[x86/SLH] Add a real Clang flag and LLVM IR attribute for Speculative

Load Hardening.

Wires up the existing pass to work with a proper IR attribute rather
than just a hidden/internal flag. The internal flag continues to work
for now, but I'll likely remove it soon.

Most of the churn here is adding the IR attribute. I talked about this
Kristof Beyls and he seemed at least initially OK with this direction.
The idea of using a full attribute here is that we *do* expect at least
some forms of this for other architectures. There isn't anything
*inherently* x86-specific about this technique, just that we only have
an implementation for x86 at the moment.

While we could potentially expose this as a Clang-level attribute as
well, that seems like a good question to defer for the moment as it
isn't 100% clear whether that or some other programmer interface (or
both?) would be best. We'll defer the programmer interface side of this
for now, but at least get to the point where the feature can be enabled
without relying on implementation details.

This also allows us to do something that was really hard before: we can
enable *just* the indirect call retpolines when using SLH. For x86, we
don't have any other way to mitigate indirect calls. Other architectures
may take a different approach of course, and none of this is surfaced to
user-level flags.

Differential Revision: https://reviews.llvm.org/D51157

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@341363 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Driver/Options.td

@@ -2009,6 +2009,10 @@ def mno_stackrealign : Flag<["-"], "mno-stackrealign">, Group<m_Group>;
 
 def mretpoline : Flag<["-"], "mretpoline">, Group<m_Group>, Flags<[CoreOption,DriverOption]>;
 def mno_retpoline : Flag<["-"], "mno-retpoline">, Group<m_Group>, Flags<[CoreOption,DriverOption]>;
+def mspeculative_load_hardening : Flag<["-"], "mspeculative-load-hardening">,
+  Group<m_Group>, Flags<[CoreOption,CC1Option]>;
+def mno_speculative_load_hardening : Flag<["-"], "mno-speculative-load-hardening">,
+  Group<m_Group>, Flags<[CoreOption]>;
 
 def mrelax : Flag<["-"], "mrelax">, Group<m_riscv_Features_Group>,
   HelpText<"Enable linker relaxation">;

include/clang/Frontend/CodeGenOptions.def

@@ -211,6 +211,7 @@ CODEGENOPT(SanitizeCoverageStackDepth, 1, 0) ///< Enable max stack depth tracing
 CODEGENOPT(SanitizeStats     , 1, 0) ///< Collect statistics for sanitizers.
 CODEGENOPT(SimplifyLibCalls  , 1, 1) ///< Set when -fbuiltin is enabled.
 CODEGENOPT(SoftFloat         , 1, 0) ///< -soft-float.
+CODEGENOPT(SpeculativeLoadHardening, 1, 0) ///< Enable speculative load hardening.
 CODEGENOPT(FineGrainedBitfieldAccesses, 1, 0) ///< Enable fine-grained bitfield accesses.
 CODEGENOPT(StrictEnums       , 1, 0) ///< Optimize based on strict enum definition.
 CODEGENOPT(StrictVTablePointers, 1, 0) ///< Optimize based on the strict vtable pointers

lib/CodeGen/CGCall.cpp

@@ -1784,6 +1784,9 @@ void CodeGenModule::ConstructDefaultFnAttrList(StringRef Name, bool HasOptnone,
       FuncAttrs.addAttribute("stackrealign");
     if (CodeGenOpts.Backchain)
       FuncAttrs.addAttribute("backchain");
+
+    if (CodeGenOpts.SpeculativeLoadHardening)
+      FuncAttrs.addAttribute(llvm::Attribute::SpeculativeLoadHardening);
   }
 
   if (getLangOpts().assumeFunctionsAreConvergent()) {

lib/Driver/ToolChains/Arch/X86.cpp

@@ -146,15 +146,23 @@ void x86::getX86TargetFeatures(const Driver &D, const llvm::Triple &Triple,
 
   // Translate the high level `-mretpoline` flag to the specific target feature
   // flags. We also detect if the user asked for retpoline external thunks but
-  // failed to ask for retpolines themselves. This is a bit hacky but keeps
-  // existing usages working. We should consider deprecated this and instead
-  // warning if the user requests external retpoline thunks and *doesn't*
-  // request some form of retpolines.
-  if (Args.hasArgNoClaim(options::OPT_mretpoline, options::OPT_mno_retpoline)) {
+  // failed to ask for retpolines themselves (through any of the different
+  // flags). This is a bit hacky but keeps existing usages working. We should
+  // consider deprecating this and instead warn if the user requests external
+  // retpoline thunks and *doesn't* request some form of retpolines.
+  if (Args.hasArgNoClaim(options::OPT_mretpoline, options::OPT_mno_retpoline,
+                         options::OPT_mspeculative_load_hardening,
+                         options::OPT_mno_speculative_load_hardening)) {
     if (Args.hasFlag(options::OPT_mretpoline, options::OPT_mno_retpoline,
                      false)) {
       Features.push_back("+retpoline-indirect-calls");
       Features.push_back("+retpoline-indirect-branches");
+    } else if (Args.hasFlag(options::OPT_mspeculative_load_hardening,
+                            options::OPT_mno_speculative_load_hardening,
+                            false)) {
+      // On x86, speculative load hardening relies on at least using retpolines
+      // for indirect calls.
+      Features.push_back("+retpoline-indirect-calls");
     }
   } else if (Args.hasFlag(options::OPT_mretpoline_external_thunk,
                           options::OPT_mno_retpoline_external_thunk, false)) {

lib/Driver/ToolChains/Clang.cpp

@@ -4132,6 +4132,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
 
   Args.AddLastArg(CmdArgs, options::OPT_pthread);
 
+  Args.AddLastArg(CmdArgs, options::OPT_mspeculative_load_hardening,
+                  options::OPT_mno_speculative_load_hardening);
+
   RenderSSPOptions(getToolChain(), Args, CmdArgs, KernelOrKext);
 
   // Translate -mstackrealign

lib/Frontend/CompilerInvocation.cpp

@@ -1147,6 +1147,8 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
 
   Opts.KeepStaticConsts = Args.hasArg(OPT_fkeep_static_consts);
 
+  Opts.SpeculativeLoadHardening = Args.hasArg(OPT_mspeculative_load_hardening);
+
   return Success;
 }
 

test/CodeGen/attr-speculative-load-hardening.c

@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -mspeculative-load-hardening -disable-llvm-passes -emit-llvm %s -o - | FileCheck %s -check-prefix=SLH
+//
+// Check that we set the attribute on each function.
+
+int test1() {
+  return 42;
+}
+// SLH: @{{.*}}test1{{.*}}[[SLH:#[0-9]+]]
+
+// SLH: attributes [[SLH]] = { {{.*}}speculative_load_hardening{{.*}} }

test/Driver/x86-target-features.c

@@ -140,6 +140,15 @@
 // RETPOLINE-EXTERNAL-THUNK: "-target-feature" "+retpoline-external-thunk"
 // NO-RETPOLINE-EXTERNAL-THUNK: "-target-feature" "-retpoline-external-thunk"
 
+// RUN: %clang -target i386-linux-gnu -mspeculative-load-hardening %s -### -o %t.o 2>&1 | FileCheck -check-prefix=SLH %s
+// RUN: %clang -target i386-linux-gnu -mretpoline -mspeculative-load-hardening %s -### -o %t.o 2>&1 | FileCheck -check-prefix=RETPOLINE %s
+// RUN: %clang -target i386-linux-gnu -mno-speculative-load-hardening %s -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-SLH %s
+// SLH-NOT: retpoline
+// SLH: "-target-feature" "+retpoline-indirect-calls"
+// SLH-NOT: retpoline
+// SLH: "-mspeculative-load-hardening"
+// NO-SLH-NOT: retpoline
+
 // RUN: %clang -target i386-linux-gnu -mwaitpkg %s -### -o %t.o 2>&1 | FileCheck -check-prefix=WAITPKG %s
 // RUN: %clang -target i386-linux-gnu -mno-waitpkg %s -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-WAITPKG %s
 // WAITPKG: "-target-feature" "+waitpkg"