[ubsan] Have -fsanitize=vptr emit a null check if -fsanitize=null isn't available

[vsk]

Summary

In r309007, I made -fsanitize=null a hard prerequisite for -fsanitize=vptr. I did not see the need for the two checks to have separate null checking logic for the same pointer. I expected the two checks to either always be enabled together, or to be mutually compatible.

In the mailing list discussion re: r309007 it became clear that that isn't the case. If a codebase is -fsanitize=vptr clean but not -fsanitize=null clean, it's useful to have -fsanitize=vptr emit its own null check. That's what this patch does: with it, -fsanitize=vptr can be used without -fsanitize=null.

[arphaman]

That makes sense. It's kinda weird not to report the null, but I guess it makes sense if the null sanitiser is off. It's not actually UB unless it's dereferenced, right, so casts are allowed?

test/CodeGenCXX/ubsan-type-checks.cpp

73 ↗

(On Diff #108988)

Can you use reuse the first invalid_cast with different checks since the code is the same?

[vsk]

In D36112#828891, @arphaman wrote:

That makes sense. It's kinda weird not to report the null, but I guess it makes sense if the null sanitiser is off.

It is kinda weird, but any such diagnostic would fit better under -fsanitize=null, and that's explicitly not enabled.

It's not actually UB unless it's dereferenced, right, so casts are allowed?

Right, even the null sanitizer doesn't issue reports when it finds an {up,down}cast of null.

@arphaman thanks for taking a look! I've fixed the test case issue you pointed out and added minor comment and doc updates. I'll go ahead and commit to unblock Nico.

cfe/trunk/docs/ReleaseNotes.rst

 Static Analyzer
 ---------------
 
 ...
 
 Undefined Behavior Sanitizer (UBSan)
 ------------------------------------
 
-The C++ dynamic type check now requires run-time null checking (i.e,
-`-fsanitize=vptr` cannot be used without `-fsanitize=null`). This change does
-not impact users who rely on UBSan check groups (e.g `-fsanitize=undefined`).
+...
 
 Core Analysis Improvements
 ==========================
 
 - ...
 
 New Issues Found
 ================

cfe/trunk/docs/UndefinedBehaviorSanitizer.rst

-  ``-fsanitize=unreachable``: If control flow reaches
      ``__builtin_unreachable``.
   -  ``-fsanitize=unsigned-integer-overflow``: Unsigned integer
      overflows. Note that unlike signed integer overflow, unsigned integer
      is not undefined behavior. However, while it has well-defined semantics,
      it is often unintentional, so UBSan offers to catch it.
   -  ``-fsanitize=vla-bound``: A variable-length array whose bound
      does not evaluate to a positive value.
   -  ``-fsanitize=vptr``: Use of an object whose vptr indicates that it is of
-     the wrong dynamic type, or that its lifetime has not begun or has ended.
-     Incompatible with ``-fno-rtti`` and ``-fno-sanitize=null``. Link must be
-     performed by ``clang++``, not ``clang``, to make sure C++-specific parts of
-     the runtime library and C++ standard libraries are present.
+    Incompatible with ``-fno-rtti``. Link must be performed by ``clang++``, not
+    ``clang``, to make sure C++-specific parts of the runtime library and C++
+    standard libraries are present.
 
 You can also use the following check groups:
   -  ``-fsanitize=undefined``: All of the checks listed above other than
      ``unsigned-integer-overflow`` and the ``nullability-*`` checks.
   -  ``-fsanitize=undefined-trap``: Deprecated alias of
      ``-fsanitize=undefined``.
   -  ``-fsanitize=integer``: Checks for undefined or suspicious integer
      behavior (e.g. unsigned integer overflow).

cfe/trunk/include/clang/Basic/DiagnosticDriverKinds.td

 def warn_debug_compression_unavailable : Warning<"cannot compress debug sections (zlib not installed)">,
   InGroup<DiagGroup<"debug-compression-unavailable">>;
 def warn_drv_enabling_rtti_with_exceptions : Warning<
   "implicitly enabling rtti for exception handling">,
   InGroup<DiagGroup<"rtti-for-exceptions">>;
 def warn_drv_disabling_vptr_no_rtti_default : Warning<
   "implicitly disabling vptr sanitizer because rtti wasn't enabled">,
   InGroup<AutoDisableVptrSanitizer>;
-def warn_drv_disabling_vptr_no_null_check : Warning<
-  "implicitly disabling vptr sanitizer because null checking wasn't enabled">,
-  InGroup<AutoDisableVptrSanitizer>;
 def warn_drv_object_size_disabled_O0 : Warning<
   "the object size sanitizer has no effect at -O0, but is explicitly enabled: %0">,
   InGroup<InvalidCommandLineArgument>;
 
 def note_drv_command_failed_diag_msg : Note<
   "diagnostic msg: %0">;
 def note_drv_t_option_is_global : Note<
   "The last /TC or /TP option takes precedence over earlier instances">;

cfe/trunk/lib/CodeGen/CGExpr.cpp

void CodeGenFunction::EmitTypeCheck(TypeCheckKind TCK, SourceLocation Loc,
   // type Ty at offset zero within this object.
   //
   // C++11 [basic.life]p5,6:
   //   [For storage which does not refer to an object within its lifetime]
   //   The program has undefined behavior if:
   //    -- the [pointer or glvalue] is used to access a non-static data member
   //       or call a non-static member function
   CXXRecordDecl *RD = Ty->getAsCXXRecordDecl();
-  bool HasNullCheck = IsGuaranteedNonNull || IsNonNull;
   if (SanOpts.has(SanitizerKind::Vptr) &&
-      !SkippedChecks.has(SanitizerKind::Vptr) && HasNullCheck &&
+      !SkippedChecks.has(SanitizerKind::Vptr) &&
       (TCK == TCK_MemberAccess || TCK == TCK_MemberCall ||
        TCK == TCK_DowncastPointer || TCK == TCK_DowncastReference ||
        TCK == TCK_UpcastToVirtualBase) &&
       RD && RD->hasDefinition() && RD->isDynamicClass()) {
     // Ensure that the pointer is non-null before loading it. If there is no
-    // compile-time guarantee, reuse the run-time null check.
+    // compile-time guarantee, reuse the run-time null check or emit a new one.
     if (!IsGuaranteedNonNull) {
-      assert(IsNonNull && "Missing run-time null check");
+      if (!IsNonNull)
+        IsNonNull = Builder.CreateIsNotNull(Ptr);
       if (!Done)
         Done = createBasicBlock("vptr.null");
       llvm::BasicBlock *VptrNotNull = createBasicBlock("vptr.not.null");
       Builder.CreateCondBr(IsNonNull, VptrNotNull, Done);
       EmitBlock(VptrNotNull);
     }
 
     // Compute a hash of the mangled name of the type.

cfe/trunk/lib/Driver/SanitizerArgs.cpp

SanitizerArgs::SanitizerArgs(const ToolChain &TC,
   // We disable the vptr sanitizer if it was enabled by group expansion but RTTI
   // is disabled.
   if ((Kinds & Vptr) &&
       (RTTIMode == ToolChain::RM_DisabledImplicitly ||
        RTTIMode == ToolChain::RM_DisabledExplicitly)) {
     Kinds &= ~Vptr;
   }
 
-  // Disable -fsanitize=vptr if -fsanitize=null is not enabled (the vptr
-  // instrumentation is broken without run-time null checks).
-  if ((Kinds & Vptr) && !(Kinds & Null)) {
-    Kinds &= ~Vptr;
-    D.Diag(diag::warn_drv_disabling_vptr_no_null_check);
-  }
-
   // Check that LTO is enabled if we need it.
   if ((Kinds & NeedsLTO) && !D.isUsingLTO()) {
     D.Diag(diag::err_drv_argument_only_allowed_with)
         << lastArgumentForMask(D, Args, Kinds & NeedsLTO) << "-flto";
   }
 
   // Report error if there are non-trapping sanitizers that require
   // c++abi-specific  parts of UBSan runtime, and they are not provided by the

cfe/trunk/test/CodeGenCXX/catch-undef-behavior.cpp

 // RUN: %clang_cc1 -std=c++11 -fsanitize=signed-integer-overflow,integer-divide-by-zero,float-divide-by-zero,shift-base,shift-exponent,unreachable,return,vla-bound,alignment,null,vptr,object-size,float-cast-overflow,bool,enum,array-bounds,function -fsanitize-recover=signed-integer-overflow,integer-divide-by-zero,float-divide-by-zero,shift-base,shift-exponent,vla-bound,alignment,null,vptr,object-size,float-cast-overflow,bool,enum,array-bounds,function -emit-llvm %s -o - -triple x86_64-linux-gnu | opt -instnamer -S | FileCheck %s
-// RUN: %clang_cc1 -std=c++11 -fsanitize=null,vptr,address -fsanitize-recover=null,vptr,address -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s --check-prefix=CHECK-ASAN
-// RUN: %clang_cc1 -std=c++11 -fsanitize=null,vptr -fsanitize-recover=null,vptr -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s --check-prefix=DOWNCAST-NULL
+// RUN: %clang_cc1 -std=c++11 -fsanitize=vptr,address -fsanitize-recover=vptr,address -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s --check-prefix=CHECK-ASAN
+// RUN: %clang_cc1 -std=c++11 -fsanitize=vptr -fsanitize-recover=vptr -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s --check-prefix=DOWNCAST-NULL
 // RUN: %clang_cc1 -std=c++11 -fsanitize=function -emit-llvm %s -o - -triple x86_64-linux-gnux32 | FileCheck %s --check-prefix=CHECK-X32
 // RUN: %clang_cc1 -std=c++11 -fsanitize=function -emit-llvm %s -o - -triple i386-linux-gnu | FileCheck %s --check-prefix=CHECK-X86
 
 struct S {
   double d;
   int a, b;
   virtual int f();
 };

cfe/trunk/test/CodeGenCXX/ubsan-type-checks.cpp

 };
 
 struct Dog : Animal {
   const char *speak() override { return "woof"; }
 };
 
 // VPTR-LABEL: define void @_Z12invalid_castP3Cat
 void invalid_cast(Cat *cat = nullptr) {
-  // First, null check the pointer:
+  // If -fsanitize=null is available, we'll reuse its check:
   //
   // VPTR: [[ICMP:%.*]] = icmp ne %struct.Dog* {{.*}}, null
   // VPTR-NEXT: br i1 [[ICMP]]
   // VPTR: call void @__ubsan_handle_type_mismatch
-  //
-  // Once we're done emitting the null check, reuse the check to see if we can
-  // proceed to the vptr check:
-  //
+  // VPTR-NOT: icmp ne %struct.Dog* {{.*}}, null
   // VPTR: br i1 [[ICMP]]
   // VPTR: call void @__ubsan_handle_dynamic_type_cache_miss
+  //
+  // Fall back to the vptr sanitizer's null check when -fsanitize=null isn't
+  // available.
+  //
+  // VPTR_NO_NULL-NOT: call void @__ubsan_handle_type_mismatch
+  // VPTR_NO_NULL: [[ICMP:%.*]] = icmp ne %struct.Dog* {{.*}}, null
+  // VPTR_NO_NULL-NEXT: br i1 [[ICMP]]
+  // VPTR_NO_NULL: call void @__ubsan_handle_dynamic_type_cache_miss
   auto *badDog = reinterpret_cast<Dog *>(cat);
   badDog->speak();
 }
 
 // VPTR_NO_NULL-LABEL: define void @_Z13invalid_cast2v
 void invalid_cast2() {
   // We've got a pointer to an alloca, so there's no run-time null check needed.
   // VPTR_NO_NULL-NOT: call void @__ubsan_handle_type_mismatch

cfe/trunk/test/Driver/fsanitize.c

 // CHECK-VPTR-TRAP-UNDEF: error: invalid argument '-fsanitize=vptr' not allowed with '-fsanitize-trap=undefined'
 
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=vptr -fno-rtti %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-VPTR-NO-RTTI
 // CHECK-VPTR-NO-RTTI: '-fsanitize=vptr' not allowed with '-fno-rtti'
 
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=undefined -fno-rtti %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-UNDEFINED-NO-RTTI
 // CHECK-UNDEFINED-NO-RTTI-NOT: vptr
 
-// RUN: %clang -target x86_64-linux-gnu -fsanitize=undefined -fno-sanitize=null %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-VPTR-NO-NULL
-// RUN: %clang -target x86_64-linux-gnu -fsanitize=vptr %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-VPTR-NO-NULL
-// CHECK-VPTR-NO-NULL: warning: implicitly disabling vptr sanitizer because null checking wasn't enabled
-
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=address,thread -fno-rtti %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-SANA-SANT
 // CHECK-SANA-SANT: '-fsanitize=address' not allowed with '-fsanitize=thread'
 
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=address,memory -pie -fno-rtti %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-SANA-SANM
 // CHECK-SANA-SANM: '-fsanitize=address' not allowed with '-fsanitize=memory'
 
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=thread,memory -pie -fno-rtti %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-SANT-SANM
 // CHECK-SANT-SANM: '-fsanitize=thread' not allowed with '-fsanitize=memory'

 // CHECK-FSAN-DARWIN: unsupported option '-fsanitize=function' for target 'x86_64-apple-darwin10'
 
 // RUN: %clang -target x86_64-apple-darwin10 -fsanitize=function -fsanitize=undefined %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-FSAN-UBSAN-DARWIN
 // CHECK-FSAN-UBSAN-DARWIN: unsupported option '-fsanitize=function' for target 'x86_64-apple-darwin10'
 
 // RUN: %clang -target x86_64-apple-darwin10 -mmacosx-version-min=10.8 -fsanitize=vptr %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-VPTR-DARWIN-OLD
 // CHECK-VPTR-DARWIN-OLD: unsupported option '-fsanitize=vptr' for target 'x86_64-apple-darwin10'
 
-// RUN: %clang -target x86_64-apple-darwin10 -mmacosx-version-min=10.9 -fsanitize=alignment,null,vptr %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-VPTR-DARWIN-NEW
-// CHECK-VPTR-DARWIN-NEW: -fsanitize=alignment,null,vptr
+// RUN: %clang -target x86_64-apple-darwin10 -mmacosx-version-min=10.9 -fsanitize=alignment,vptr %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-VPTR-DARWIN-NEW
+// CHECK-VPTR-DARWIN-NEW: -fsanitize=alignment,vptr
 
 // RUN: %clang -target armv7-apple-ios7 -miphoneos-version-min=7.0 -fsanitize=address %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-ASAN-IOS
 // CHECK-ASAN-IOS: -fsanitize=address
 
 // RUN: %clang -target i386-pc-openbsd -fsanitize=address %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-ASAN-OPENBSD
 // CHECK-ASAN-OPENBSD: unsupported option '-fsanitize=address' for target 'i386-pc-openbsd'
 
 // RUN: %clang -target x86_64-apple-darwin -fsanitize=leak %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-LSAN-X86-64-DARWIN

cfe/trunk/test/Driver/rtti-options.cpp

 // Make sure we keep the last -frtti/-fno-rtti argument
 // RUN: %clang -### -c -fno-rtti -frtti %s 2>&1 | FileCheck -check-prefix=CHECK-RTTI %s
 // RUN: %clang -### -c -frtti -fno-rtti %s 2>&1 | FileCheck -check-prefix=CHECK-NO-RTTI %s
 
 // -fsanitize=vptr
 // Make sure we only error/warn once, when trying to enable vptr and
 // undefined and have -fno-rtti
 // RUN: %clang -### -c -target x86_64-unknown-linux -fsanitize=undefined -fsanitize=vptr -fno-rtti %s 2>&1 | FileCheck -check-prefix=CHECK-SAN-ERROR -check-prefix=CHECK-OK %s
-// RUN: %clang -### -c -target x86_64-unknown-linux -fsanitize=null,vptr %s 2>&1 | FileCheck -check-prefix=CHECK-OK %s
-// RUN: %clang -### -c -target x86_64-unknown-linux -fsanitize=null,vptr -frtti %s 2>&1 | FileCheck -check-prefix=CHECK-OK %s
-// RUN: %clang -### -c -target x86_64-unknown-linux -fsanitize=null,vptr -fno-rtti %s 2>&1 | FileCheck -check-prefix=CHECK-SAN-ERROR %s
+// RUN: %clang -### -c -target x86_64-unknown-linux -fsanitize=vptr %s 2>&1 | FileCheck -check-prefix=CHECK-OK %s
+// RUN: %clang -### -c -target x86_64-unknown-linux -fsanitize=vptr -frtti %s 2>&1 | FileCheck -check-prefix=CHECK-OK %s
+// RUN: %clang -### -c -target x86_64-unknown-linux -fsanitize=vptr -fno-rtti %s 2>&1 | FileCheck -check-prefix=CHECK-SAN-ERROR %s
 // RUN: %clang -### -c -target x86_64-unknown-linux -fsanitize=undefined %s 2>&1 | FileCheck -check-prefix=CHECK-OK %s
 // RUN: %clang -### -c -target x86_64-unknown-linux -fsanitize=undefined -frtti %s 2>&1 | FileCheck -check-prefix=CHECK-OK %s
-// RUN: %clang -### -c -target x86_64-scei-ps4 -fsanitize=null,vptr %s 2>&1 | FileCheck -check-prefix=CHECK-SAN-WARN %s
-// RUN: %clang -### -c -target x86_64-scei-ps4 -fsanitize=null,vptr -frtti %s 2>&1 | FileCheck -check-prefix=CHECK-OK %s
-// RUN: %clang -### -c -target x86_64-scei-ps4 -fsanitize=null,vptr -fno-rtti %s 2>&1 | FileCheck -check-prefix=CHECK-SAN-ERROR %s
+// RUN: %clang -### -c -target x86_64-scei-ps4 -fsanitize=vptr %s 2>&1 | FileCheck -check-prefix=CHECK-SAN-WARN %s
+// RUN: %clang -### -c -target x86_64-scei-ps4 -fsanitize=vptr -frtti %s 2>&1 | FileCheck -check-prefix=CHECK-OK %s
+// RUN: %clang -### -c -target x86_64-scei-ps4 -fsanitize=vptr -fno-rtti %s 2>&1 | FileCheck -check-prefix=CHECK-SAN-ERROR %s
 // RUN: %clang -### -c -target x86_64-scei-ps4 -fsanitize=undefined -frtti %s 2>&1 | FileCheck -check-prefix=CHECK-OK %s
 
 // Exceptions + no/default rtti
 // RUN: %clang -### -c -target x86_64-scei-ps4 -fcxx-exceptions -fno-rtti %s 2>&1 | FileCheck -check-prefix=CHECK-EXC-ERROR-CXX %s
 // RUN: %clang -### -c -target x86_64-scei-ps4 -fcxx-exceptions %s 2>&1 | FileCheck -check-prefix=CHECK-EXC-WARN %s
 // RUN: %clang -### -c -target x86_64-unknown-unknown -fcxx-exceptions -fno-rtti %s 2>&1 | FileCheck -check-prefix=CHECK-OK %s
 // RUN: %clang -### -c -target x86_64-unknown-unknown -fcxx-exceptions %s 2>&1 | FileCheck -check-prefix=CHECK-OK %s
 // In C++, -fexceptions implies -fcxx-exceptions