Cross-DSO control flow integrity (compiler-rt part).

This is an initial version of the runtime cross-DSO CFI support
library.

It contains a number of FIXMEs, ex. it does not support the
diagnostic mode nor dlopen/dlclose, but it works and can be tested.
Diagnostic mode, in particular, would require some refactoring (we'd
like to gather all CFI hooks in the UBSan library into one function
so that we could easier pass the diagnostic information down to
__cfi_check). It will be implemented later.

Once the diagnostic mode is in, I plan to create a second test
configuration to run all existing tests in both modes. For now, this
patch includes only a few new cross-DSO tests.

llvm-svn: 255695

compiler-rt/cmake/config-ix.cmake

@@ -282,6 +282,7 @@ set(ALL_TSAN_SUPPORTED_ARCH ${X86_64} ${MIPS64} ${ARM64} ${PPC64})
 set(ALL_UBSAN_SUPPORTED_ARCH ${X86} ${X86_64} ${ARM32} ${ARM64}
     ${MIPS32} ${MIPS64} ${PPC64})
 set(ALL_SAFESTACK_SUPPORTED_ARCH ${X86} ${X86_64} ${ARM64})
+set(ALL_CFI_SUPPORTED_ARCH ${X86} ${X86_64})
 
 if(APPLE)
   include(CompilerRTDarwinUtils)
@@ -471,6 +472,9 @@ if(APPLE)
   list_union(SAFESTACK_SUPPORTED_ARCH
     ALL_SAFESTACK_SUPPORTED_ARCH
     SANITIZER_COMMON_SUPPORTED_ARCH)
+  list_union(CFI_SUPPORTED_ARCH
+    ALL_CFI_SUPPORTED_ARCH
+    SANITIZER_COMMON_SUPPORTED_ARCH)
 else()
   # Architectures supported by compiler-rt libraries.
   filter_available_targets(BUILTIN_SUPPORTED_ARCH
@@ -492,6 +496,7 @@ else()
   filter_available_targets(UBSAN_SUPPORTED_ARCH ${ALL_UBSAN_SUPPORTED_ARCH})
   filter_available_targets(SAFESTACK_SUPPORTED_ARCH
     ${ALL_SAFESTACK_SUPPORTED_ARCH})
+  filter_available_targets(CFI_SUPPORTED_ARCH ${ALL_CFI_SUPPORTED_ARCH})
 endif()
 
 message(STATUS "Compiler-RT supported architectures: ${COMPILER_RT_SUPPORTED_ARCH}")
@@ -580,3 +585,10 @@ if (COMPILER_RT_HAS_SANITIZER_COMMON AND SAFESTACK_SUPPORTED_ARCH AND
 else()
   set(COMPILER_RT_HAS_SAFESTACK FALSE)
 endif()
+
+if (COMPILER_RT_HAS_SANITIZER_COMMON AND CFI_SUPPORTED_ARCH AND
+    OS_NAME MATCHES "Linux")
+  set(COMPILER_RT_HAS_CFI TRUE)
+else()
+  set(COMPILER_RT_HAS_CFI FALSE)
+endif()

compiler-rt/lib/CMakeLists.txt

@@ -19,8 +19,6 @@ if(COMPILER_RT_BUILD_SANITIZERS)
     add_subdirectory(ubsan)
   endif()
 
-  add_subdirectory(cfi)
-
   if(COMPILER_RT_HAS_ASAN)
     add_subdirectory(asan)
   endif()
@@ -45,4 +43,8 @@ if(COMPILER_RT_BUILD_SANITIZERS)
   if(COMPILER_RT_HAS_SAFESTACK)
     add_subdirectory(safestack)
   endif()
+
+  if(COMPILER_RT_HAS_CFI)
+    add_subdirectory(cfi)
+  endif()
 endif()

compiler-rt/lib/cfi/CMakeLists.txt

@@ -1,4 +1,27 @@
 add_custom_target(cfi)
+
+set(CFI_SOURCES cfi.cc)
+
+include_directories(..)
+
+set(CFI_CFLAGS
+  ${SANITIZER_COMMON_CFLAGS}
+)
+
+foreach(arch ${CFI_SUPPORTED_ARCH})
+  add_compiler_rt_runtime(clang_rt.cfi
+    STATIC
+    ARCHS ${arch}
+    SOURCES ${CFI_SOURCES}
+    OBJECT_LIBS RTInterception
+                RTSanitizerCommon
+                RTSanitizerCommonLibc
+		RTUbsan
+		RTUbsan_cxx
+    CFLAGS ${CFI_CFLAGS}
+    PARENT_TARGET cfi)
+endforeach()
+
 add_compiler_rt_resource_file(cfi_blacklist cfi_blacklist.txt)
 add_dependencies(cfi cfi_blacklist)
 add_dependencies(compiler-rt cfi)

compiler-rt/lib/cfi/cfi.cc

@@ -0,0 +1,265 @@
+//===-------- cfi.cc ------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the runtime support for the cross-DSO CFI.
+//
+//===----------------------------------------------------------------------===//
+
+// FIXME: Intercept dlopen/dlclose.
+// FIXME: Support diagnostic mode.
+// FIXME: Harden:
+//  * mprotect shadow, use mremap for updates
+//  * something else equally important
+
+#include <assert.h>
+#include <elf.h>
+#include <link.h>
+#include <string.h>
+
+typedef ElfW(Phdr) Elf_Phdr;
+typedef ElfW(Ehdr) Elf_Ehdr;
+
+#include "interception/interception.h"
+#include "sanitizer_common/sanitizer_common.h"
+#include "sanitizer_common/sanitizer_flag_parser.h"
+#include "ubsan/ubsan_init.h"
+#include "ubsan/ubsan_flags.h"
+
+static uptr __cfi_shadow;
+static constexpr uptr kShadowGranularity = 12;
+static constexpr uptr kShadowAlign = 1UL << kShadowGranularity; // 4096
+
+static constexpr uint16_t kInvalidShadow = 0;
+static constexpr uint16_t kUncheckedShadow = 0xFFFFU;
+
+static uint16_t *mem_to_shadow(uptr x) {
+  return (uint16_t *)(__cfi_shadow + ((x >> kShadowGranularity) << 1));
+}
+
+typedef int (*CFICheckFn)(uptr, void *);
+
+class ShadowValue {
+  uptr addr;
+  uint16_t v;
+  explicit ShadowValue(uptr addr, uint16_t v) : addr(addr), v(v) {}
+
+public:
+  bool is_invalid() const { return v == kInvalidShadow; }
+
+  bool is_unchecked() const { return v == kUncheckedShadow; }
+
+  CFICheckFn get_cfi_check() const {
+    assert(!is_invalid() && !is_unchecked());
+    uptr aligned_addr = addr & ~(kShadowAlign - 1);
+    uptr p = aligned_addr - (((uptr)v - 1) << kShadowGranularity);
+    return reinterpret_cast<CFICheckFn>(p);
+  }
+
+  // Load a shadow valud for the given application memory address.
+  static const ShadowValue load(uptr addr) {
+    return ShadowValue(addr, *mem_to_shadow(addr));
+  }
+};
+
+static void fill_shadow_constant(uptr begin, uptr end, uint16_t v) {
+  assert(v == kInvalidShadow || v == kUncheckedShadow);
+  uint16_t *shadow_begin = mem_to_shadow(begin);
+  uint16_t *shadow_end = mem_to_shadow(end - 1) + 1;
+  memset(shadow_begin, v, (shadow_end - shadow_begin) * sizeof(*shadow_begin));
+}
+
+static void fill_shadow(uptr begin, uptr end, uptr cfi_check) {
+  assert((cfi_check & (kShadowAlign - 1)) == 0);
+
+  // Don't fill anything below cfi_check. We can not represent those addresses
+  // in the shadow, and must make sure at codegen to place all valid call
+  // targets above cfi_check.
+  uptr p = Max(begin, cfi_check);
+  uint16_t *s = mem_to_shadow(p);
+  uint16_t *s_end = mem_to_shadow(end - 1) + 1;
+  uint16_t sv = ((p - cfi_check) >> kShadowGranularity) + 1;
+  for (; s < s_end; s++, sv++)
+    *s = sv;
+
+  // Sanity checks.
+  for (; p < end; p += kShadowAlign) {
+    assert((uptr)ShadowValue::load(p).get_cfi_check() == cfi_check);
+    assert((uptr)ShadowValue::load(p + kShadowAlign / 2).get_cfi_check() ==
+           cfi_check);
+    assert((uptr)ShadowValue::load(p + kShadowAlign - 1).get_cfi_check() ==
+           cfi_check);
+  }
+}
+
+// This is a workaround for a glibc bug:
+// https://sourceware.org/bugzilla/show_bug.cgi?id=15199
+// Other platforms can, hopefully, just do
+//    dlopen(RTLD_NOLOAD | RTLD_LAZY)
+//    dlsym("__cfi_check").
+static uptr find_cfi_check_in_dso(dl_phdr_info *info) {
+  const ElfW(Dyn) *dynamic = nullptr;
+  for (int i = 0; i < info->dlpi_phnum; ++i) {
+    if (info->dlpi_phdr[i].p_type == PT_DYNAMIC) {
+      dynamic =
+          (const ElfW(Dyn) *)(info->dlpi_addr + info->dlpi_phdr[i].p_vaddr);
+      break;
+    }
+  }
+  if (!dynamic) return 0;
+  uptr strtab = 0, symtab = 0;
+  for (const ElfW(Dyn) *p = dynamic; p->d_tag != PT_NULL; ++p) {
+    if (p->d_tag == DT_SYMTAB)
+      symtab = p->d_un.d_ptr;
+    else if (p->d_tag == DT_STRTAB)
+      strtab = p->d_un.d_ptr;
+  }
+
+  if (symtab > strtab) {
+    VReport(1, "Can not handle: symtab > strtab (%p > %zx)\n", symtab, strtab);
+    return 0;
+  }
+
+  // Verify that strtab and symtab are inside of the same LOAD segment.
+  // This excludes VDSO, which has (very high) bogus strtab and symtab pointers.
+  int phdr_idx;
+  for (phdr_idx = 0; phdr_idx < info->dlpi_phnum; phdr_idx++) {
+    const Elf_Phdr *phdr = &info->dlpi_phdr[phdr_idx];
+    if (phdr->p_type == PT_LOAD) {
+      uptr beg = info->dlpi_addr + phdr->p_vaddr;
+      uptr end = beg + phdr->p_memsz;
+      if (strtab >= beg && strtab < end && symtab >= beg && symtab < end)
+        break;
+    }
+  }
+  if (phdr_idx == info->dlpi_phnum) {
+    // Nope, either different segments or just bogus pointers.
+    // Can not handle this.
+    VReport(1, "Can not handle: symtab %p, strtab %zx\n", symtab, strtab);
+    return 0;
+  }
+
+  for (const ElfW(Sym) *p = (const ElfW(Sym) *)symtab; (ElfW(Addr))p < strtab;
+       ++p) {
+    char *name = (char*)(strtab + p->st_name);
+    if (strcmp(name, "__cfi_check") == 0) {
+      assert(p->st_info == ELF32_ST_INFO(STB_GLOBAL, STT_FUNC));
+      uptr addr = info->dlpi_addr + p->st_value;
+      return addr;
+    }
+  }
+  return 0;
+}
+
+static int dl_iterate_phdr_cb(dl_phdr_info *info, size_t size, void *data) {
+  uptr cfi_check = find_cfi_check_in_dso(info);
+  if (cfi_check)
+    VReport(1, "Module '%s' __cfi_check %zx\n", info->dlpi_name, cfi_check);
+
+  for (int i = 0; i < info->dlpi_phnum; i++) {
+    const Elf_Phdr *phdr = &info->dlpi_phdr[i];
+    if (phdr->p_type == PT_LOAD) {
+      // Jump tables are in the executable segment.
+      // VTables are in the non-executable one.
+      // Need to fill shadow for both.
+      // FIXME: reject writable if vtables are in the r/o segment. Depend on
+      // PT_RELRO?
+      uptr cur_beg = info->dlpi_addr + phdr->p_vaddr;
+      uptr cur_end = cur_beg + phdr->p_memsz;
+      if (cfi_check) {
+        VReport(1, "   %zx .. %zx\n", cur_beg, cur_end);
+        fill_shadow(cur_beg, cur_end, cfi_check ? cfi_check : (uptr)(-1));
+      } else {
+        fill_shadow_constant(cur_beg, cur_end, kInvalidShadow);
+      }
+    }
+  }
+  return 0;
+}
+
+// Fill shadow for the initial libraries.
+static void init_shadow() {
+  dl_iterate_phdr(dl_iterate_phdr_cb, nullptr);
+}
+
+SANITIZER_INTERFACE_ATTRIBUTE extern "C"
+void __cfi_slowpath(uptr CallSiteTypeId, void *Ptr) {
+  uptr Addr = (uptr)Ptr;
+  VReport(3, "__cfi_slowpath: %zx, %p\n", CallSiteTypeId, Ptr);
+  ShadowValue sv = ShadowValue::load(Addr);
+  if (sv.is_invalid()) {
+    VReport(2, "CFI: invalid memory region for a function pointer (shadow==0): %p\n", Ptr);
+    Die();
+  }
+  if (sv.is_unchecked()) {
+    VReport(2, "CFI: unchecked call (shadow=FFFF): %p\n", Ptr);
+    return;
+  }
+  CFICheckFn cfi_check = sv.get_cfi_check();
+  VReport(2, "__cfi_check at %p\n", cfi_check);
+  cfi_check(CallSiteTypeId, Ptr);
+}
+
+static void InitializeFlags() {
+  SetCommonFlagsDefaults();
+  __ubsan::Flags *uf = __ubsan::flags();
+  uf->SetDefaults();
+
+  FlagParser cfi_parser;
+  RegisterCommonFlags(&cfi_parser);
+
+  FlagParser ubsan_parser;
+  __ubsan::RegisterUbsanFlags(&ubsan_parser, uf);
+  RegisterCommonFlags(&ubsan_parser);
+
+  const char *ubsan_default_options = __ubsan::MaybeCallUbsanDefaultOptions();
+  ubsan_parser.ParseString(ubsan_default_options);
+
+  cfi_parser.ParseString(GetEnv("CFI_OPTIONS"));
+  ubsan_parser.ParseString(GetEnv("UBSAN_OPTIONS"));
+
+  SetVerbosity(common_flags()->verbosity);
+
+  if (Verbosity()) ReportUnrecognizedFlags();
+
+  if (common_flags()->help) {
+    cfi_parser.PrintFlagDescriptions();
+  }
+}
+
+extern "C" __attribute__((visibility("default")))
+#if !SANITIZER_CAN_USE_PREINIT_ARRAY
+// On ELF platforms, the constructor is invoked using .preinit_array (see below)
+__attribute__((constructor(0)))
+#endif
+void __cfi_init() {
+  SanitizerToolName = "CFI";
+  InitializeFlags();
+
+  uptr vma = GetMaxVirtualAddress();
+  // Shadow is 2 -> 2**kShadowGranularity.
+  uptr shadow_size = (vma >> (kShadowGranularity - 1)) + 1;
+  VReport(1, "CFI: VMA size %zx, shadow size %zx\n", vma, shadow_size);
+  void *shadow = MmapNoReserveOrDie(shadow_size, "CFI shadow");
+  VReport(1, "CFI: shadow at %zx .. %zx\n", shadow,
+          reinterpret_cast<uptr>(shadow) + shadow_size);
+  __cfi_shadow = (uptr)shadow;
+  init_shadow();
+
+  __ubsan::InitAsPlugin();
+}
+
+#if SANITIZER_CAN_USE_PREINIT_ARRAY
+// On ELF platforms, run cfi initialization before any other constructors.
+// On other platforms we use the constructor attribute to arrange to run our
+// initialization early.
+extern "C" {
+__attribute__((section(".preinit_array"),
+               used)) void (*__cfi_preinit)(void) = __cfi_init;
+}
+#endif

compiler-rt/test/cfi/CMakeLists.txt

@@ -6,6 +6,7 @@ configure_lit_site_cfg(
 set(CFI_TEST_DEPS ${SANITIZER_COMMON_LIT_TEST_DEPS})
 if(NOT COMPILER_RT_STANDALONE_BUILD)
   list(APPEND CFI_TEST_DEPS
+    cfi
     opt
     ubsan
   )

compiler-rt/test/cfi/cross-dso/icall/icall-from-dso.cpp

@@ -0,0 +1,26 @@
+// RUN: %clangxx_cfi_dso -DSHARED_LIB %s -fPIC -shared -o %t-so.so
+// RUN: %clangxx_cfi_dso %s -o %t %t-so.so && %expect_crash %t 2>&1 | FileCheck %s
+
+#include <stdio.h>
+
+#ifdef SHARED_LIB
+void g();
+void f() {
+  // CHECK: =1=
+  fprintf(stderr, "=1=\n");
+  ((void (*)(void))g)();
+  // CHECK: =2=
+  fprintf(stderr, "=2=\n");
+  ((void (*)(int))g)(42); // UB here
+  // CHECK-NOT: =3=
+  fprintf(stderr, "=3=\n");
+}
+#else
+void f();
+void g() {
+}
+
+int main() {
+  f();
+}
+#endif

compiler-rt/test/cfi/cross-dso/icall/icall.cpp

@@ -0,0 +1,21 @@
+// RUN: %clangxx_cfi_dso -DSHARED_LIB %s -fPIC -shared -o %t-so.so
+// RUN: %clangxx_cfi_dso %s -o %t %t-so.so && %expect_crash %t 2>&1 | FileCheck %s
+
+#include <stdio.h>
+
+#ifdef SHARED_LIB
+void f() {
+}
+#else
+void f();
+int main() {
+  // CHECK: =1=
+  fprintf(stderr, "=1=\n");
+  ((void (*)(void))f)();
+  // CHECK: =2=
+  fprintf(stderr, "=2=\n");
+  ((void (*)(int))f)(42); // UB here
+  // CHECK-NOT: =3=
+  fprintf(stderr, "=3=\n");
+}
+#endif

compiler-rt/test/cfi/cross-dso/icall/lit.local.cfg

@@ -0,0 +1,3 @@
+# The cfi-icall checker is only supported on x86 and x86_64 for now.
+if config.root.host_arch not in ['x86', 'x86_64']:
+  config.unsupported = True

compiler-rt/test/cfi/cross-dso/simple-fail.cpp

@@ -0,0 +1,87 @@
+// RUN: %clangxx_cfi_dso -DSHARED_LIB %s -fPIC -shared -o %t1-so.so
+// RUN: %clangxx_cfi_dso %s -o %t1 %t1-so.so
+// RUN: %expect_crash %t1 2>&1 | FileCheck --check-prefix=CFI %s
+// RUN: %expect_crash %t1 x 2>&1 | FileCheck --check-prefix=CFI-CAST %s
+
+// RUN: %clangxx_cfi_dso -DB32 -DSHARED_LIB %s -fPIC -shared -o %t2-so.so
+// RUN: %clangxx_cfi_dso -DB32 %s -o %t2 %t2-so.so
+// RUN: %expect_crash %t2 2>&1 | FileCheck --check-prefix=CFI %s
+// RUN: %expect_crash %t2 x 2>&1 | FileCheck --check-prefix=CFI-CAST %s
+
+// RUN: %clangxx_cfi_dso -DB64 -DSHARED_LIB %s -fPIC -shared -o %t3-so.so
+// RUN: %clangxx_cfi_dso -DB64 %s -o %t3 %t3-so.so
+// RUN: %expect_crash %t3 2>&1 | FileCheck --check-prefix=CFI %s
+// RUN: %expect_crash %t3 x 2>&1 | FileCheck --check-prefix=CFI-CAST %s
+
+// RUN: %clangxx_cfi_dso -DBM -DSHARED_LIB %s -fPIC -shared -o %t4-so.so
+// RUN: %clangxx_cfi_dso -DBM %s -o %t4 %t4-so.so
+// RUN: %expect_crash %t4 2>&1 | FileCheck --check-prefix=CFI %s
+// RUN: %expect_crash %t4 x 2>&1 | FileCheck --check-prefix=CFI-CAST %s
+
+// RUN: %clangxx -DBM -DSHARED_LIB %s -fPIC -shared -o %t5-so.so
+// RUN: %clangxx -DBM %s -o %t5 %t5-so.so
+// RUN: %t5 2>&1 | FileCheck --check-prefix=NCFI %s
+// RUN: %t5 x 2>&1 | FileCheck --check-prefix=NCFI %s
+
+// Tests that the CFI mechanism crashes the program when making a virtual call
+// to an object of the wrong class but with a compatible vtable, by casting a
+// pointer to such an object and attempting to make a call through it.
+
+// REQUIRES: cxxabi
+
+#include <stdio.h>
+#include <string.h>
+
+struct A {
+  virtual void f();
+};
+
+void *create_B();
+
+#ifdef SHARED_LIB
+
+#include "../utils.h"
+struct B {
+  virtual void f();
+};
+void B::f() {}
+
+void *create_B() {
+  create_derivers<B>();
+  return (void *)(new B());
+}
+
+#else
+
+void A::f() {}
+
+int main(int argc, char *argv[]) {
+  void *p = create_B();
+  A *a;
+
+  // CFI: =0=
+  // CFI-CAST: =0=
+  // NCFI: =0=
+  fprintf(stderr, "=0=\n");
+
+  if (argc > 1 && argv[1][0] == 'x') {
+    // Test cast. BOOM.
+    a = (A*)p;
+  } else {
+    // Invisible to CFI. Test virtual call later.
+    memcpy(&a, &p, sizeof(a));
+  }
+
+  // CFI: =1=
+  // CFI-CAST-NOT: =1=
+  // NCFI: =1=
+  fprintf(stderr, "=1=\n");
+
+  a->f(); // UB here
+
+  // CFI-NOT: =2=
+  // CFI-CAST-NOT: =2=
+  // NCFI: =2=
+  fprintf(stderr, "=2=\n");
+}
+#endif

compiler-rt/test/cfi/cross-dso/simple-pass.cpp

@@ -0,0 +1,65 @@
+// RUN: %clangxx_cfi_dso -DSHARED_LIB %s -fPIC -shared -o %t1-so.so
+// RUN: %clangxx_cfi_dso %s -o %t1 %t1-so.so
+// RUN: %t1 2>&1 | FileCheck --check-prefix=CFI %s
+
+// RUN: %clangxx_cfi_dso -DB32 -DSHARED_LIB %s -fPIC -shared -o %t2-so.so
+// RUN: %clangxx_cfi_dso -DB32 %s -o %t2 %t2-so.so
+// RUN: %t2 2>&1 | FileCheck --check-prefix=CFI %s
+
+// RUN: %clangxx_cfi_dso -DB64 -DSHARED_LIB %s -fPIC -shared -o %t3-so.so
+// RUN: %clangxx_cfi_dso -DB64 %s -o %t3 %t3-so.so
+// RUN: %t3 2>&1 | FileCheck --check-prefix=CFI %s
+
+// RUN: %clangxx_cfi_dso -DBM -DSHARED_LIB %s -fPIC -shared -o %t4-so.so
+// RUN: %clangxx_cfi_dso -DBM %s -o %t4 %t4-so.so
+// RUN: %t4 2>&1 | FileCheck --check-prefix=CFI %s
+
+// RUN: %clangxx -DBM -DSHARED_LIB %s -fPIC -shared -o %t5-so.so
+// RUN: %clangxx -DBM %s -o %t5 %t5-so.so
+// RUN: %t5 2>&1 | FileCheck --check-prefix=NCFI %s
+// RUN: %t5 x 2>&1 | FileCheck --check-prefix=NCFI %s
+
+// Tests that the CFI mechanism crashes the program when making a virtual call
+// to an object of the wrong class but with a compatible vtable, by casting a
+// pointer to such an object and attempting to make a call through it.
+
+// REQUIRES: cxxabi
+
+#include <stdio.h>
+#include <string.h>
+
+struct A {
+  virtual void f();
+};
+
+A *create_B();
+
+#ifdef SHARED_LIB
+
+#include "../utils.h"
+struct B : public A {
+  virtual void f();
+};
+void B::f() {}
+
+A *create_B() {
+  create_derivers<B>();
+  return new B();
+}
+
+#else
+
+void A::f() {}
+
+int main(int argc, char *argv[]) {
+  A *a = create_B();
+
+  // CFI: =1=
+  // NCFI: =1=
+  fprintf(stderr, "=1=\n");
+  a->f(); // OK
+  // CFI: =2=
+  // NCFI: =2=
+  fprintf(stderr, "=2=\n");
+}
+#endif

compiler-rt/test/cfi/lit.cfg

@@ -10,8 +10,11 @@ clangxx = ' '.join([config.clang] + config.cxx_mode_flags)
 config.substitutions.append((r"%clangxx ", clangxx + ' '))
 if config.lto_supported:
   clangxx_cfi = ' '.join(config.lto_launch + [clangxx] + config.lto_flags + ['-flto -fsanitize=cfi '])
+  clangxx_cfi_diag = clangxx_cfi + '-fno-sanitize-trap=cfi -fsanitize-recover=cfi '
   config.substitutions.append((r"%clangxx_cfi ", clangxx_cfi))
-  config.substitutions.append((r"%clangxx_cfi_diag ", clangxx_cfi + '-fno-sanitize-trap=cfi -fsanitize-recover=cfi '))
+  config.substitutions.append((r"%clangxx_cfi_diag ", clangxx_cfi_diag))
+  config.substitutions.append((r"%clangxx_cfi_dso ", clangxx_cfi + '-fsanitize-cfi-cross-dso '))
+  config.substitutions.append((r"%clangxx_cfi_dso_diag ", clangxx_cfi_diag + '-fsanitize-cfi-cross-dso '))
 else:
   config.unsupported = True