[PATCH] D25857: [tsan] Introduce a function attribute to disable TSan checking at run time

[Anna Zaks via cfe-commits]

zaks.anna created this revision.
zaks.anna added reviewers: kcc, kubabrecka, dvyukov.
zaks.anna added a subscriber: cfe-commits.

This introduces a function annotation that disables TSan checking for the function at run time. The benefit over __attribute__((no_sanitize("thread"))) is that the accesses within the callees will also be suppressed.

The motivation for this attribute is a guarantee given by the objective C language that the calls to the reference count decrement and object deallocation will be synchronized. To model this properly, we would need to intercept all ref count decrement calls (which are very common in ObjC due to use of ARC) and also every single message send. Instead, we propose to just ignore all accesses made from within dealloc at run time. The main downside is that this still does not introduce any synchronization, which means we might still report false positives if the code that relies on this synchronization is not executed from within dealloc. However, we have not seen this in practice so far and think these cases will be very rare.

(This problem is similar in nature to https://reviews.llvm.org/D21609; unfortunately, the same solution does not apply here.)


https://reviews.llvm.org/D25857

Files:
  lib/CodeGen/CodeGenFunction.cpp
  test/CodeGen/sanitize-thread-attr-dealloc.m


Index: test/CodeGen/sanitize-thread-attr-dealloc.m
===================================================================
--- /dev/null
+++ test/CodeGen/sanitize-thread-attr-dealloc.m
@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -triple x86_64-apple-darwin -emit-llvm -o - %s | FileCheck -check-prefix=WITHOUT %s
+// RUN: %clang_cc1 -triple x86_64-apple-darwin -emit-llvm -o - %s -fsanitize=thread | FileCheck -check-prefix=TSAN %s
+
+__attribute__((objc_root_class))
+ at interface NSObject
+- (void)dealloc;
+ at end
+
+ at interface MyObject : NSObject
+- (void) dealloc;
+ at end
+
+ at implementation MyObject
+- (void)dealloc {
+  [super dealloc];
+}
+ at end
+
+// WITHOUT-NOT: "sanitize_thread_no_checking_at_run_time"
+
+// TSAN: dealloc{{.*}}) [[ATTR:#[0-9]+]]
+// TSAN: attributes [[ATTR]] = { nounwind sanitize_thread {{.*}} "sanitize_thread_no_checking_at_run_time" {{.*}} }
+
Index: lib/CodeGen/CodeGenFunction.cpp
===================================================================
--- lib/CodeGen/CodeGenFunction.cpp
+++ lib/CodeGen/CodeGenFunction.cpp
@@ -714,6 +714,13 @@
   if (SanOpts.has(SanitizerKind::SafeStack))
     Fn->addFnAttr(llvm::Attribute::SafeStack);
 
+  // Ignore TSan memory acesses from within dealloc and all of its calees at
+  // run time.
+  if (SanOpts.has(SanitizerKind::Thread))
+    if (const auto *M = dyn_cast_or_null<ObjCMethodDecl>(D))
+      if (M->getMethodFamily() == OMF_dealloc)
+        Fn->addFnAttr("sanitize_thread_no_checking_at_run_time");
+
   // Apply xray attributes to the function (as a string, for now)
   if (D && ShouldXRayInstrumentFunction()) {
     if (const auto *XRayAttr = D->getAttr<XRayInstrumentAttr>()) {


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