
===================================================================
--- /dev/null
+++ clang/test/Driver/Inputs/actual_file.txt
@@ -0,0 +1 @@
+
Index: clang/test/Driver/Inputs/vfsoverlay.yaml
===================================================================
--- /dev/null
+++ clang/test/Driver/Inputs/vfsoverlay.yaml
@@ -0,0 +1,12 @@
+{
+  'version': 0,
+  'roots': [
+    { 'name': 'DIR', 'type': 'directory',
+      'contents': [
+        { 'name': 'only_virtual_file.txt', 'type': 'file',
+          'external-contents': 'DIR/actual_file.txt'
+        }
+      ]
+    }
+  ]
+}
Index: clang/test/Driver/vfsoverlay-sanitizers-blacklists.c
===================================================================
--- /dev/null
+++ clang/test/Driver/vfsoverlay-sanitizers-blacklists.c
@@ -0,0 +1,8 @@
+// RUN: rm -rf %t.dir
+// RUN: mkdir -p %t.dir
+// RUN: cp %S/Inputs/actual_file.txt %T/actual_file.txt
+// RUN: sed -e "s|DIR|%T|g" %S/Inputs/vfsoverlay.yaml > %T/vfsoverlay.yaml
+// RUN: mkdir %t.dir/Inputs
+
+// RUN: %clang -### -ivfsoverlay %T/vfsoverlay.yaml -fsanitize=undefined -fsanitize-blacklist=%T/only_virtual_file.txt %s 2>&1 | FileCheck %s
+// CHECK: only_virtual_file.txt
Index: clang/tools/driver/driver.cpp
===================================================================
--- clang/tools/driver/driver.cpp
+++ clang/tools/driver/driver.cpp
@@ -447,7 +447,40 @@
 
   ProcessWarningOptions(Diags, *DiagOpts, /*ReportDiags=*/false);
 
-  Driver TheDriver(Path, llvm::sys::getDefaultTargetTriple(), Diags);
+  IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS =
+      llvm::vfs::getRealFileSystem();
+  bool ExpectingVFSOverlayFile = false;
+  for (const auto *A : argv) {
+    if (StringRef(A) == "-ivfsoverlay") {
+      ExpectingVFSOverlayFile = true;
+      continue;
+    }
+
+    if (ExpectingVFSOverlayFile) {
+      llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Buffer =
+          VFS->getBufferForFile(A);
+      if (!Buffer) {
+        Diags.Report(diag::err_drv_force_crash)
+            << A; // TODO err_missing_vfs_overlay_file
+        continue;
+      }
+      IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFSFromA =
+          llvm::vfs::getVFSFromYAML(std::move(Buffer.get()),
+                                    /*DiagHandler*/ nullptr, A,
+                                    /*DiagContext*/ nullptr, VFS);
+      if (!VFSFromA) {
+        Diags.Report(diag::err_drv_force_crash)
+            << A; // TODO err_invalid_vfs_overlay
+        continue;
+      }
+      VFS = VFSFromA;
+    }
+
+    ExpectingVFSOverlayFile = false;
+  }
+  // TODO if(ExpectingVFSOverlayFile) report there's missing value
+
+  Driver TheDriver(Path, llvm::sys::getDefaultTargetTriple(), Diags, VFS);
   SetInstallDir(argv, TheDriver, CanonicalPrefixes);
   TheDriver.setTargetAndMode(TargetAndMode);