[PATCH] D139296: [msan][CodeGen] Set noundef for C return value

[Vitaly Buka via Phabricator via cfe-commits]

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Msan needs noundef consistency between interface and implementation. If
we call C++ from C we can have noundef on C++ size, and no noundef on
caller C side, noundef implementation will not set TLS for return value,
no noundef caller will expect it. Then we have false reports in msan.

The workaround could be set TLS to zero even for noundef return values.
However if we do that always it will increase binary size by about 10%.
If we do that selectively we need to handle "address is taken"
functions, any non local functions, and probably all function which have
musttail callers. Which is still a lot.

The existing implemetation of HasStrictReturn refers to C standard as
the reason not enforcing noundef. I believe it applies only to the case
when return statement is omited. Testing on Google codebase I never see
such cases, however I've see tens of cases where C code returns actual
uninitialized variables, but we ignore that it because of "omited
return" case.

So this patch will:

1. fix false-positives with TLS missmatch.
2. detect bugs returning uninitialized variables for C as well.
3. report "omited return" cases stricted than C, which is alredy a warning and very likely a bug in a code anyway.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D139296

Files:
  clang/lib/CodeGen/CGCall.cpp
  clang/test/CodeGen/cleanup-destslot-simple.c
  clang/test/CodeGen/msan-param-retval.c


Index: clang/test/CodeGen/msan-param-retval.c
===================================================================
--- clang/test/CodeGen/msan-param-retval.c
+++ clang/test/CodeGen/msan-param-retval.c
@@ -23,13 +23,20 @@
   return 1;
 }
 
-// CHECK: define dso_local i32 @foo() #0 {
-// CHECK:   @__msan_retval_tls
+// CLEAN:   define dso_local i32 @foo() #0 {
+// NOUNDEF: define dso_local noundef i32 @foo() #0 {
+// CLEAN:        @__msan_retval_tls
+// NOUNDEF_ONLY: @__msan_retval_tls
+// EAGER-NOT:    @__msan_retval_tls
 // CHECK: }
 
 int noret() {
 }
 
-// CHECK: define dso_local i32 @noret() #0 {
+// CLEAN:   define dso_local i32 @noret() #0 {   
+// NOUNDEF: define dso_local noundef i32 @noret() #0 {
 // CHECK:   %retval = alloca
+// CLEAN:        @__msan_retval_tls
+// NOUNDEF_ONLY: @__msan_retval_tls
+// EAGER-NOT:    @__msan_retval_tls
 // CHECK: }
\ No newline at end of file
Index: clang/test/CodeGen/cleanup-destslot-simple.c
===================================================================
--- clang/test/CodeGen/cleanup-destslot-simple.c
+++ clang/test/CodeGen/cleanup-destslot-simple.c
@@ -64,7 +64,12 @@
 // CHECK-MSAN-NEXT:    [[_MSLD1:%.*]] = load i32, ptr [[TMP11]], align 4, !dbg [[DBG20]]
 // CHECK-MSAN-NEXT:    call void @llvm.lifetime.end.p0(i64 8, ptr nonnull [[P]]), !dbg [[DBG22:![0-9]+]]
 // CHECK-MSAN-NEXT:    call void @llvm.lifetime.end.p0(i64 4, ptr nonnull [[X]]) #[[ATTR2]], !dbg [[DBG22]]
-// CHECK-MSAN-NEXT:    store i32 [[_MSLD1]], ptr @__msan_retval_tls, align 8, !dbg [[DBG23:![0-9]+]]
+// CHECK-MSAN-NEXT:    [[_MSCMP2_NOT:%.*]] = icmp eq i32 [[_MSLD1]], 0, !dbg [[DBG23:![0-9]+]]
+// CHECK-MSAN-NEXT:    br i1 [[_MSCMP2_NOT]], label [[TMP13:%.*]], label [[TMP12:%.*]], !dbg [[DBG23]], !prof [[PROF21]]
+// CHECK-MSAN:       12:
+// CHECK-MSAN-NEXT:    call void @__msan_warning_noreturn() #[[ATTR3]], !dbg [[DBG23]]
+// CHECK-MSAN-NEXT:    unreachable, !dbg [[DBG23]]
+// CHECK-MSAN:       13:
 // CHECK-MSAN-NEXT:    ret i32 [[TMP8]], !dbg [[DBG23]]
 //
 // CHECK-KMSAN-LABEL: @test(
Index: clang/lib/CodeGen/CGCall.cpp
===================================================================
--- clang/lib/CodeGen/CGCall.cpp
+++ clang/lib/CodeGen/CGCall.cpp
@@ -1798,6 +1798,13 @@
 
 static bool HasStrictReturn(const CodeGenModule &Module, QualType RetTy,
                             const Decl *TargetDecl) {
+  // As-is msan can not tolerate noundef mismatch between caller and
+  // implementation. Mismatch is possible for e.g. indirect calls from C-caller
+  // into C++. Such mismatches lead to confusing false reports. To avoid
+  // expensive workaround on msan we enforce initialization event in uncommon
+  // cases where it's allowed.
+  if (Module.getLangOpts().Sanitize.has(SanitizerKind::Memory))
+    return true;
   // C++ explicitly makes returning undefined values UB. C's rule only applies
   // to used values, so we never mark them noundef for now.
   if (!Module.getLangOpts().CPlusPlus)


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