[clang][slh] add Clang attr no_speculative_load_hardening

Summary:
This attribute will allow users to opt specific functions out of
speculative load hardening. This compliments the Clang attribute
named speculative_load_hardening. When this attribute or the attribute
speculative_load_hardening is used in combination with the flags
-mno-speculative-load-hardening or -mspeculative-load-hardening,
the function level attribute will override the default during LLVM IR
generation. For example, in the case, where the flag opposes the
function attribute, the function attribute will take precendence.
The sticky inlining behavior of the speculative_load_hardening attribute
may cause a function with the no_speculative_load_hardening attribute
to be tagged with the speculative_load_hardening tag in
subsequent compiler phases which is desired behavior since the
speculative_load_hardening LLVM attribute is designed to be maximally
conservative.

If both attributes are specified for a function, then an error will be
thrown.

Reviewers: chandlerc, echristo, kristof.beyls, aaron.ballman

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54909

llvm-svn: 351565

Author: Zola Bridges

clang/include/clang/Basic/Attr.td

@@ -3149,6 +3149,12 @@ def SpeculativeLoadHardening : InheritableAttr {
   let Documentation = [SpeculativeLoadHardeningDocs];
 }
 
+def NoSpeculativeLoadHardening : InheritableAttr {
+  let Spellings = [Clang<"no_speculative_load_hardening">];
+  let Subjects = SubjectList<[Function, ObjCMethod], ErrorDiag>;
+  let Documentation = [NoSpeculativeLoadHardeningDocs];
+}
+
 def Uninitialized : InheritableAttr {
   let Spellings = [Clang<"uninitialized", 0>];
   let Subjects = SubjectList<[LocalVar]>;

clang/include/clang/Basic/AttrDocs.td

@@ -3822,7 +3822,8 @@ def SpeculativeLoadHardeningDocs : Documentation {
   This attribute can be applied to a function declaration in order to indicate
   that `Speculative Load Hardening <https://llvm.org/docs/SpeculativeLoadHardening.html>`_
   should be enabled for the function body. This can also be applied to a method
-  in Objective C.
+  in Objective C. This attribute will take precedence over the command line flag in
+  the case where `-mno-speculative-load-hardening <https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mspeculative-load-hardening>`_ is specified.
 
   Speculative Load Hardening is a best-effort mitigation against
   information leak attacks that make use of control flow
@@ -3840,6 +3841,42 @@ def SpeculativeLoadHardeningDocs : Documentation {
   }];
 }
 
+def NoSpeculativeLoadHardeningDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+  This attribute can be applied to a function declaration in order to indicate
+  that `Speculative Load Hardening <https://llvm.org/docs/SpeculativeLoadHardening.html>`_
+  is *not* needed for the function body. This can also be applied to a method
+  in Objective C. This attribute will take precedence over the command line flag in
+  the case where `-mspeculative-load-hardening <https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mspeculative-load-hardening>`_ is specified.
+
+  Warning: This attribute may not prevent Speculative Load Hardening from being
+  enabled for a function which inlines a function that has the
+  'speculative_load_hardening' attribute. This is intended to provide a
+  maximally conservative model where the code that is marked with the
+  'speculative_load_hardening' attribute will always (even when inlined)
+  be hardened. A user of this attribute may want to mark functions called by
+  a function they do not want to be hardened with the 'noinline' attribute.
+
+  For example:
+
+  .. code-block:: c
+
+    __attribute__((speculative_load_hardening))
+    int foo(int i) {
+      return i;
+    }
+
+    // Note: bar() may still have speculative load hardening enabled if
+    // foo() is inlined into bar(). Mark foo() with __attribute__((noinline))
+    // to avoid this situation.
+    __attribute__((no_speculative_load_hardening))
+    int bar(int i) {
+      return foo(i);
+    }
+  }];
+}
+
 def ObjCExternallyRetainedDocs : Documentation {
   let Category = DocCatVariable;
   let Content = [{

clang/include/clang/Sema/Sema.h

@@ -2496,6 +2496,12 @@ class Sema {
                                           unsigned AttrSpellingListIndex);
   MinSizeAttr *mergeMinSizeAttr(Decl *D, SourceRange Range,
                                 unsigned AttrSpellingListIndex);
+  NoSpeculativeLoadHardeningAttr *
+  mergeNoSpeculativeLoadHardeningAttr(Decl *D,
+                                      const NoSpeculativeLoadHardeningAttr &AL);
+  SpeculativeLoadHardeningAttr *
+  mergeSpeculativeLoadHardeningAttr(Decl *D,
+                                    const SpeculativeLoadHardeningAttr &AL);
   OptimizeNoneAttr *mergeOptimizeNoneAttr(Decl *D, SourceRange Range,
                                           unsigned AttrSpellingListIndex);
   InternalLinkageAttr *mergeInternalLinkageAttr(Decl *D, const ParsedAttr &AL);

clang/lib/CodeGen/CGCall.cpp

@@ -1793,8 +1793,6 @@ void CodeGenModule::ConstructDefaultFnAttrList(StringRef Name, bool HasOptnone,
     if (CodeGenOpts.Backchain)
       FuncAttrs.addAttribute("backchain");
 
-    // FIXME: The interaction of this attribute with the SLH command line flag
-    // has not been determined.
     if (CodeGenOpts.SpeculativeLoadHardening)
       FuncAttrs.addAttribute(llvm::Attribute::SpeculativeLoadHardening);
   }
@@ -1864,8 +1862,6 @@ void CodeGenModule::ConstructAttributeList(
       FuncAttrs.addAttribute(llvm::Attribute::NoDuplicate);
     if (TargetDecl->hasAttr<ConvergentAttr>())
       FuncAttrs.addAttribute(llvm::Attribute::Convergent);
-    if (TargetDecl->hasAttr<SpeculativeLoadHardeningAttr>())
-      FuncAttrs.addAttribute(llvm::Attribute::SpeculativeLoadHardening);
 
     if (const FunctionDecl *Fn = dyn_cast<FunctionDecl>(TargetDecl)) {
       AddAttributesFromFunctionProtoType(
@@ -1910,6 +1906,16 @@ void CodeGenModule::ConstructAttributeList(
 
   ConstructDefaultFnAttrList(Name, HasOptnone, AttrOnCallSite, FuncAttrs);
 
+  // This must run after constructing the default function attribute list
+  // to ensure that the speculative load hardening attribute is removed
+  // in the case where the -mspeculative-load-hardening flag was passed.
+  if (TargetDecl) {
+    if (TargetDecl->hasAttr<NoSpeculativeLoadHardeningAttr>())
+      FuncAttrs.removeAttribute(llvm::Attribute::SpeculativeLoadHardening);
+    if (TargetDecl->hasAttr<SpeculativeLoadHardeningAttr>())
+      FuncAttrs.addAttribute(llvm::Attribute::SpeculativeLoadHardening);
+  }
+
   if (CodeGenOpts.EnableSegmentedStacks &&
       !(TargetDecl && TargetDecl->hasAttr<NoSplitStackAttr>()))
     FuncAttrs.addAttribute("split-stack");

clang/lib/Sema/SemaDecl.cpp

@@ -2489,6 +2489,10 @@ static bool mergeDeclAttribute(Sema &S, NamedDecl *D,
   else if (const auto *UA = dyn_cast<UuidAttr>(Attr))
     NewAttr = S.mergeUuidAttr(D, UA->getRange(), AttrSpellingListIndex,
                               UA->getGuid());
+  else if (const auto *SLHA = dyn_cast<SpeculativeLoadHardeningAttr>(Attr))
+    NewAttr = S.mergeSpeculativeLoadHardeningAttr(D, *SLHA);
+  else if (const auto *SLHA = dyn_cast<NoSpeculativeLoadHardeningAttr>(Attr))
+    NewAttr = S.mergeNoSpeculativeLoadHardeningAttr(D, *SLHA);
   else if (Attr->shouldInheritEvenIfAlreadyPresent() || !DeclHasAttr(D, Attr))
     NewAttr = cast<InheritableAttr>(Attr->clone(S.Context));
 

clang/lib/Sema/SemaDeclAttr.cpp

@@ -4157,6 +4157,15 @@ MinSizeAttr *Sema::mergeMinSizeAttr(Decl *D, SourceRange Range,
   return ::new (Context) MinSizeAttr(Range, Context, AttrSpellingListIndex);
 }
 
+NoSpeculativeLoadHardeningAttr *Sema::mergeNoSpeculativeLoadHardeningAttr(
+    Decl *D, const NoSpeculativeLoadHardeningAttr &AL) {
+  if (checkAttrMutualExclusion<SpeculativeLoadHardeningAttr>(*this, D, AL))
+    return nullptr;
+
+  return ::new (Context) NoSpeculativeLoadHardeningAttr(
+      AL.getRange(), Context, AL.getSpellingListIndex());
+}
+
 OptimizeNoneAttr *Sema::mergeOptimizeNoneAttr(Decl *D, SourceRange Range,
                                               unsigned AttrSpellingListIndex) {
   if (AlwaysInlineAttr *Inline = D->getAttr<AlwaysInlineAttr>()) {
@@ -4177,6 +4186,15 @@ OptimizeNoneAttr *Sema::mergeOptimizeNoneAttr(Decl *D, SourceRange Range,
                                           AttrSpellingListIndex);
 }
 
+SpeculativeLoadHardeningAttr *Sema::mergeSpeculativeLoadHardeningAttr(
+    Decl *D, const SpeculativeLoadHardeningAttr &AL) {
+  if (checkAttrMutualExclusion<NoSpeculativeLoadHardeningAttr>(*this, D, AL))
+    return nullptr;
+
+  return ::new (Context) SpeculativeLoadHardeningAttr(
+      AL.getRange(), Context, AL.getSpellingListIndex());
+}
+
 static void handleAlwaysInlineAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   if (checkAttrMutualExclusion<NotTailCalledAttr>(S, D, AL))
     return;
@@ -6618,7 +6636,13 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
     handleSectionAttr(S, D, AL);
     break;
   case ParsedAttr::AT_SpeculativeLoadHardening:
-    handleSimpleAttribute<SpeculativeLoadHardeningAttr>(S, D, AL);
+    handleSimpleAttributeWithExclusions<SpeculativeLoadHardeningAttr,
+                                        NoSpeculativeLoadHardeningAttr>(S, D,
+                                                                        AL);
+    break;
+  case ParsedAttr::AT_NoSpeculativeLoadHardening:
+    handleSimpleAttributeWithExclusions<NoSpeculativeLoadHardeningAttr,
+                                        SpeculativeLoadHardeningAttr>(S, D, AL);
     break;
   case ParsedAttr::AT_CodeSeg:
     handleCodeSegAttr(S, D, AL);

clang/test/CodeGen/attr-speculative-load-hardening.cpp

@@ -0,0 +1,62 @@
+// Check that we correctly set or did not set the attribute for each function.
+// RUN: %clang_cc1 -std=c++11 -disable-llvm-passes -emit-llvm %s -o - | FileCheck %s --check-prefix=CHECK1
+// RUN: %clang_cc1 -std=c++11 -disable-llvm-passes -emit-llvm %s -o - | FileCheck %s --check-prefix=CHECK2
+
+// Check that we correctly set or did not set the attribute on each function despite the
+// -mspeculative-load-hardening flag.
+// RUN: %clang_cc1 -mspeculative-load-hardening -std=c++11 -disable-llvm-passes -emit-llvm %s -o - | FileCheck %s --check-prefix=CHECK3
+// RUN: %clang_cc1 -mspeculative-load-hardening -std=c++11 -disable-llvm-passes -emit-llvm %s -o - | FileCheck %s --check-prefix=CHECK4
+
+
+// Check that we correctly set or did not set the attribute on each function despite the
+// -mno-speculative-load-hardening flag.
+// RUN: %clang -mno-speculative-load-hardening -S -std=c++11 -disable-llvm-passes -emit-llvm %s -o - | FileCheck %s --check-prefix=CHECK5
+// RUN: %clang -mno-speculative-load-hardening -S -std=c++11 -disable-llvm-passes -emit-llvm %s -o - | FileCheck %s --check-prefix=CHECK6
+
+
+[[clang::speculative_load_hardening]]
+int test1() {
+  return 42;
+}
+
+int __attribute__((speculative_load_hardening)) test2() {
+  return 42;
+}
+
+[[clang::no_speculative_load_hardening]]
+int test3() {
+  return 42;
+}
+
+int __attribute__((no_speculative_load_hardening)) test4() {
+  return 42;
+}
+// CHECK1: @{{.*}}test1{{.*}}[[SLH:#[0-9]+]]
+// CHECK1: @{{.*}}test3{{.*}}[[NOSLH:#[0-9]+]]
+// CHECK1: attributes [[SLH]] = { {{.*}}speculative_load_hardening{{.*}} }
+// CHECK1-NOT: attributes [[NOSLH]] = { {{.*}}speculative_load_hardening{{.*}} }
+
+// CHECK2: @{{.*}}test2{{.*}}[[SLH:#[0-9]+]]
+// CHECK2: @{{.*}}test4{{.*}}[[NOSLH:#[0-9]+]]
+// CHECK2: attributes [[SLH]] = { {{.*}}speculative_load_hardening{{.*}} }
+// CHECK2-NOT: attributes [[NOSLH]] = { {{.*}}speculative_load_hardening{{.*}} }
+
+// CHECK3: @{{.*}}test1{{.*}}[[SLH:#[0-9]+]]
+// CHECK3: @{{.*}}test3{{.*}}[[NOSLH:#[0-9]+]]
+// CHECK3: attributes [[SLH]] = { {{.*}}speculative_load_hardening{{.*}} }
+// CHECK3-NOT: attributes [[NOSLH]] = { {{.*}}speculative_load_hardening{{.*}} }
+
+// CHECK4: @{{.*}}test2{{.*}}[[SLH:#[0-9]+]]
+// CHECK4: @{{.*}}test4{{.*}}[[NOSLH:#[0-9]+]]
+// CHECK4: attributes [[SLH]] = { {{.*}}speculative_load_hardening{{.*}} }
+// CHECK4-NOT: attributes [[NOSLH]] = { {{.*}}speculative_load_hardening{{.*}} }
+
+// CHECK5: @{{.*}}test1{{.*}}[[SLH:#[0-9]+]]
+// CHECK5: @{{.*}}test3{{.*}}[[NOSLH:#[0-9]+]]
+// CHECK5: attributes [[SLH]] = { {{.*}}speculative_load_hardening{{.*}} }
+// CHECK5-NOT: attributes [[NOSLH]] = { {{.*}}speculative_load_hardening{{.*}} }
+
+// CHECK6: @{{.*}}test2{{.*}}[[SLH:#[0-9]+]]
+// CHECK6: @{{.*}}test4{{.*}}[[NOSLH:#[0-9]+]]
+// CHECK6: attributes [[SLH]] = { {{.*}}speculative_load_hardening{{.*}} }
+// CHECK6-NOT: attributes [[NOSLH]] = { {{.*}}speculative_load_hardening{{.*}} }

clang/test/CodeGenCXX/attr-speculative-load-hardening.cpp

@@ -4,6 +4,11 @@ int main() __attribute__((speculative_load_hardening)) {
   return 0;
 }
 
-// SLH: @{{.*}}main{{.*}}[[SLH:#[0-9]+]]
+int test() __attribute__((no_speculative_load_hardening)) {
+  return 0;
+}
 
+// SLH: @{{.*}}main{{.*}}[[SLH:#[0-9]+]]
+// SLH: @{{.*}}test{{.*}}[[NOSLH:#[0-9]+]]
 // SLH: attributes [[SLH]] = { {{.*}}speculative_load_hardening{{.*}} }
+// SLH-NOT: attributes [[NOSLH]] = { {{.*}}speculative_load_hardening{{.*}} }

clang/test/CodeGen/attr-speculative-load-hardening.m renamed to clang/test/CodeGenObjC/attr-speculative-load-hardening.m

@@ -80,6 +80,7 @@
 // CHECK-NEXT: NoMips16 (SubjectMatchRule_function)
 // CHECK-NEXT: NoSanitize (SubjectMatchRule_function, SubjectMatchRule_objc_method, SubjectMatchRule_variable_is_global)
 // CHECK-NEXT: NoSanitizeSpecific (SubjectMatchRule_function, SubjectMatchRule_variable_is_global)
+// CHECK-NEXT: NoSpeculativeLoadHardening (SubjectMatchRule_function, SubjectMatchRule_objc_method)
 // CHECK-NEXT: NoSplitStack (SubjectMatchRule_function)
 // CHECK-NEXT: NoStackProtector (SubjectMatchRule_function)
 // CHECK-NEXT: NoThreadSafetyAnalysis (SubjectMatchRule_function)