[clang][slh] add attribute for speculative load hardening

Summary:
LLVM IR already has an attribute for speculative_load_hardening. Before
this commit, when a user passed the -mspeculative-load-hardening flag to
Clang, every function would have this attribute added to it. This Clang
attribute will allow users to opt into SLH on a function by function basis.

This can be applied to functions and Objective C methods.

Reviewers: chandlerc, echristo

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54555

llvm-svn: 347586

clang/include/clang/Basic/Attr.td

@@ -3091,3 +3091,9 @@ def AlwaysDestroy : InheritableAttr {
   let Subjects = SubjectList<[Var]>;
   let Documentation = [AlwaysDestroyDocs];
 }
+
+def SpeculativeLoadHardening : InheritableAttr {
+  let Spellings = [Clang<"speculative_load_hardening">];
+  let Subjects = SubjectList<[Function, ObjCMethod], ErrorDiag>;
+  let Documentation = [SpeculativeLoadHardeningDocs];
+}

clang/include/clang/Basic/AttrDocs.td

@@ -3629,3 +3629,27 @@ GNU inline semantics are the default behavior with ``-std=gnu89``,
 ``-std=c89``, ``-std=c94``, or ``-fgnu89-inline``.
   }];
 }
+
+def SpeculativeLoadHardeningDocs : 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>`_
+  should be enabled for the function body. This can also be applied to a method
+  in Objective C.
+
+  Speculative Load Hardening is a best-effort mitigation against
+  information leak attacks that make use of control flow
+  miss-speculation - specifically miss-speculation of whether a branch
+  is taken or not. Typically vulnerabilities enabling such attacks are
+  classified as "Spectre variant #1". Notably, this does not attempt to
+  mitigate against miss-speculation of branch target, classified as
+  "Spectre variant #2" vulnerabilities.
+
+  When inlining, the attribute is sticky. Inlining a function that
+  carries this attribute will cause the caller to gain the
+  attribute. This is intended to provide a maximally conservative model
+  where the code in a function annotated with this attribute will always
+  (even after inlining) end up hardened.
+  }];
+}

clang/lib/CodeGen/CGCall.cpp

@@ -1791,6 +1791,8 @@ 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);
   }
@@ -1854,6 +1856,8 @@ 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(

clang/lib/Sema/SemaDeclAttr.cpp

@@ -6373,6 +6373,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case ParsedAttr::AT_Section:
     handleSectionAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_SpeculativeLoadHardening:
+    handleSimpleAttribute<SpeculativeLoadHardeningAttr>(S, D, AL);
+    break;
   case ParsedAttr::AT_CodeSeg:
     handleCodeSegAttr(S, D, AL);
     break;

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

@@ -0,0 +1,18 @@
+// 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 set the attribute on each function.
+
+[[clang::speculative_load_hardening]]
+int test1() {
+  return 42;
+}
+
+int __attribute__((speculative_load_hardening)) test2() {
+  return 42;
+}
+// CHECK1: @{{.*}}test1{{.*}}[[SLH1:#[0-9]+]]
+// CHECK1: attributes [[SLH1]] = { {{.*}}speculative_load_hardening{{.*}} }
+
+// CHECK2: @{{.*}}test2{{.*}}[[SLH2:#[0-9]+]]
+// CHECK2: attributes [[SLH2]] = { {{.*}}speculative_load_hardening{{.*}} }

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

@@ -0,0 +1,9 @@
+// RUN: %clang -emit-llvm %s -o - -S | FileCheck %s -check-prefix=SLH
+
+int main() __attribute__((speculative_load_hardening)) {
+  return 0;
+}
+
+// SLH: @{{.*}}main{{.*}}[[SLH:#[0-9]+]]
+
+// SLH: attributes [[SLH]] = { {{.*}}speculative_load_hardening{{.*}} }

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

@@ -0,0 +1,34 @@
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify %s
+
+int i __attribute__((speculative_load_hardening)); // expected-error {{'speculative_load_hardening' attribute only applies to functions}}
+
+void f1() __attribute__((speculative_load_hardening));
+void f2() __attribute__((speculative_load_hardening(1))); // expected-error {{'speculative_load_hardening' attribute takes no arguments}}
+
+template <typename T>
+void tf1() __attribute__((speculative_load_hardening));
+
+int f3(int __attribute__((speculative_load_hardening)), int); // expected-error {{'speculative_load_hardening' attribute only applies to functions}}
+
+struct A {
+  int f __attribute__((speculative_load_hardening));  // expected-error {{'speculative_load_hardening' attribute only applies to functions}}
+  void mf1() __attribute__((speculative_load_hardening));
+  static void mf2() __attribute__((speculative_load_hardening));
+};
+
+int ci [[speculative_load_hardening]]; // expected-error {{'speculative_load_hardening' attribute only applies to functions}}
+
+[[speculative_load_hardening]] void cf1();
+[[speculative_load_hardening(1)]] void cf2(); // expected-error {{'speculative_load_hardening' attribute takes no arguments}}
+
+template <typename T>
+[[speculative_load_hardening]]
+void ctf1();
+
+int cf3(int c[[speculative_load_hardening]], int); // expected-error {{'speculative_load_hardening' attribute only applies to functions}}
+
+struct CA {
+  int f [[speculative_load_hardening]];  // expected-error {{'speculative_load_hardening' attribute only applies to functions}}
+  [[speculative_load_hardening]] void mf1();
+  [[speculative_load_hardening]] static void mf2();
+};

llvm/docs/LangRef.rst

@@ -1643,19 +1643,15 @@ example:
 ``speculative_load_hardening``
     This attribute indicates that
     `Speculative Load Hardening <https://llvm.org/docs/SpeculativeLoadHardening.html>`_
-    should be enabled for the function body. This is a best-effort attempt to
-    mitigate all known speculative execution information leak vulnerabilities
-    that are based on the fundamental principles of modern processors'
-    speculative execution. These vulnerabilities are classified as "Spectre
-    variant #1" vulnerabilities typically. Notably, this does not attempt to
-    mitigate any vulnerabilities where the speculative execution and/or
-    prediction devices of specific processors can be *completely* undermined
-    (such as "Branch Target Injection", a.k.a, "Spectre variant #2"). Instead,
-    this is a target-independent request to harden against the completely
-    generic risk posed by speculative execution to incorrectly load secret data,
-    making it available to some micro-architectural side-channel for information
-    leak. For a processor without any speculative execution or predictors, this
-    is expected to be a no-op.
+    should be enabled for the function body.
+
+    Speculative Load Hardening is a best-effort mitigation against
+    information leak attacks that make use of control flow
+    miss-speculation - specifically miss-speculation of whether a branch
+    is taken or not. Typically vulnerabilities enabling such attacks are
+    classified as "Spectre variant #1". Notably, this does not attempt to
+    mitigate against miss-speculation of branch target, classified as
+    "Spectre variant #2" vulnerabilities.
 
     When inlining, the attribute is sticky. Inlining a function that carries
     this attribute will cause the caller to gain the attribute. This is intended