[asan] Use dynamic shadow on 32-bit Android, try 2.

Summary:
This change reverts r318575 and changes FindDynamicShadowStart() to
keep the memory range it found mapped PROT_NONE to make sure it is
not reused. We also skip MemoryRangeIsAvailable() check, because it
is (a) unnecessary, and (b) would fail anyway.

Reviewers: pcc, vitalybuka, kcc

Subscribers: srhines, kubamracek, mgorny, llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D40203

llvm-svn: 318666

14 files changed, 310 insertions, 22 deletions

diff --git a/compiler-rt/lib/asan/CMakeLists.txt b/compiler-rt/lib/asan/CMakeLists.txt
index 78872cd9f6b..da82e485b58 100644
--- a/compiler-rt/lib/asan/CMakeLists.txt
+++ b/compiler-rt/lib/asan/CMakeLists.txt
@@ -21,6 +21,7 @@ set(ASAN_SOURCES
   asan_memory_profile.cc
   asan_poisoning.cc
   asan_posix.cc
+  asan_premap_shadow.cc
   asan_report.cc
   asan_rtl.cc
   asan_shadow_setup.cc
diff --git a/compiler-rt/lib/asan/asan_init_version.h b/compiler-rt/lib/asan/asan_init_version.h
index f48cc19cc51..c49fcd74024 100644
--- a/compiler-rt/lib/asan/asan_init_version.h
+++ b/compiler-rt/lib/asan/asan_init_version.h
@@ -15,6 +15,8 @@
 #ifndef ASAN_INIT_VERSION_H
 #define ASAN_INIT_VERSION_H
 
+#include "sanitizer_common/sanitizer_platform.h"
+
 extern "C" {
   // Every time the ASan ABI changes we also change the version number in the
   // __asan_init function name.  Objects built with incompatible ASan ABI
@@ -32,7 +34,12 @@ extern "C" {
   // v6=>v7: added 'odr_indicator' to __asan_global
   // v7=>v8: added '__asan_(un)register_image_globals' functions for dead
   //         stripping support on Mach-O platforms
+#if SANITIZER_WORDSIZE == 32 && SANITIZER_ANDROID
+  // v8=>v9: 32-bit Android switched to dynamic shadow
+  #define __asan_version_mismatch_check __asan_version_mismatch_check_v9
+#else
   #define __asan_version_mismatch_check __asan_version_mismatch_check_v8
+#endif
 }
 
 #endif  // ASAN_INIT_VERSION_H
diff --git a/compiler-rt/lib/asan/asan_linux.cc b/compiler-rt/lib/asan/asan_linux.cc
index a949a9888e2..5cf466661f0 100644
--- a/compiler-rt/lib/asan/asan_linux.cc
+++ b/compiler-rt/lib/asan/asan_linux.cc
@@ -17,6 +17,7 @@
 
 #include "asan_interceptors.h"
 #include "asan_internal.h"
+#include "asan_premap_shadow.h"
 #include "asan_thread.h"
 #include "sanitizer_common/sanitizer_flags.h"
 #include "sanitizer_common/sanitizer_freebsd.h"
@@ -81,9 +82,51 @@ void *AsanDoesNotSupportStaticLinkage() {
   return &_DYNAMIC;  // defined in link.h
 }
 
+static void UnmapFromTo(uptr from, uptr to) {
+  CHECK(to >= from);
+  if (to == from) return;
+  uptr res = internal_munmap(reinterpret_cast<void *>(from), to - from);
+  if (UNLIKELY(internal_iserror(res))) {
+    Report(
+        "ERROR: AddresSanitizer failed to unmap 0x%zx (%zd) bytes at address "
+        "%p\n",
+        to - from, to - from, from);
+    CHECK("unable to unmap" && 0);
+  }
+}
+
+#if ASAN_PREMAP_SHADOW
+uptr FindPremappedShadowStart() {
+  uptr granularity = GetMmapGranularity();
+  uptr shadow_start = reinterpret_cast<uptr>(&__asan_shadow);
+  uptr premap_shadow_size = PremapShadowSize();
+  uptr shadow_size = RoundUpTo(kHighShadowEnd, granularity);
+  // We may have mapped too much. Release extra memory.
+  UnmapFromTo(shadow_start + shadow_size, shadow_start + premap_shadow_size);
+  return shadow_start;
+}
+#endif
+
 uptr FindDynamicShadowStart() {
-  UNREACHABLE("FindDynamicShadowStart is not available");
-  return 0;
+#if ASAN_PREMAP_SHADOW
+  if (!PremapShadowFailed())
+    return FindPremappedShadowStart();
+#endif
+
+  uptr granularity = GetMmapGranularity();
+  uptr alignment = granularity * 8;
+  uptr left_padding = granularity;
+  uptr shadow_size = RoundUpTo(kHighShadowEnd, granularity);
+  uptr map_size = shadow_size + left_padding + alignment;
+
+  uptr map_start = (uptr)MmapNoAccess(map_size);
+  CHECK_NE(map_start, ~(uptr)0);
+
+  uptr shadow_start = RoundUpTo(map_start + left_padding, alignment);
+  UnmapFromTo(map_start, shadow_start - left_padding);
+  UnmapFromTo(shadow_start + shadow_size, map_start + map_size);
+
+  return shadow_start;
 }
 
 void AsanApplyToGlobals(globals_op_fptr op, const void *needle) {
diff --git a/compiler-rt/lib/asan/asan_mapping.h b/compiler-rt/lib/asan/asan_mapping.h
index acdaa60b9c5..c51c960c5ab 100644
--- a/compiler-rt/lib/asan/asan_mapping.h
+++ b/compiler-rt/lib/asan/asan_mapping.h
@@ -161,7 +161,7 @@ static const u64 kWindowsShadowOffset32 = 3ULL << 28;  // 0x30000000
 #  define SHADOW_OFFSET (0)
 #elif SANITIZER_WORDSIZE == 32
 #  if SANITIZER_ANDROID
-#    define SHADOW_OFFSET (0)
+#    define SHADOW_OFFSET __asan_shadow_memory_dynamic_address
 #  elif defined(__mips__)
 #    define SHADOW_OFFSET kMIPS32_ShadowOffset32
 #  elif SANITIZER_FREEBSD
@@ -205,6 +205,12 @@ static const u64 kWindowsShadowOffset32 = 3ULL << 28;  // 0x30000000
 #  endif
 #endif
 
+#if SANITIZER_ANDROID && defined(__arm__)
+# define ASAN_PREMAP_SHADOW 1
+#else
+# define ASAN_PREMAP_SHADOW 0
+#endif
+
 #define SHADOW_GRANULARITY (1ULL << SHADOW_SCALE)
 #define MEM_TO_SHADOW(mem) (((mem) >> SHADOW_SCALE) + (SHADOW_OFFSET))
 
diff --git a/compiler-rt/lib/asan/asan_premap_shadow.cc b/compiler-rt/lib/asan/asan_premap_shadow.cc
new file mode 100644
index 00000000000..229eba99fe0
--- /dev/null
+++ b/compiler-rt/lib/asan/asan_premap_shadow.cc
@@ -0,0 +1,79 @@
+//===-- asan_premap_shadow.cc ---------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file is a part of AddressSanitizer, an address sanity checker.
+//
+// Reserve shadow memory with an ifunc resolver.
+//===----------------------------------------------------------------------===//
+
+#include "asan_mapping.h"
+
+#if ASAN_PREMAP_SHADOW
+
+#include "asan_premap_shadow.h"
+#include "sanitizer_common/sanitizer_posix.h"
+
+namespace __asan {
+
+// The code in this file needs to run in an unrelocated binary. It may not
+// access any external symbol, including its own non-hidden globals.
+
+// Conservative upper limit.
+uptr PremapShadowSize() {
+  uptr granularity = GetMmapGranularity();
+  return RoundUpTo(GetMaxVirtualAddress() >> SHADOW_SCALE, granularity);
+}
+
+// Returns an address aligned to 8 pages, such that one page on the left and
+// PremapShadowSize() bytes on the right of it are mapped r/o.
+uptr PremapShadow() {
+  uptr granularity = GetMmapGranularity();
+  uptr alignment = granularity * 8;
+  uptr left_padding = granularity;
+  uptr shadow_size = PremapShadowSize();
+  uptr map_size = shadow_size + left_padding + alignment;
+
+  uptr map_start = (uptr)MmapNoAccess(map_size);
+  CHECK_NE(map_start, ~(uptr)0);
+
+  uptr shadow_start = RoundUpTo(map_start + left_padding, alignment);
+  uptr shadow_end = shadow_start + shadow_size;
+  internal_munmap(reinterpret_cast<void *>(map_start),
+                  shadow_start - left_padding - map_start);
+  internal_munmap(reinterpret_cast<void *>(shadow_end),
+                  map_start + map_size - shadow_end);
+  return shadow_start;
+}
+
+bool PremapShadowFailed() {
+  uptr shadow = reinterpret_cast<uptr>(&__asan_shadow);
+  uptr resolver = reinterpret_cast<uptr>(&__asan_premap_shadow);
+  // shadow == resolver is how Android KitKat and older handles ifunc.
+  // shadow == 0 just in case.
+  if (shadow == 0 || shadow == resolver)
+    return true;
+  return false;
+}
+} // namespace __asan
+
+extern "C" {
+decltype(__asan_shadow)* __asan_premap_shadow() {
+  // The resolver may be called multiple times. Map the shadow just once.
+  static uptr premapped_shadow = 0;
+  if (!premapped_shadow) premapped_shadow = __asan::PremapShadow();
+  return reinterpret_cast<decltype(__asan_shadow)*>(premapped_shadow);
+}
+
+// __asan_shadow is a "function" that has the same address as the first byte of
+// the shadow mapping.
+INTERFACE_ATTRIBUTE __attribute__((ifunc("__asan_premap_shadow"))) void
+__asan_shadow();
+}
+
+#endif // ASAN_PREMAP_SHADOW
diff --git a/compiler-rt/lib/asan/asan_premap_shadow.h b/compiler-rt/lib/asan/asan_premap_shadow.h
new file mode 100644
index 00000000000..41acbdbbb69
--- /dev/null
+++ b/compiler-rt/lib/asan/asan_premap_shadow.h
@@ -0,0 +1,30 @@
+//===-- asan_mapping.h ------------------------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file is a part of AddressSanitizer, an address sanity checker.
+//
+// Premap shadow range with an ifunc resolver.
+//===----------------------------------------------------------------------===//
+
+
+#ifndef ASAN_PREMAP_SHADOW_H
+#define ASAN_PREMAP_SHADOW_H
+
+#if ASAN_PREMAP_SHADOW
+namespace __asan {
+// Conservative upper limit.
+uptr PremapShadowSize();
+bool PremapShadowFailed();
+}
+#endif
+
+extern "C" INTERFACE_ATTRIBUTE void __asan_shadow();
+extern "C" decltype(__asan_shadow)* __asan_premap_shadow();
+
+#endif // ASAN_PREMAP_SHADOW_H
diff --git a/compiler-rt/lib/asan/asan_shadow_setup.cc b/compiler-rt/lib/asan/asan_shadow_setup.cc
index 08c009184f8..b3cf0b5c1ce 100644
--- a/compiler-rt/lib/asan/asan_shadow_setup.cc
+++ b/compiler-rt/lib/asan/asan_shadow_setup.cc
@@ -99,17 +99,21 @@ void InitializeShadowMemory() {
   // when necessary. When dynamic address is used, the macro |kLowShadowBeg|
   // expands to |__asan_shadow_memory_dynamic_address| which is
   // |kDefaultShadowSentinel|.
+  bool full_shadow_is_available = false;
   if (shadow_start == kDefaultShadowSentinel) {
     __asan_shadow_memory_dynamic_address = 0;
     CHECK_EQ(0, kLowShadowBeg);
     shadow_start = FindDynamicShadowStart();
+    if (SANITIZER_LINUX) full_shadow_is_available = true;
   }
   // Update the shadow memory address (potentially) used by instrumentation.
   __asan_shadow_memory_dynamic_address = shadow_start;
 
   if (kLowShadowBeg) shadow_start -= GetMmapGranularity();
-  bool full_shadow_is_available =
-      MemoryRangeIsAvailable(shadow_start, kHighShadowEnd);
+
+  if (!full_shadow_is_available)
+    full_shadow_is_available =
+        MemoryRangeIsAvailable(shadow_start, kHighShadowEnd);
 
 #if SANITIZER_LINUX && defined(__x86_64__) && defined(_LP64) && \
     !ASAN_FIXED_MAPPING
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_common.h b/compiler-rt/lib/sanitizer_common/sanitizer_common.h
index 9fb615f9668..746ba226cd5 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_common.h
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_common.h
@@ -73,6 +73,7 @@ INLINE uptr GetPageSizeCached() {
   return PageSizeCached;
 }
 uptr GetMmapGranularity();
+uptr GetMaxVirtualAddress();
 uptr GetMaxUserVirtualAddress();
 // Threads
 tid_t GetTid();
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cc b/compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cc
index 554bc458cbe..c130c10c2dc 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cc
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cc
@@ -191,6 +191,10 @@ uptr GetMaxUserVirtualAddress() {
   return ShadowBounds.memory_limit - 1;
 }
 
+uptr GetMaxVirtualAddress() {
+  return GetMaxUserVirtualAddress();
+}
+
 static void *DoAnonymousMmapOrDie(uptr size, const char *mem_type,
                                   bool raw_report, bool die_for_nomem) {
   size = RoundUpTo(size, PAGE_SIZE);
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_linux.cc b/compiler-rt/lib/sanitizer_common/sanitizer_linux.cc
index ea1e79556ab..6274b248edb 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_linux.cc
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_linux.cc
@@ -954,7 +954,7 @@ static uptr GetKernelAreaSize() {
 }
 #endif  // SANITIZER_WORDSIZE == 32
 
-uptr GetMaxUserVirtualAddress() {
+uptr GetMaxVirtualAddress() {
 #if SANITIZER_NETBSD && defined(__x86_64__)
   return 0x7f7ffffff000ULL;  // (0x00007f8000000000 - PAGE_SIZE)
 #elif SANITIZER_WORDSIZE == 64
@@ -978,15 +978,21 @@ uptr GetMaxUserVirtualAddress() {
 # if defined(__s390__)
   return (1ULL << 31) - 1;  // 0x7fffffff;
 # else
-  uptr res = (1ULL << 32) - 1;  // 0xffffffff;
-  if (!common_flags()->full_address_space)
-    res -= GetKernelAreaSize();
-  CHECK_LT(reinterpret_cast<uptr>(&res), res);
-  return res;
+  return (1ULL << 32) - 1;  // 0xffffffff;
 # endif
 #endif  // SANITIZER_WORDSIZE
 }
 
+uptr GetMaxUserVirtualAddress() {
+  uptr addr = GetMaxVirtualAddress();
+#if SANITIZER_WORDSIZE == 32 && !defined(__s390__)
+  if (!common_flags()->full_address_space)
+    addr -= GetKernelAreaSize();
+  CHECK_LT(reinterpret_cast<uptr>(&addr), addr);
+#endif
+  return addr;
+}
+
 uptr GetPageSize() {
 // Android post-M sysconf(_SC_PAGESIZE) crashes if called from .preinit_array.
 #if SANITIZER_ANDROID
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_mac.cc b/compiler-rt/lib/sanitizer_common/sanitizer_mac.cc
index 284b6cfc75f..195a52c4771 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_mac.cc
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_mac.cc
@@ -864,6 +864,10 @@ uptr GetMaxUserVirtualAddress() {
 #endif  // SANITIZER_WORDSIZE
 }
 
+uptr GetMaxVirtualAddress() {
+  return GetMaxUserVirtualAddress();
+}
+
 uptr FindAvailableMemoryRange(uptr shadow_size,
                               uptr alignment,
                               uptr left_padding,
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_win.cc b/compiler-rt/lib/sanitizer_common/sanitizer_win.cc
index ef69ad45e9f..0d72b494d25 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_win.cc
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_win.cc
@@ -70,6 +70,10 @@ uptr GetMaxUserVirtualAddress() {
   return (uptr)si.lpMaximumApplicationAddress;
 }
 
+uptr GetMaxVirtualAddress() {
+  return GetMaxUserVirtualAddress();
+}
+
 bool FileExists(const char *filename) {
   return ::GetFileAttributesA(filename) != INVALID_FILE_ATTRIBUTES;
 }
diff --git a/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp b/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
index 26bea031463..c707dfc0b50 100644
--- a/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -137,8 +137,8 @@ static const char *const kAsanUnregisterElfGlobalsName =
 static const char *const kAsanPoisonGlobalsName = "__asan_before_dynamic_init";
 static const char *const kAsanUnpoisonGlobalsName = "__asan_after_dynamic_init";
 static const char *const kAsanInitName = "__asan_init";
-static const char *const kAsanVersionCheckName =
-    "__asan_version_mismatch_check_v8";
+static const char *const kAsanVersionCheckNamePrefix =
+    "__asan_version_mismatch_check_v";
 static const char *const kAsanPtrCmp = "__sanitizer_ptr_cmp";
 static const char *const kAsanPtrSub = "__sanitizer_ptr_sub";
 static const char *const kAsanHandleNoReturnName = "__asan_handle_no_return";
@@ -208,6 +208,18 @@ static cl::opt<bool> ClForceDynamicShadow(
     cl::desc("Load shadow address into a local variable for each function"),
     cl::Hidden, cl::init(false));
 
+static cl::opt<bool>
+    ClWithIfunc("asan-with-ifunc",
+                cl::desc("Access dynamic shadow through an ifunc global on "
+                         "platforms that support this"),
+                cl::Hidden, cl::init(true));
+
+static cl::opt<bool> ClWithIfuncSuppressRemat(
+    "asan-with-ifunc-suppress-remat",
+    cl::desc("Suppress rematerialization of dynamic shadow address by passing "
+             "it through inline asm in prologue."),
+    cl::Hidden, cl::init(true));
+
 // This flag limits the number of instructions to be instrumented
 // in any given BB. Normally, this should be set to unlimited (INT_MAX),
 // but due to http://llvm.org/bugs/show_bug.cgi?id=12652 we temporary
@@ -448,10 +460,14 @@ private:
 
 /// This struct defines the shadow mapping using the rule:
 ///   shadow = (mem >> Scale) ADD-or-OR Offset.
+/// If InGlobal is true, then
+///   extern char __asan_shadow[];
+///   shadow = (mem >> Scale) + &__asan_shadow
 struct ShadowMapping {
   int Scale;
   uint64_t Offset;
   bool OrShadowOffset;
+  bool InGlobal;
 };
 
 } // end anonymous namespace
@@ -473,6 +489,7 @@ static ShadowMapping getShadowMapping(Triple &TargetTriple, int LongSize,
                   TargetTriple.getArch() == Triple::mipsel;
   bool IsMIPS64 = TargetTriple.getArch() == Triple::mips64 ||
                   TargetTriple.getArch() == Triple::mips64el;
+  bool IsArmOrThumb = TargetTriple.isARM() || TargetTriple.isThumb();
   bool IsAArch64 = TargetTriple.getArch() == Triple::aarch64;
   bool IsWindows = TargetTriple.isOSWindows();
   bool IsFuchsia = TargetTriple.isOSFuchsia();
@@ -485,10 +502,8 @@ static ShadowMapping getShadowMapping(Triple &TargetTriple, int LongSize,
   }
 
   if (LongSize == 32) {
-    // Android is always PIE, which means that the beginning of the address
-    // space is always available.
     if (IsAndroid)
-      Mapping.Offset = 0;
+      Mapping.Offset = kDynamicShadowSentinel;
     else if (IsMIPS32)
       Mapping.Offset = kMIPS32_ShadowOffset32;
     else if (IsFreeBSD)
@@ -552,6 +567,9 @@ static ShadowMapping getShadowMapping(Triple &TargetTriple, int LongSize,
   Mapping.OrShadowOffset = !IsAArch64 && !IsPPC64 && !IsSystemZ && !IsPS4CPU &&
                            !(Mapping.Offset & (Mapping.Offset - 1)) &&
                            Mapping.Offset != kDynamicShadowSentinel;
+  bool IsAndroidWithIfuncSupport =
+      IsAndroid && !TargetTriple.isAndroidVersionLT(21);
+  Mapping.InGlobal = ClWithIfunc && IsAndroidWithIfuncSupport && IsArmOrThumb;
 
   return Mapping;
 }
@@ -674,6 +692,7 @@ private:
   DominatorTree *DT;
   Function *AsanHandleNoReturnFunc;
   Function *AsanPtrCmpFunction, *AsanPtrSubFunction;
+  Constant *AsanShadowGlobal;
 
   // These arrays is indexed by AccessIsWrite, Experiment and log2(AccessSize).
   Function *AsanErrorCallback[2][2][kNumberOfAccessSizes];
@@ -746,6 +765,7 @@ private:
   size_t MinRedzoneSizeForGlobal() const {
     return RedzoneSizeForScale(Mapping.Scale);
   }
+  int GetAsanVersion(const Module &M) const;
 
   GlobalsMetadata GlobalsMD;
   bool CompileKernel;
@@ -978,8 +998,9 @@ struct FunctionStackPoisoner : public InstVisitor<FunctionStackPoisoner> {
   void visitCallSite(CallSite CS) {
     Instruction *I = CS.getInstruction();
     if (CallInst *CI = dyn_cast<CallInst>(I)) {
-      HasNonEmptyInlineAsm |=
-          CI->isInlineAsm() && !CI->isIdenticalTo(EmptyInlineAsm.get());
+      HasNonEmptyInlineAsm |= CI->isInlineAsm() &&
+                              !CI->isIdenticalTo(EmptyInlineAsm.get()) &&
+                              I != ASan.LocalDynamicShadow;
       HasReturnsTwiceCall |= CI->canReturnTwice();
     }
   }
@@ -2160,6 +2181,16 @@ bool AddressSanitizerModule::InstrumentGlobals(IRBuilder<> &IRB, Module &M, bool
   return true;
 }
 
+int AddressSanitizerModule::GetAsanVersion(const Module &M) const {
+  int LongSize = M.getDataLayout().getPointerSizeInBits();
+  bool isAndroid = Triple(M.getTargetTriple()).isAndroid();
+  int Version = 8;
+  // 32-bit Android is one version ahead because of the switch to dynamic
+  // shadow.
+  Version += (LongSize == 32 && isAndroid);
+  return Version;
+}
+
 bool AddressSanitizerModule::runOnModule(Module &M) {
   C = &(M.getContext());
   int LongSize = M.getDataLayout().getPointerSizeInBits();
@@ -2173,9 +2204,11 @@ bool AddressSanitizerModule::runOnModule(Module &M) {
 
   // Create a module constructor. A destructor is created lazily because not all
   // platforms, and not all modules need it.
+  std::string VersionCheckName =
+      kAsanVersionCheckNamePrefix + std::to_string(GetAsanVersion(M));
   std::tie(AsanCtorFunction, std::ignore) = createSanitizerCtorAndInitFunctions(
       M, kAsanModuleCtorName, kAsanInitName, /*InitArgTypes=*/{},
-      /*InitArgs=*/{}, kAsanVersionCheckName);
+      /*InitArgs=*/{}, VersionCheckName);
 
   bool CtorComdat = true;
   bool Changed = false;
@@ -2274,6 +2307,9 @@ void AddressSanitizer::initializeCallbacks(Module &M) {
   EmptyAsm = InlineAsm::get(FunctionType::get(IRB.getVoidTy(), false),
                             StringRef(""), StringRef(""),
                             /*hasSideEffects=*/true);
+  if (Mapping.InGlobal)
+    AsanShadowGlobal = M.getOrInsertGlobal("__asan_shadow",
+                                           ArrayType::get(IRB.getInt8Ty(), 0));
 }
 
 // virtual
@@ -2319,9 +2355,25 @@ void AddressSanitizer::maybeInsertDynamicShadowAtFunctionEntry(Function &F) {
     return;
 
   IRBuilder<> IRB(&F.front().front());
-  Value *GlobalDynamicAddress = F.getParent()->getOrInsertGlobal(
-      kAsanShadowMemoryDynamicAddress, IntptrTy);
-  LocalDynamicShadow = IRB.CreateLoad(GlobalDynamicAddress);
+  if (Mapping.InGlobal) {
+    if (ClWithIfuncSuppressRemat) {
+      // An empty inline asm with input reg == output reg.
+      // An opaque pointer-to-int cast, basically.
+      InlineAsm *Asm = InlineAsm::get(
+          FunctionType::get(IntptrTy, {AsanShadowGlobal->getType()}, false),
+          StringRef(""), StringRef("=r,0"),
+          /*hasSideEffects=*/false);
+      LocalDynamicShadow =
+          IRB.CreateCall(Asm, {AsanShadowGlobal}, ".asan.shadow");
+    } else {
+      LocalDynamicShadow =
+          IRB.CreatePointerCast(AsanShadowGlobal, IntptrTy, ".asan.shadow");
+    }
+  } else {
+    Value *GlobalDynamicAddress = F.getParent()->getOrInsertGlobal(
+        kAsanShadowMemoryDynamicAddress, IntptrTy);
+    LocalDynamicShadow = IRB.CreateLoad(GlobalDynamicAddress);
+  }
 }
 
 void AddressSanitizer::markEscapedLocalAllocas(Function &F) {
diff --git a/llvm/test/Instrumentation/AddressSanitizer/with-ifunc.ll b/llvm/test/Instrumentation/AddressSanitizer/with-ifunc.ll
new file mode 100644
index 00000000000..4771a967401
--- /dev/null
+++ b/llvm/test/Instrumentation/AddressSanitizer/with-ifunc.ll
@@ -0,0 +1,47 @@
+; Test -asan-with-ifunc flag.
+;
+; RUN: opt -asan -asan-module -S -asan-with-ifunc=0 < %s | \
+; RUN:     FileCheck %s --check-prefixes=CHECK,CHECK-NOIFUNC
+; RUN: opt -asan -asan-module -S -asan-with-ifunc=1 -asan-with-ifunc-suppress-remat=0 < %s | \
+; RUN:     FileCheck %s --check-prefixes=CHECK,CHECK-IFUNC
+; RUN: opt -asan -asan-module -S -asan-with-ifunc=1 -asan-with-ifunc-suppress-remat=1 < %s | \
+; RUN:     FileCheck %s --check-prefixes=CHECK,CHECK-IFUNC-NOREMAT
+
+; Pre-Lollipop Android does not support ifunc.
+; RUN: opt -asan -asan-module -S -asan-with-ifunc=1 -asan-with-ifunc-suppress-remat=0 -mtriple=armv7-linux-android20 < %s | \
+; RUN:     FileCheck %s --check-prefixes=CHECK,CHECK-NOIFUNC
+; RUN: opt -asan -asan-module -S -asan-with-ifunc=1 -asan-with-ifunc-suppress-remat=0 -mtriple=armv7-linux-android < %s | \
+; RUN:     FileCheck %s --check-prefixes=CHECK,CHECK-NOIFUNC
+; RUN: opt -asan -asan-module -S -asan-with-ifunc=1 -asan-with-ifunc-suppress-remat=0 -mtriple=armv7-linux-android21 < %s | \
+; RUN:     FileCheck %s --check-prefixes=CHECK,CHECK-IFUNC
+
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "armv7--linux-android22"
+
+; CHECK-IFUNC: @__asan_shadow = external global [0 x i8]
+; CHECK-NOIFUNC: @__asan_shadow_memory_dynamic_address = external global i32
+
+define i32 @test_load(i32* %a) sanitize_address {
+; First instrumentation in the function must be to load the dynamic shadow
+; address into a local variable.
+; CHECK-LABEL: @test_load
+; CHECK: entry:
+
+; CHECK-IFUNC-NEXT: %[[A:[^ ]*]] = ptrtoint i32* %a to i32
+; CHECK-IFUNC-NEXT: %[[B:[^ ]*]] = lshr i32 %[[A]], 3
+; CHECK-IFUNC-NEXT: %[[C:[^ ]*]] = add i32 %[[B]], ptrtoint ([0 x i8]* @__asan_shadow to i32)
+
+; CHECK-IFUNC-NOREMAT-NEXT: %[[S:[^ ]*]] = call i32 asm "", "=r,0"([0 x i8]* @__asan_shadow)
+; CHECK-IFUNC-NOREMAT-NEXT: %[[A:[^ ]*]] = ptrtoint i32* %a to i32
+; CHECK-IFUNC-NOREMAT-NEXT: %[[B:[^ ]*]] = lshr i32 %[[A]], 3
+; CHECK-IFUNC-NOREMAT-NEXT: %[[C:[^ ]*]] = add i32 %[[B]], %[[S]]
+
+; CHECK-NOIFUNC-NEXT: %[[SHADOW:[^ ]*]] = load i32, i32* @__asan_shadow_memory_dynamic_address
+; CHECK-NOIFUNC-NEXT: %[[A:[^ ]*]] = ptrtoint i32* %a to i32
+; CHECK-NOIFUNC-NEXT: %[[B:[^ ]*]] = lshr i32 %[[A]], 3
+; CHECK-NOIFUNC-NEXT: %[[C:[^ ]*]] = add i32 %[[B]], %[[SHADOW]]
+
+entry:
+  %x = load i32, i32* %a, align 4
+  ret i32 %x
+}