Add API to temporalily disable usage of ASAN's fake stack #160135

happyCoder92 · 2025-09-22T15:47:38Z

Intended use-case is for threads that use (or switch to) stack with special properties e.g. backed by MADV_DONTDUMP memory.

llvmbot · 2025-09-22T15:48:16Z

@llvm/pr-subscribers-compiler-rt-sanitizer

Author: None (happyCoder92)

Changes

Intended use-case is for threads that use (or switch to) stack with special properties e.g. backed by MADV_DONTDUMP memory.

Full diff: https://github.com/llvm/llvm-project/pull/160135.diff

10 Files Affected:

(modified) compiler-rt/include/sanitizer/asan_interface.h (+7)
(modified) compiler-rt/lib/asan/asan_fake_stack.cpp (+30-7)
(modified) compiler-rt/lib/asan/asan_fake_stack.h (+3-1)
(modified) compiler-rt/lib/asan/asan_interface.inc (+2)
(modified) compiler-rt/lib/asan/asan_thread.cpp (+4-3)
(modified) compiler-rt/lib/asan/asan_thread.h (+7-1)
(modified) compiler-rt/lib/asan_abi/asan_abi.cpp (+2)
(modified) compiler-rt/lib/asan_abi/asan_abi.h (+3)
(modified) compiler-rt/lib/asan_abi/asan_abi_shim.cpp (+6)
(added) compiler-rt/test/asan/TestCases/disable_fake_stack.cpp (+27)

diff --git a/compiler-rt/include/sanitizer/asan_interface.h b/compiler-rt/include/sanitizer/asan_interface.h
index 37b6d08f4db19..06deb9116709f 100644
--- a/compiler-rt/include/sanitizer/asan_interface.h
+++ b/compiler-rt/include/sanitizer/asan_interface.h
@@ -333,6 +333,13 @@ void SANITIZER_CDECL __asan_handle_no_return(void);
 /// trace. Returns 1 if successful, 0 if not.
 int SANITIZER_CDECL __asan_update_allocation_context(void *addr);
 
+/// Disables fake stack usage for the current thread.
+/// Temporarily disables use-after-return detection for current thread.
+void SANITIZER_CDECL __asan_disable_fake_stack_usage(void);
+
+/// (Re)enables fake stack usage for the current thread.
+void SANITIZER_CDECL __asan_enable_fake_stack_usage(void);
+
 #ifdef __cplusplus
 } // extern "C"
 #endif
diff --git a/compiler-rt/lib/asan/asan_fake_stack.cpp b/compiler-rt/lib/asan/asan_fake_stack.cpp
index c3ed2526f0ed4..e1772997d681f 100644
--- a/compiler-rt/lib/asan/asan_fake_stack.cpp
+++ b/compiler-rt/lib/asan/asan_fake_stack.cpp
@@ -217,26 +217,44 @@ static THREADLOCAL FakeStack *fake_stack_tls;
 FakeStack *GetTLSFakeStack() {
   return fake_stack_tls;
 }
-void SetTLSFakeStack(FakeStack *fs) {
+void SetTLSFakeStack(AsanThread* t, FakeStack *fs) {
+  if (fs && !t->fake_stack_usage_enabled()) {
+    return;
+  }
   fake_stack_tls = fs;
 }
 #else
 FakeStack *GetTLSFakeStack() { return 0; }
-void SetTLSFakeStack(FakeStack *fs) { }
+void SetTLSFakeStack(AsanThread* t, FakeStack *fs) { }
 #endif  // (SANITIZER_LINUX && !SANITIZER_ANDROID) || SANITIZER_FUCHSIA
 
-static FakeStack *GetFakeStack() {
+static void DisableFakeStackUsage() {
   AsanThread *t = GetCurrentThread();
-  if (!t) return nullptr;
+  if (t) {
+    t->set_fake_stack_usage_enabled(false);
+  }
+  SetTLSFakeStack(t, nullptr);
+}
+
+static void EnableFakeStackUsage() {
+  AsanThread *t = GetCurrentThread();
+  if (t) {
+    t->set_fake_stack_usage_enabled(true);
+  }
+}
+
+static FakeStack *GetFakeStack(bool for_allocation = true) {
+  AsanThread *t = GetCurrentThread();
+  if (!t || (for_allocation && !t->fake_stack_usage_enabled())) return nullptr;
   return t->get_or_create_fake_stack();
 }
 
-static FakeStack *GetFakeStackFast() {
+static FakeStack *GetFakeStackFast(bool for_allocation = true) {
   if (FakeStack *fs = GetTLSFakeStack())
     return fs;
   if (!__asan_option_detect_stack_use_after_return)
     return nullptr;
-  return GetFakeStack();
+  return GetFakeStack(for_allocation);
 }
 
 static FakeStack *GetFakeStackFastAlways() {
@@ -311,7 +329,7 @@ extern "C" {
 // -asan-use-after-return=never, after modal UAR flag lands
 // (https://github.com/google/sanitizers/issues/1394)
 SANITIZER_INTERFACE_ATTRIBUTE
-void *__asan_get_current_fake_stack() { return GetFakeStackFast(); }
+void *__asan_get_current_fake_stack() { return GetFakeStackFast(/*for_allocation=*/false); }
 
 SANITIZER_INTERFACE_ATTRIBUTE
 void *__asan_addr_is_in_fake_stack(void *fake_stack, void *addr, void **beg,
@@ -349,4 +367,9 @@ void __asan_allocas_unpoison(uptr top, uptr bottom) {
   (reinterpret_cast<void *>(MemToShadow(top)), 0,
    (bottom - top) / ASAN_SHADOW_GRANULARITY);
 }
+
+SANITIZER_INTERFACE_ATTRIBUTE
+void __asan_disable_fake_stack_usage() { return DisableFakeStackUsage(); }
+SANITIZER_INTERFACE_ATTRIBUTE
+void __asan_enable_fake_stack_usage() { return EnableFakeStackUsage(); }
 } // extern "C"
diff --git a/compiler-rt/lib/asan/asan_fake_stack.h b/compiler-rt/lib/asan/asan_fake_stack.h
index 50706e6e5876c..9f1f452834b97 100644
--- a/compiler-rt/lib/asan/asan_fake_stack.h
+++ b/compiler-rt/lib/asan/asan_fake_stack.h
@@ -18,6 +18,8 @@
 
 namespace __asan {
 
+class AsanThread;
+
 // Fake stack frame contains local variables of one function.
 struct FakeFrame {
   uptr magic;  // Modified by the instrumented code.
@@ -196,7 +198,7 @@ class FakeStack {
 };
 
 FakeStack *GetTLSFakeStack();
-void SetTLSFakeStack(FakeStack *fs);
+void SetTLSFakeStack(AsanThread* t, FakeStack *fs);
 
 }  // namespace __asan
 
diff --git a/compiler-rt/lib/asan/asan_interface.inc b/compiler-rt/lib/asan/asan_interface.inc
index bfc44b4619623..a279ae834549a 100644
--- a/compiler-rt/lib/asan/asan_interface.inc
+++ b/compiler-rt/lib/asan/asan_interface.inc
@@ -15,6 +15,8 @@ INTERFACE_FUNCTION(__asan_alloca_poison)
 INTERFACE_FUNCTION(__asan_allocas_unpoison)
 INTERFACE_FUNCTION(__asan_before_dynamic_init)
 INTERFACE_FUNCTION(__asan_describe_address)
+INTERFACE_FUNCTION(__asan_disable_fake_stack_usage)
+INTERFACE_FUNCTION(__asan_enable_fake_stack_usage)
 INTERFACE_FUNCTION(__asan_exp_load1)
 INTERFACE_FUNCTION(__asan_exp_load2)
 INTERFACE_FUNCTION(__asan_exp_load4)
diff --git a/compiler-rt/lib/asan/asan_thread.cpp b/compiler-rt/lib/asan/asan_thread.cpp
index 2627ae1289012..f6f54022bf377 100644
--- a/compiler-rt/lib/asan/asan_thread.cpp
+++ b/compiler-rt/lib/asan/asan_thread.cpp
@@ -163,7 +163,7 @@ void AsanThread::StartSwitchFiber(FakeStack **fake_stack_save, uptr bottom,
   if (fake_stack_save)
     *fake_stack_save = fake_stack_;
   fake_stack_ = nullptr;
-  SetTLSFakeStack(nullptr);
+  SetTLSFakeStack(this, nullptr);
   // if fake_stack_save is null, the fiber will die, delete the fakestack
   if (!fake_stack_save && current_fake_stack)
     current_fake_stack->Destroy(this->tid());
@@ -177,7 +177,7 @@ void AsanThread::FinishSwitchFiber(FakeStack *fake_stack_save, uptr *bottom_old,
   }
 
   if (fake_stack_save) {
-    SetTLSFakeStack(fake_stack_save);
+    SetTLSFakeStack(this, fake_stack_save);
     fake_stack_ = fake_stack_save;
   }
 
@@ -242,7 +242,7 @@ FakeStack *AsanThread::AsyncSignalSafeLazyInitFakeStack() {
         Max(stack_size_log, static_cast<uptr>(flags()->min_uar_stack_size_log));
     fake_stack_ = FakeStack::Create(stack_size_log);
     DCHECK_EQ(GetCurrentThread(), this);
-    SetTLSFakeStack(fake_stack_);
+    SetTLSFakeStack(this, fake_stack_);
     return fake_stack_;
   }
   return nullptr;
@@ -251,6 +251,7 @@ FakeStack *AsanThread::AsyncSignalSafeLazyInitFakeStack() {
 void AsanThread::Init(const InitOptions *options) {
   DCHECK_NE(tid(), kInvalidTid);
   next_stack_top_ = next_stack_bottom_ = 0;
+  fake_stack_usage_enabled_ = true;
   atomic_store(&stack_switching_, false, memory_order_release);
   CHECK_EQ(this->stack_size(), 0U);
   SetThreadStackAndTls(options);
diff --git a/compiler-rt/lib/asan/asan_thread.h b/compiler-rt/lib/asan/asan_thread.h
index 12f0cc7a62dae..c54bbb1c64604 100644
--- a/compiler-rt/lib/asan/asan_thread.h
+++ b/compiler-rt/lib/asan/asan_thread.h
@@ -104,7 +104,7 @@ class AsanThread {
     if (!fake_stack_) return;
     FakeStack *t = fake_stack_;
     fake_stack_ = nullptr;
-    SetTLSFakeStack(nullptr);
+    SetTLSFakeStack(this, nullptr);
     t->Destroy(tid);
   }
 
@@ -144,6 +144,11 @@ class AsanThread {
     GetStartData(&data, sizeof(data));
   }
 
+  bool fake_stack_usage_enabled() const { return fake_stack_usage_enabled_; }
+  void set_fake_stack_usage_enabled(bool enabled) {
+    fake_stack_usage_enabled_ = enabled;
+  }
+
  private:
   // NOTE: There is no AsanThread constructor. It is allocated
   // via mmap() and *must* be valid in zero-initialized state.
@@ -179,6 +184,7 @@ class AsanThread {
   DTLS *dtls_;
 
   FakeStack *fake_stack_;
+  bool fake_stack_usage_enabled_;
   AsanThreadLocalMallocStorage malloc_storage_;
   AsanStats stats_;
   bool unwinding_;
diff --git a/compiler-rt/lib/asan_abi/asan_abi.cpp b/compiler-rt/lib/asan_abi/asan_abi.cpp
index cf8663024eb73..c2bf2f52dd4e5 100644
--- a/compiler-rt/lib/asan_abi/asan_abi.cpp
+++ b/compiler-rt/lib/asan_abi/asan_abi.cpp
@@ -73,6 +73,8 @@ void *__asan_abi_addr_is_in_fake_stack(void *fake_stack, void *addr, void **beg,
                                        void **end) {
   return NULL;
 }
+void __asan_abi_disable_fake_stack_usage(void) {}
+void __asan_abi_enable_fake_stack_usage(void) {}
 
 // Functions concerning poisoning and unpoisoning fake stack alloca
 void __asan_abi_alloca_poison(void *addr, size_t size) {}
diff --git a/compiler-rt/lib/asan_abi/asan_abi.h b/compiler-rt/lib/asan_abi/asan_abi.h
index 8702bcd133919..372e44e755cc5 100644
--- a/compiler-rt/lib/asan_abi/asan_abi.h
+++ b/compiler-rt/lib/asan_abi/asan_abi.h
@@ -76,6 +76,9 @@ void *__asan_abi_load_cxx_array_cookie(void **p);
 void *__asan_abi_get_current_fake_stack();
 void *__asan_abi_addr_is_in_fake_stack(void *fake_stack, void *addr, void **beg,
                                        void **end);
+void *__asan_abi_disable_fake_stack_usage();
+void *__asan_abi_enable_fake_stack_usage();
+
 // Functions concerning poisoning and unpoisoning fake stack alloca
 void __asan_abi_alloca_poison(void *addr, size_t size);
 void __asan_abi_allocas_unpoison(void *top, void *bottom);
diff --git a/compiler-rt/lib/asan_abi/asan_abi_shim.cpp b/compiler-rt/lib/asan_abi/asan_abi_shim.cpp
index 2512abc641250..0b4a170015b61 100644
--- a/compiler-rt/lib/asan_abi/asan_abi_shim.cpp
+++ b/compiler-rt/lib/asan_abi/asan_abi_shim.cpp
@@ -365,6 +365,12 @@ void *__asan_addr_is_in_fake_stack(void *fake_stack, void *addr, void **beg,
                                    void **end) {
   return __asan_abi_addr_is_in_fake_stack(fake_stack, addr, beg, end);
 }
+void __asan_disable_fake_stack_usage(void) {
+  return __asan_abi_disable_fake_stack_usage();
+}
+void __asan_enable_fake_stack_usage(void) {
+  return __asan_abi_enable_fake_stack_usage();
+}
 
 // Functions concerning poisoning and unpoisoning fake stack alloca
 void __asan_alloca_poison(uptr addr, uptr size) {
diff --git a/compiler-rt/test/asan/TestCases/disable_fake_stack.cpp b/compiler-rt/test/asan/TestCases/disable_fake_stack.cpp
new file mode 100644
index 0000000000000..b1ee8d72e2b58
--- /dev/null
+++ b/compiler-rt/test/asan/TestCases/disable_fake_stack.cpp
@@ -0,0 +1,27 @@
+// RUN: %clangxx_asan %s -o %t && %run %t
+
+#include "defines.h"
+
+#include <sanitizer/asan_interface.h>
+
+volatile char *saved;
+
+ATTRIBUTE_NOINLINE bool IsOnStack() {
+  volatile char temp = ' ';
+  void* fake_stack = __asan_get_current_fake_stack();
+  void* real = __asan_addr_is_in_fake_stack(fake_stack, const_cast<char*>(&temp), nullptr, nullptr);
+  saved = &temp;
+  return real == nullptr;
+}
+
+int main(int argc, char *argv[]) {
+  __asan_disable_fake_stack_usage();
+  if (!IsOnStack()) {
+    return 1;
+  }
+  __asan_enable_fake_stack_usage();
+  if (IsOnStack()) {
+    return 2;
+  }
+  return 0;
+}

github-actions · 2025-09-22T15:51:18Z

✅ With the latest revision this PR passed the C/C++ code formatter.

compiler-rt/include/sanitizer/asan_interface.h

compiler-rt/lib/asan/asan_thread.h

vitalybuka · 2025-09-22T16:55:06Z

I thought StartSwitchFiber was added for that

I am not sure I understand what is special with MADV_DONTDUMP
Can you add a test which essentially reproduces the issue?
You don't need to remove the existing test.

vitalybuka

comments above

happyCoder92 · 2025-09-23T12:49:31Z

Addressed the comments.
We switch to a MADV_DONTDUMP stack and don't want any of the local variable contents to end up in a core dump.
As fake stack is not allocated with the same flags, its usage will result in those variables ending up in the core dump.

compiler-rt/lib/asan/asan_fake_stack.cpp

vitalybuka · 2025-10-14T23:56:15Z

compiler-rt/lib/asan/asan_fake_stack.cpp

@@ -311,7 +329,9 @@ extern "C" {
 // -asan-use-after-return=never, after modal UAR flag lands
 // (https://github.com/google/sanitizers/issues/1394)
 SANITIZER_INTERFACE_ATTRIBUTE
-void *__asan_get_current_fake_stack() { return GetFakeStackFast(); }
+void* __asan_get_current_fake_stack() {


There is TODO for to remove this one
if this is just for test, can we instead rely on __asan_get_current_fake_stack
Use __builtin_frame_address

__builtin_frame_address always return real stack address

Removed the argument and adjusted the test.

Did you forget to upload the patch?

I have rebased and updated it

vitalybuka · 2025-10-15T00:38:10Z

compiler-rt/lib/asan/asan_fake_stack.cpp

@@ -217,26 +217,44 @@ static THREADLOCAL FakeStack *fake_stack_tls;
 FakeStack *GetTLSFakeStack() {
  return fake_stack_tls;
 }
-void SetTLSFakeStack(FakeStack *fs) {
+void SetTLSFakeStack(AsanThread* t, FakeStack* fs) {


Will #163481 help to simplify this patch?

Slightly. By only calling SetTLSFakeStack from GetFakeStack #163481 also indirectly ensures the tls var won't be set if fake stack is disabled. I added the thread arg and put the logic into SetTLSFakeStack to make it more explicit.

Intended use-case is for threads that use (or switch to) stack with special properties e.g. backed by MADV_DONTDUMP memory.

…ake_stack_tls (#163481) To simplify implementation of #160135

…163674) To simplify implementation of #160135 To keep the logic of figuring out what should be in TLS to one place. The rest of the code should just reset it and rely on GetFakeStackFast()/GetFakeStackFastAlways().

Intended use-case is for threads that use (or switch to) stack with special properties e.g. backed by MADV_DONTDUMP memory.

This reverts commit 4e72801.

vitalybuka · 2025-10-16T02:36:46Z

@happyCoder92 can you please elaborate description message?

…SFakeStack (#163674) To simplify implementation of llvm/llvm-project#160135 To keep the logic of figuring out what should be in TLS to one place. The rest of the code should just reset it and rely on GetFakeStackFast()/GetFakeStackFastAlways().

thurstond

What is the intended interaction of this API with -mllvm -asan-use-after-return={never,runtime,always} and ASAN_OPTIONS=detect_stack_use_after_return={0,1}? Please update the description and TestCases/disable_fake_stack.cpp to clarify.

Also, if the behavior of __asan_disable_fake_stack()/__asan_enable_fake_stack() depends on those flags, the naming could be confusing.

happyCoder92 · 2025-10-17T13:31:58Z

The behavior does depend on the flags. Would __asan_suppress_fake_stack and __asan_unsuppress_fake_stack be less confusing?

llvmbot added compiler-rt compiler-rt:asan Address sanitizer compiler-rt:sanitizer labels Sep 22, 2025

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happyCoder92 force-pushed the main branch from de92c31 to f6e0711 Compare September 23, 2025 12:42

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vitalybuka mentioned this pull request Oct 15, 2025

[asan] Make GetFakeStackFast()/GetFakeStackFastAlways() lazily init fake_stack_tls #163481

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Add API to temporalily disable usage of ASAN's fake stack

4e72801

Intended use-case is for threads that use (or switch to) stack with special properties e.g. backed by MADV_DONTDUMP memory.

happyCoder92 force-pushed the main branch from 10972a7 to 4e72801 Compare October 15, 2025 13:11

vitalybuka mentioned this pull request Oct 16, 2025

[asan] Hide SetTLSFakeStack and replaces uses with ResetTLSFakeStack #163674

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vitalybuka added a commit that referenced this pull request Oct 16, 2025

[asan] Make GetFakeStackFast()/GetFakeStackFastAlways() lazily init f…

082efba

…ake_stack_tls (#163481) To simplify implementation of #160135

happyCoder92 and others added 6 commits October 15, 2025 18:58

Add API to temporalily disable usage of ASAN's fake stack

c10e568

Intended use-case is for threads that use (or switch to) stack with special properties e.g. backed by MADV_DONTDUMP memory.

Revert "Add API to temporalily disable usage of ASAN's fake stack"

0769e3d

This reverts commit 4e72801.

Merge branch 'happyCoder92/main' into HEAD

41c302e

Update test

7657030

format

f56cd99

format

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vitalybuka requested review from fmayer and thurstond October 16, 2025 02:34

format

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Add API to temporalily disable usage of ASAN's fake stack #160135

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