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

This reverts commit 801eaf9.

llvm-svn: 347588

Author: Zola Bridges

clang/include/clang/Basic/Attr.td

@@ -3091,9 +3091,3 @@ 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

@@ -3629,27 +3629,3 @@ 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

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

clang/lib/Sema/SemaDeclAttr.cpp

@@ -6373,9 +6373,6 @@ 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

@@ -1643,15 +1643,19 @@ example:
 ``speculative_load_hardening``
     This attribute indicates that
     `Speculative Load Hardening <https://llvm.org/docs/SpeculativeLoadHardening.html>`_
-    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.
+    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.
 
     When inlining, the attribute is sticky. Inlining a function that carries
     this attribute will cause the caller to gain the attribute. This is intended