[PATCH] D22227: [ubsan] Disable bounds-check for flexible array ivars

Vedant Kumar via cfe-commits cfe-commits at lists.llvm.org
Mon Jul 11 10:35:02 PDT 2016

vsk created this revision.
vsk added reviewers: rsmith, samsonov.
vsk added a subscriber: cfe-commits.

Ubsan does not emit bounds checks for flexible array members, e.g:
  struct Foo { char arr[0]; };
  char bar(struct Foo *F) { return F->arr[1]; }

Teach ubsan to skip the bounds check for flexible array ivars as well.

This reduces the false-positive rate when instrumenting Objective-C frameworks.

Ubsan tests are typically added to compiler-rt. I chose to create a test in clang instead because I didn't want to introduce a libobjc dependency. The test works by checking for the "nosanitize" attribute: hopefully this is less brittle than checking for the name of the relevant runtime handler.



Index: test/CodeGenObjC/ubsan.m
--- /dev/null
+++ test/CodeGenObjC/ubsan.m
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -x objective-c -emit-llvm -triple x86_64-apple-macosx10.10.0 -Wno-objc-root-class -fsanitize=array-bounds %s -o - | FileCheck %s
+ at interface HasFlexibleArray {
+  @public char chars[0];
+ at end
+ at implementation HasFlexibleArray @end
+// CHECK-LABEL: do_not_instrument_flexible_array_members
+char do_not_instrument_flexible_array_members(HasFlexibleArray *HFA) {
+// CHECK-NOT: !nosanitize
+  return HFA->chars[1];
+// CHECK: }
Index: lib/CodeGen/CGExpr.cpp
--- lib/CodeGen/CGExpr.cpp
+++ lib/CodeGen/CGExpr.cpp
@@ -708,6 +708,8 @@
           DeclContext::decl_iterator(const_cast<FieldDecl *>(FD)));
       return ++FI == FD->getParent()->field_end();
+  } else if (const auto *IRE = dyn_cast<ObjCIvarRefExpr>(E)) {
+    return IRE->getDecl()->getNextIvar() == nullptr;
   return false;

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