[X86] initial -mfunction-return=thunk-extern support

Adds support for:
* `-mfunction-return=<value>` command line flag, and
* `__attribute__((function_return("<value>")))` function attribute

Where the supported <value>s are:
* keep (disable)
* thunk-extern (enable)

thunk-extern enables clang to change ret instructions into jmps to an
external symbol named __x86_return_thunk, implemented as a new
MachineFunctionPass named "x86-return-thunks", keyed off the new IR
attribute fn_ret_thunk_extern.

The symbol __x86_return_thunk is expected to be provided by the runtime
the compiled code is linked against and is not defined by the compiler.
Enabling this option alone doesn't provide mitigations without
corresponding definitions of __x86_return_thunk!

This new MachineFunctionPass is very similar to "x86-lvi-ret".

The <value>s "thunk" and "thunk-inline" are currently unsupported. It's
not clear yet that they are necessary: whether the thunk pattern they
would emit is beneficial or used anywhere.

Should the <value>s "thunk" and "thunk-inline" become necessary,
x86-return-thunks could probably be merged into x86-retpoline-thunks
which has pre-existing machinery for emitting thunks (which could be
used to implement the <value> "thunk").

Has been found to build+boot with corresponding Linux
kernel patches. This helps the Linux kernel mitigate RETBLEED.
* CVE-2022-23816
* CVE-2022-28693
* CVE-2022-29901

See also:
* "RETBLEED: Arbitrary Speculative Code Execution with Return
Instructions."
* AMD SECURITY NOTICE AMD-SN-1037: AMD CPU Branch Type Confusion
* TECHNICAL GUIDANCE FOR MITIGATING BRANCH TYPE CONFUSION REVISION 1.0
  2022-07-12
* Return Stack Buffer Underflow / Return Stack Buffer Underflow /
  CVE-2022-29901, CVE-2022-28693 / INTEL-SA-00702

SystemZ may eventually want to support "thunk-extern" and "thunk"; both
options are used by the Linux kernel's CONFIG_EXPOLINE.

This functionality has been available in GCC since the 8.1 release, and
was backported to the 7.3 release.

Many thanks for folks that provided discrete review off list due to the
embargoed nature of this hardware vulnerability. Many Bothans died to
bring us this information.

Link: https://www.youtube.com/watch?v=IF6HbCKQHK8
Link: https://github.com/llvm/llvm-project/issues/54404
Link: https://gcc.gnu.org/legacy-ml/gcc-patches/2018-01/msg01197.html
Link: https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/advisory-guidance/return-stack-buffer-underflow.html
Link: https://arstechnica.com/information-technology/2022/07/intel-and-amd-cpus-vulnerable-to-a-new-speculative-execution-attack/?comments=1
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ce114c866860aa9eae3f50974efc68241186ba60
Link: https://www.intel.com/content/www/us/en/security-center/advisory/intel-sa-00702.html
Link: https://www.intel.com/content/www/us/en/security-center/advisory/intel-sa-00707.html

Reviewed By: aaron.ballman, craig.topper

Differential Revision: https://reviews.llvm.org/D129572

clang/docs/ReleaseNotes.rst

@@ -404,6 +404,12 @@ Attribute Changes in Clang
   format string must correctly format the fixed parameter types of the function.
   Using the attribute this way emits a GCC compatibility diagnostic.
 
+- Support was added for ``__attribute__((function_return("thunk-extern")))``
+  to X86 to replace ``ret`` instructions with ``jmp __x86_return_thunk``. The
+  corresponding attribute to disable this,
+  ``__attribute__((function_return("keep")))`` was added. This is intended to
+  be used by the Linux kernel to mitigate RETBLEED.
+
 Windows Support
 ---------------
 
@@ -556,6 +562,9 @@ X86 Support in Clang
   this instruction (see rdpruintrin.h).
 - Support ``-mstack-protector-guard-symbol=[SymbolName]`` to use the given
   symbol for addressing the stack protector guard.
+- ``-mfunction-return=thunk-extern`` support was added to clang for x86. This
+  will be used by Linux kernel mitigations for RETBLEED. The corresponding flag
+  ``-mfunction-return=keep`` may be appended to disable the feature.
 
 DWARF Support in Clang
 ----------------------

clang/include/clang/Basic/Attr.td

@@ -4036,3 +4036,14 @@ def NoRandomizeLayout : InheritableAttr {
   let LangOpts = [COnly];
 }
 def : MutualExclusions<[RandomizeLayout, NoRandomizeLayout]>;
+
+def FunctionReturnThunks : InheritableAttr,
+    TargetSpecificAttr<TargetAnyX86> {
+  let Spellings = [GCC<"function_return">];
+  let Args = [EnumArgument<"ThunkType", "Kind",
+    ["keep", "thunk-extern"],
+    ["Keep", "Extern"]
+  >];
+  let Subjects = SubjectList<[Function]>;
+  let Documentation = [FunctionReturnThunksDocs];
+}

clang/include/clang/Basic/AttrDocs.td

@@ -6606,6 +6606,27 @@ evaluate to NULL.
       }
       return 0;
   }
-
+  }];
+}
+
+def FunctionReturnThunksDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+The attribute ``function_return`` can replace return instructions with jumps to
+target-specific symbols. This attribute supports 2 possible values,
+corresponding to the values supported by the ``-mfunction-return=`` command
+line flag:
+* ``__attribute__((function_return("keep")))`` to disable related transforms.
+  This is useful for undoing global setting from ``-mfunction-return=`` locally
+  for individual functions.
+* ``__attribute__((function_return("thunk-extern")))`` to replace returns with
+  jumps, while NOT emitting the thunk.
+
+The values ``thunk`` and ``thunk-inline`` from GCC are not supported.
+
+The symbol used for ``thunk-extern`` is target specific:
+* X86: ``__x86_return_thunk``
+
+As such, this function attribute is currently only supported on X86 targets.
   }];
 }

clang/include/clang/Basic/CodeGenOptions.def

@@ -107,6 +107,7 @@ CODEGENOPT(CFProtectionReturn , 1, 0) ///< if -fcf-protection is
 CODEGENOPT(CFProtectionBranch , 1, 0) ///< if -fcf-protection is
                                       ///< set to full or branch.
 CODEGENOPT(IBTSeal, 1, 0)             ///< set to optimize CFProtectionBranch.
+CODEGENOPT(FunctionReturnThunks, 1, 0) ///< -mfunction-return={keep|thunk-extern}
 
 CODEGENOPT(XRayInstrumentFunctions , 1, 0) ///< Set when -fxray-instrument is
                                            ///< enabled.

clang/include/clang/Driver/Options.td

@@ -1998,6 +1998,13 @@ def fcf_protection : Flag<["-"], "fcf-protection">, Group<f_Group>, Flags<[CoreO
   HelpText<"Enable cf-protection in 'full' mode">;
 def mibt_seal : Flag<["-"], "mibt-seal">, Group<m_Group>, Flags<[CoreOption, CC1Option]>,
   HelpText<"Optimize fcf-protection=branch/full (requires LTO).">;
+def mfunction_return_EQ : Joined<["-"], "mfunction-return=">,
+  Group<m_Group>, Flags<[CoreOption, CC1Option]>,
+  HelpText<"Replace returns with jumps to ``__x86_return_thunk`` (x86 only, error otherwise)">,
+  Values<"keep,thunk-extern">,
+  NormalizedValues<["Keep", "Extern"]>,
+  NormalizedValuesScope<"llvm::FunctionReturnThunksKind">,
+  MarshallingInfoEnum<CodeGenOpts<"FunctionReturnThunks">, "Keep">;
 
 defm xray_instrument : BoolFOption<"xray-instrument",
   LangOpts<"XRayInstrument">, DefaultFalse,

clang/lib/CodeGen/CodeGenFunction.cpp

@@ -898,6 +898,20 @@ void CodeGenFunction::StartFunction(GlobalDecl GD, QualType RetTy,
   if (D && D->hasAttr<NoProfileFunctionAttr>())
     Fn->addFnAttr(llvm::Attribute::NoProfile);
 
+  if (D) {
+    // Function attributes take precedence over command line flags.
+    if (auto *A = D->getAttr<FunctionReturnThunksAttr>()) {
+      switch (A->getThunkType()) {
+      case FunctionReturnThunksAttr::Kind::Keep:
+        break;
+      case FunctionReturnThunksAttr::Kind::Extern:
+        Fn->addFnAttr(llvm::Attribute::FnRetThunkExtern);
+        break;
+      }
+    } else if (CGM.getCodeGenOpts().FunctionReturnThunks)
+      Fn->addFnAttr(llvm::Attribute::FnRetThunkExtern);
+  }
+
   if (FD && (getLangOpts().OpenCL ||
              (getLangOpts().HIP && getLangOpts().CUDAIsDevice))) {
     // Add metadata for a kernel function.

clang/lib/Driver/ToolChains/Clang.cpp

@@ -6344,6 +6344,10 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   if (IsUsingLTO)
     Args.AddLastArg(CmdArgs, options::OPT_mibt_seal);
 
+  if (Arg *A = Args.getLastArg(options::OPT_mfunction_return_EQ))
+    CmdArgs.push_back(
+        Args.MakeArgString(Twine("-mfunction-return=") + A->getValue()));
+
   // Forward -f options with positive and negative forms; we translate these by
   // hand.  Do not propagate PGO options to the GPU-side compilations as the
   // profile info is for the host-side compilation only.

clang/lib/Frontend/CompilerInvocation.cpp

@@ -1485,6 +1485,9 @@ void CompilerInvocation::GenerateCodeGenArgs(
   if (Opts.IBTSeal)
     GenerateArg(Args, OPT_mibt_seal, SA);
 
+  if (Opts.FunctionReturnThunks)
+    GenerateArg(Args, OPT_mfunction_return_EQ, "thunk-extern", SA);
+
   for (const auto &F : Opts.LinkBitcodeFiles) {
     bool Builtint = F.LinkFlags == llvm::Linker::Flags::LinkOnlyNeeded &&
                     F.PropagateAttrs && F.Internalize;
@@ -1825,6 +1828,27 @@ bool CompilerInvocation::ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args,
       Diags.Report(diag::err_drv_invalid_value) << A->getAsString(Args) << Name;
   }
 
+  if (const Arg *A = Args.getLastArg(OPT_mfunction_return_EQ)) {
+    auto Val = llvm::StringSwitch<llvm::FunctionReturnThunksKind>(A->getValue())
+                   .Case("keep", llvm::FunctionReturnThunksKind::Keep)
+                   .Case("thunk-extern", llvm::FunctionReturnThunksKind::Extern)
+                   .Default(llvm::FunctionReturnThunksKind::Invalid);
+    // SystemZ might want to add support for "expolines."
+    if (!T.isX86())
+      Diags.Report(diag::err_drv_argument_not_allowed_with)
+          << A->getSpelling() << T.getTriple();
+    else if (Val == llvm::FunctionReturnThunksKind::Invalid)
+      Diags.Report(diag::err_drv_invalid_value)
+          << A->getAsString(Args) << A->getValue();
+    else if (Val == llvm::FunctionReturnThunksKind::Extern &&
+             Args.getLastArgValue(OPT_mcmodel_EQ).equals("large"))
+      Diags.Report(diag::err_drv_argument_not_allowed_with)
+          << A->getAsString(Args)
+          << Args.getLastArg(OPT_mcmodel_EQ)->getAsString(Args);
+    else
+      Opts.FunctionReturnThunks = static_cast<unsigned>(Val);
+  }
+
   if (Opts.PrepareForLTO && Args.hasArg(OPT_mibt_seal))
     Opts.IBTSeal = 1;
 

clang/lib/Sema/SemaDeclAttr.cpp

@@ -8000,6 +8000,26 @@ static void handleZeroCallUsedRegsAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   D->addAttr(ZeroCallUsedRegsAttr::Create(S.Context, Kind, AL));
 }
 
+static void handleFunctionReturnThunksAttr(Sema &S, Decl *D,
+                                           const ParsedAttr &AL) {
+  StringRef KindStr;
+  SourceLocation LiteralLoc;
+  if (!S.checkStringLiteralArgumentAttr(AL, 0, KindStr, &LiteralLoc))
+    return;
+
+  FunctionReturnThunksAttr::Kind Kind;
+  if (!FunctionReturnThunksAttr::ConvertStrToKind(KindStr, Kind)) {
+    S.Diag(LiteralLoc, diag::warn_attribute_type_not_supported)
+        << AL << KindStr;
+    return;
+  }
+  // FIXME: it would be good to better handle attribute merging rather than
+  // silently replacing the existing attribute, so long as it does not break
+  // the expected codegen tests.
+  D->dropAttr<FunctionReturnThunksAttr>();
+  D->addAttr(FunctionReturnThunksAttr::Create(S.Context, Kind, AL));
+}
+
 static void handleSYCLKernelAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   // The 'sycl_kernel' attribute applies only to function templates.
   const auto *FD = cast<FunctionDecl>(D);
@@ -8866,6 +8886,9 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_ZeroCallUsedRegs:
     handleZeroCallUsedRegsAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_FunctionReturnThunks:
+    handleFunctionReturnThunksAttr(S, D, AL);
+    break;
 
   // Microsoft attributes:
   case ParsedAttr::AT_LayoutVersion:

clang/test/CodeGen/attr-function-return.c

@@ -0,0 +1,96 @@
+// RUN: %clang_cc1 -std=gnu2x -triple x86_64-linux-gnu %s -emit-llvm -o - \
+// RUN:   | FileCheck %s --check-prefixes=CHECK,CHECK-NOM
+// RUN: %clang_cc1 -std=gnu2x -triple x86_64-linux-gnu %s -emit-llvm -o - \
+// RUN:   -mfunction-return=keep | FileCheck %s \
+// RUN:   --check-prefixes=CHECK,CHECK-KEEP
+// RUN: %clang_cc1 -std=gnu2x -triple x86_64-linux-gnu %s -emit-llvm -o - \
+// RUN:  -mfunction-return=thunk-extern | FileCheck %s \
+// RUN:  --check-prefixes=CHECK,CHECK-EXTERN
+
+#if !__has_attribute(function_return)
+#error "missing attribute support for function_return"
+#endif
+
+// CHECK: @keep() [[KEEP:#[0-9]+]]
+__attribute__((function_return("keep"))) void keep(void) {}
+
+// CHECK: @keep2() [[KEEP:#[0-9]+]]
+[[gnu::function_return("keep")]] void keep2(void) {}
+
+// CHECK: @thunk_extern() [[EXTERN:#[0-9]+]]
+__attribute__((function_return("thunk-extern"))) void thunk_extern(void) {}
+
+// CHECK: @thunk_extern2() [[EXTERN:#[0-9]+]]
+[[gnu::function_return("thunk-extern")]] void thunk_extern2(void) {}
+
+// CHECK: @double_thunk_keep() [[KEEP]]
+// clang-format off
+__attribute__((function_return("thunk-extern")))
+__attribute__((function_return("keep")))
+void double_thunk_keep(void) {}
+
+// CHECK: @double_thunk_keep2() [[KEEP]]
+[[gnu::function_return("thunk-extern")]][[gnu::function_return("keep")]]
+void double_thunk_keep2(void) {}
+
+// CHECK: @double_keep_thunk() [[EXTERN]]
+__attribute__((function_return("keep")))
+__attribute__((function_return("thunk-extern")))
+void double_keep_thunk(void) {}
+
+// CHECK: @double_keep_thunk2() [[EXTERN]]
+[[gnu::function_return("thunk-keep")]][[gnu::function_return("thunk-extern")]]
+void double_keep_thunk2(void) {}
+
+// CHECK: @thunk_keep() [[KEEP]]
+__attribute__((function_return("thunk-extern"), function_return("keep")))
+void thunk_keep(void) {}
+
+// CHECK: @thunk_keep2() [[KEEP]]
+[[gnu::function_return("thunk-extern"), gnu::function_return("keep")]]
+void thunk_keep2(void) {}
+
+// CHECK: @keep_thunk() [[EXTERN]]
+__attribute__((function_return("keep"), function_return("thunk-extern")))
+void keep_thunk(void) {}
+
+// CHECK: @keep_thunk2() [[EXTERN]]
+[[gnu::function_return("keep"), gnu::function_return("thunk-extern")]]
+void keep_thunk2(void) {}
+// clang-format on
+
+void undef(void);
+// CHECK: @undef() [[KEEP]]
+__attribute__((function_return("keep"))) void undef(void) {}
+
+void undef2(void);
+// CHECK: @undef2() [[EXTERN]]
+__attribute__((function_return("thunk-extern"))) void undef2(void) {}
+
+__attribute__((function_return("thunk-extern"))) void change_def(void);
+// CHECK: @change_def() [[KEEP]]
+__attribute__((function_return("keep"))) void change_def(void) {}
+
+__attribute__((function_return("keep"))) void change_def2(void);
+// CHECK: @change_def2() [[EXTERN]]
+__attribute__((function_return("thunk-extern"))) void change_def2(void) {}
+
+__attribute__((function_return("thunk-extern"))) void change_def3(void);
+// CHECK: @change_def3() [[KEEP]]
+[[gnu::function_return("keep")]] void change_def3(void) {}
+
+[[gnu::function_return("keep")]] void change_def4(void);
+// CHECK: @change_def4() [[EXTERN]]
+__attribute__((function_return("thunk-extern"))) void change_def4(void) {}
+
+// When there is no -mfunction-return= flag set (NOM) or it's set to keep,
+// we don't emit anything into the IR for unattributed functions.
+
+// CHECK-NOM:    @no_attrs() [[NOATTR:#[0-9]+]]
+// CHECK-KEEP:   @no_attrs() [[NOATTR:#[0-9]+]]
+// CHECK-EXTERN: @no_attrs() [[EXTERN]]
+void no_attrs(void) {}
+
+// CHECK-NOM-NOT:  [[NOATTR]] = {{.*}}fn_ret_thunk_extern
+// CHECK-KEEP-NOT: [[NOATTR]] = {{.*}}fn_ret_thunk_extern
+// CHECK: [[EXTERN]] = {{.*}}fn_ret_thunk_extern

clang/test/CodeGen/attr-function-return.cpp

@@ -0,0 +1,52 @@
+// RUN: %clang_cc1 -triple x86_64-linux-gnu %s -emit-llvm -o - \
+// RUN:   | FileCheck %s --check-prefixes=CHECK,CHECK-NOM
+// RUN: %clang_cc1 -triple x86_64-linux-gnu %s -emit-llvm -o - \
+// RUN:   -mfunction-return=keep | FileCheck %s \
+// RUN:   --check-prefixes=CHECK,CHECK-KEEP
+// RUN: %clang_cc1 -triple x86_64-linux-gnu %s -emit-llvm -o - \
+// RUN:  -mfunction-return=thunk-extern | FileCheck %s \
+// RUN:  --check-prefixes=CHECK,CHECK-EXTERN
+
+int foo(void) {
+  // CHECK: @"_ZZ3foovENK3$_0clEv"({{.*}}) [[NOATTR:#[0-9]+]]
+  return []() {
+    return 42;
+  }();
+}
+int bar(void) {
+  // CHECK: @"_ZZ3barvENK3$_1clEv"({{.*}}) [[EXTERN:#[0-9]+]]
+  return []() __attribute__((function_return("thunk-extern"))) {
+    return 42;
+  }
+  ();
+}
+int baz(void) {
+  // CHECK: @"_ZZ3bazvENK3$_2clEv"({{.*}}) [[KEEP:#[0-9]+]]
+  return []() __attribute__((function_return("keep"))) {
+    return 42;
+  }
+  ();
+}
+
+class Foo {
+public:
+  // CHECK: @_ZN3Foo3fooEv({{.*}}) [[EXTERN]]
+  __attribute__((function_return("thunk-extern"))) int foo() { return 42; }
+};
+
+int quux() {
+  Foo my_foo;
+  return my_foo.foo();
+}
+
+// CHECK: @extern_c() [[EXTERN]]
+extern "C" __attribute__((function_return("thunk-extern"))) void extern_c() {}
+extern "C" {
+// CHECK: @extern_c2() [[EXTERN]]
+__attribute__((function_return("thunk-extern"))) void extern_c2() {}
+}
+
+// CHECK-NOM-NOT:   [[NOATTR]] = {{.*}}fn_ret_thunk_extern
+// CHECK-KEEP-NOT:  [[NOATTR]] = {{.*}}fn_ret_thunk_extern
+// CHECK-KEEP-NOT:  [[KEEP]] = {{.*}}fn_ret_thunk_extern
+// CHECK-EXTERN:    [[EXTERN]] = {{.*}}fn_ret_thunk_extern

clang/test/Driver/mfunction-return.c

@@ -0,0 +1,22 @@
+// RUN: %clang -mfunction-return= -### %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-VALID %s
+// RUN: not %clang -mfunction-return -### %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-INVALID %s
+
+// RUN: %clang -mfunction-return=keep -### %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-KEEP %s
+// RUN: %clang -mfunction-return=thunk-extern -### %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-EXTERN %s
+
+// RUN: %clang -mfunction-return=keep -mfunction-return=thunk-extern -### %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-EXTERN %s
+// RUN: %clang -mfunction-return=thunk-extern -mfunction-return=keep -### %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-KEEP %s
+
+// CHECK-VALID:   "-mfunction-return="
+// CHECK-INVALID: error: unknown argument: '-mfunction-return'
+
+// CHECK-KEEP:       "-mfunction-return=keep"
+// CHECK-KEEP-NOT:   "-mfunction-return=thunk-extern"
+// CHECK-EXTERN:     "-mfunction-return=thunk-extern"
+// CHECK-EXTERN-NOT: "-mfunction-return=keep"

clang/test/Frontend/mfunction-return.c

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -mfunction-return=keep -triple x86_64-linux-gnu %s
+// RUN: %clang_cc1 -mfunction-return=thunk-extern -triple x86_64-linux-gnu %s
+
+// RUN: not %clang_cc1 -mfunction-return=thunk -triple x86_64-linux-gnu %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-THUNK %s
+// RUN: not %clang_cc1 -mfunction-return=thunk-inline -triple x86_64-linux-gnu %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-INLINE %s
+// RUN: not %clang_cc1 -mfunction-return=invalid -triple x86_64-linux-gnu %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-INVALID %s
+// RUN: not %clang_cc1 -mfunction-return=thunk-extern -triple s390x-linux-gnu %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-TARGET %s
+// RUN: not %clang_cc1 -mfunction-return=thunk-extern -mcmodel=large \
+// RUN:   -triple x86_64-linux-gnu %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-LARGE %s
+
+// CHECK-THUNK: error: invalid value 'thunk' in '-mfunction-return=thunk'
+// CHECK-INLINE: error: invalid value 'thunk-inline' in '-mfunction-return=thunk-inline'
+// CHECK-INVALID: error: invalid value 'invalid' in '-mfunction-return=invalid'
+// CHECK-TARGET: error: invalid argument '-mfunction-return=' not allowed with 's390x-unknown-linux-gnu'
+// CHECK-LARGE: error: invalid argument '-mfunction-return=thunk-extern' not allowed with '-mcmodel=large'

clang/test/Misc/pragma-attribute-supported-attributes-list.test

@@ -69,6 +69,7 @@
 // CHECK-NEXT: ExternalSourceSymbol ((SubjectMatchRule_record, SubjectMatchRule_enum, SubjectMatchRule_enum_constant, SubjectMatchRule_field, SubjectMatchRule_function, SubjectMatchRule_namespace, SubjectMatchRule_objc_category, SubjectMatchRule_objc_implementation, SubjectMatchRule_objc_interface, SubjectMatchRule_objc_method, SubjectMatchRule_objc_property, SubjectMatchRule_objc_protocol, SubjectMatchRule_record, SubjectMatchRule_type_alias, SubjectMatchRule_variable))
 // CHECK-NEXT: FlagEnum (SubjectMatchRule_enum)
 // CHECK-NEXT: Flatten (SubjectMatchRule_function)
+// CHECK-NEXT: FunctionReturnThunks (SubjectMatchRule_function)
 // CHECK-NEXT: GNUInline (SubjectMatchRule_function)
 // CHECK-NEXT: HIPManaged (SubjectMatchRule_variable)
 // CHECK-NEXT: Hot (SubjectMatchRule_function)

clang/test/Sema/attr-function-return-unsupported-target.c

@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -triple s390x-linux-gnu -fsyntax-only -verify %s
+
+// expected-warning@+1 {{unknown attribute 'function_return' ignored}}
+__attribute__((function_return("keep"))) void x(void) {}
+
+// expected-warning@+1 {{unknown attribute 'function_return' ignored}}
+__attribute__((function_return("thunk"))) void y(void) {}
+
+// expected-warning@+1 {{unknown attribute 'function_return' ignored}}
+__attribute__((function_return("thunk-inline"))) void z(void) {}
+
+// expected-warning@+1 {{unknown attribute 'function_return' ignored}}
+__attribute__((function_return("thunk-extern"))) void w(void) {}
+
+// expected-warning@+1 {{unknown attribute 'function_return' ignored}}
+__attribute__((function_return("invalid"))) void v(void) {}

clang/test/Sema/attr-function-return.c

@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -fsyntax-only -verify %s
+
+__attribute__((function_return("keep"))) void x(void) {}
+
+// expected-warning@+1 {{'function_return' attribute argument not supported: thunk}}
+__attribute__((function_return("thunk"))) void y(void) {}
+
+// expected-warning@+1 {{'function_return' attribute argument not supported: thunk-inline}}
+__attribute__((function_return("thunk-inline"))) void z(void) {}
+
+__attribute__((function_return("thunk-extern"))) void w(void) {}
+
+// expected-warning@+1 {{'function_return' attribute argument not supported: invalid}}
+__attribute__((function_return("invalid"))) void v(void) {}
+
+// expected-error@+1 {{'function_return' attribute requires a string}}
+__attribute__((function_return(5))) void a(void) {}
+
+// expected-error@+1 {{'function_return' attribute takes one argument}}
+__attribute__((function_return)) void b(void) {}
+
+// expected-warning@+1 {{'function_return' attribute only applies to functions}}
+__attribute__((function_return)) int c;

llvm/docs/LangRef.rst

@@ -1678,6 +1678,10 @@ example:
     Front ends can provide optional ``srcloc`` metadata nodes on call sites of
     such callees to attach information about where in the source language such a
     call came from. A string value can be provided as a note.
+``fn_ret_thunk_extern``
+    This attribute tells the code generator that returns from functions should
+    be replaced with jumps to externally-defined architecture-specific symbols.
+    For X86, this symbol's identifier is ``__x86_return_thunk``.
 ``"frame-pointer"``
     This attribute tells the code generator whether the function
     should keep the frame pointer. The code generator may emit the frame pointer

llvm/include/llvm/Bitcode/LLVMBitCodes.h

@@ -688,6 +688,7 @@ enum AttributeKindCodes {
   ATTR_KIND_ALLOCATED_POINTER = 81,
   ATTR_KIND_ALLOC_KIND = 82,
   ATTR_KIND_PRESPLIT_COROUTINE = 83,
+  ATTR_KIND_FNRETTHUNK_EXTERN = 84,
 };
 
 enum ComdatSelectionKindCodes {

llvm/include/llvm/IR/Attributes.td

@@ -102,6 +102,10 @@ def DisableSanitizerInstrumentation: EnumAttr<"disable_sanitizer_instrumentation
 /// Provide pointer element type to intrinsic.
 def ElementType : TypeAttr<"elementtype", [ParamAttr]>;
 
+/// Whether to keep return instructions, or replace with a jump to an external
+/// symbol.
+def FnRetThunkExtern : EnumAttr<"fn_ret_thunk_extern", [FnAttr]>;
+
 /// Function may only access memory that is inaccessible from IR.
 def InaccessibleMemOnly : EnumAttr<"inaccessiblememonly", [FnAttr]>;
 

llvm/include/llvm/Support/CodeGen.h

@@ -103,6 +103,13 @@ namespace llvm {
     Async = 2, ///< "Asynchronous" unwind tables (instr precise)
     Default = 2,
   };
+
+  enum class FunctionReturnThunksKind : unsigned int {
+    Keep = 0,    ///< No function return thunk.
+    Extern = 1,  ///< Replace returns with jump to thunk, don't emit thunk.
+    Invalid = 2, ///< Not used.
+  };
+
   } // namespace llvm
 
 #endif

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1855,6 +1855,8 @@ static Attribute::AttrKind getAttrFromCode(uint64_t Code) {
     return Attribute::DisableSanitizerInstrumentation;
   case bitc::ATTR_KIND_ELEMENTTYPE:
     return Attribute::ElementType;
+  case bitc::ATTR_KIND_FNRETTHUNK_EXTERN:
+    return Attribute::FnRetThunkExtern;
   case bitc::ATTR_KIND_INACCESSIBLEMEM_ONLY:
     return Attribute::InaccessibleMemOnly;
   case bitc::ATTR_KIND_INACCESSIBLEMEM_OR_ARGMEMONLY:

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -634,6 +634,8 @@ static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
     return bitc::ATTR_KIND_COLD;
   case Attribute::DisableSanitizerInstrumentation:
     return bitc::ATTR_KIND_DISABLE_SANITIZER_INSTRUMENTATION;
+  case Attribute::FnRetThunkExtern:
+    return bitc::ATTR_KIND_FNRETTHUNK_EXTERN;
   case Attribute::Hot:
     return bitc::ATTR_KIND_HOT;
   case Attribute::ElementType:

llvm/lib/Target/X86/CMakeLists.txt

@@ -75,6 +75,7 @@ set(sources
   X86PartialReduction.cpp
   X86RegisterBankInfo.cpp
   X86RegisterInfo.cpp
+  X86ReturnThunks.cpp
   X86SelectionDAGInfo.cpp
   X86ShuffleDecodeConstantPool.cpp
   X86SpeculativeLoadHardening.cpp

llvm/lib/Target/X86/X86.h

@@ -132,6 +132,9 @@ FunctionPass *createX86EvexToVexInsts();
 /// This pass creates the thunks for the retpoline feature.
 FunctionPass *createX86IndirectThunksPass();
 
+/// This pass replaces ret instructions with jmp's to __x86_return thunk.
+FunctionPass *createX86ReturnThunksPass();
+
 /// This pass ensures instructions featuring a memory operand
 /// have distinctive <LineNumber, Discriminator> (with respect to eachother)
 FunctionPass *createX86DiscriminateMemOpsPass();
@@ -185,6 +188,7 @@ void initializeX86LowerAMXTypeLegacyPassPass(PassRegistry &);
 void initializeX86PreAMXConfigPassPass(PassRegistry &);
 void initializeX86LowerTileCopyPass(PassRegistry &);
 void initializeX86LowerAMXIntrinsicsLegacyPassPass(PassRegistry &);
+void initializeX86ReturnThunksPass(PassRegistry &);
 
 namespace X86AS {
 enum : unsigned {

llvm/lib/Target/X86/X86ReturnThunks.cpp

@@ -0,0 +1,92 @@
+//==- X86ReturnThunks.cpp - Replace rets with thunks or inline thunks --=//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+/// \file
+///
+/// Pass that replaces ret instructions with a jmp to __x86_return_thunk.
+///
+/// This corresponds to -mfunction-return=thunk-extern or
+/// __attribute__((function_return("thunk-extern").
+///
+/// This pass is a minimal implementation necessary to help mitigate
+/// RetBleed for the Linux kernel.
+///
+/// Should support for thunk or thunk-inline be necessary in the future, then
+/// this pass should be combined with x86-retpoline-thunks which already has
+/// machinery to emit thunks. Until then, YAGNI.
+///
+/// This pass is very similar to x86-lvi-ret.
+///
+//===----------------------------------------------------------------------===//
+
+#include "X86.h"
+#include "X86InstrInfo.h"
+#include "X86Subtarget.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/Triple.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstr.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/MC/MCInstrDesc.h"
+#include "llvm/Support/Debug.h"
+
+using namespace llvm;
+
+#define PASS_KEY "x86-return-thunks"
+#define DEBUG_TYPE PASS_KEY
+
+struct X86ReturnThunks final : public MachineFunctionPass {
+  static char ID;
+  X86ReturnThunks() : MachineFunctionPass(ID) {}
+  StringRef getPassName() const override { return "X86 Return Thunks"; }
+  bool runOnMachineFunction(MachineFunction &MF) override;
+};
+
+char X86ReturnThunks::ID = 0;
+
+bool X86ReturnThunks::runOnMachineFunction(MachineFunction &MF) {
+  LLVM_DEBUG(dbgs() << getPassName() << "\n");
+
+  bool Modified = false;
+
+  if (!MF.getFunction().hasFnAttribute(llvm::Attribute::FnRetThunkExtern))
+    return Modified;
+
+  StringRef ThunkName = "__x86_return_thunk";
+  if (MF.getFunction().getName() == ThunkName)
+    return Modified;
+
+  const auto &ST = MF.getSubtarget<X86Subtarget>();
+  const bool Is64Bit = ST.getTargetTriple().getArch() == Triple::x86_64;
+  const unsigned RetOpc = Is64Bit ? X86::RET64 : X86::RET32;
+  SmallVector<MachineInstr *, 16> Rets;
+
+  for (MachineBasicBlock &MBB : MF)
+    for (MachineInstr &Term : MBB.terminators())
+      if (Term.getOpcode() == RetOpc)
+        Rets.push_back(&Term);
+
+  const MCInstrDesc &JMP = ST.getInstrInfo()->get(X86::TAILJMPd);
+
+  for (MachineInstr *Ret : Rets) {
+    BuildMI(Ret->getParent(), Ret->getDebugLoc(), JMP)
+        .addExternalSymbol(ThunkName.data());
+    Ret->eraseFromParent();
+    Modified = true;
+  }
+
+  return Modified;
+}
+
+INITIALIZE_PASS(X86ReturnThunks, PASS_KEY, "X86 Return Thunks", false, false)
+
+FunctionPass *llvm::createX86ReturnThunksPass() {
+  return new X86ReturnThunks();
+}

llvm/lib/Target/X86/X86TargetMachine.cpp

@@ -100,6 +100,7 @@ extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeX86Target() {
   initializeX86OptimizeLEAPassPass(PR);
   initializeX86PartialReductionPass(PR);
   initializePseudoProbeInserterPass(PR);
+  initializeX86ReturnThunksPass(PR);
 }
 
 static std::unique_ptr<TargetLoweringObjectFile> createTLOF(const Triple &TT) {
@@ -575,6 +576,7 @@ void X86PassConfig::addPreEmitPass2() {
   // hand inspection of the codegen output.
   addPass(createX86SpeculativeExecutionSideEffectSuppression());
   addPass(createX86IndirectThunksPass());
+  addPass(createX86ReturnThunksPass());
 
   // Insert extra int3 instructions after trailing call instructions to avoid
   // issues in the unwinder.

llvm/lib/Transforms/Utils/CodeExtractor.cpp

@@ -927,6 +927,7 @@ Function *CodeExtractor::constructFunction(const ValueSet &inputs,
       case Attribute::AlwaysInline:
       case Attribute::Cold:
       case Attribute::DisableSanitizerInstrumentation:
+      case Attribute::FnRetThunkExtern:
       case Attribute::Hot:
       case Attribute::NoRecurse:
       case Attribute::InlineHint:

llvm/test/Bitcode/attributes.ll

@@ -532,6 +532,9 @@ define void @f86() nosanitize_bounds
         ret void;
 }
 
+; CHECK: define void @f87() [[FNRETTHUNKEXTERN:#[0-9]+]]
+define void @f87() fn_ret_thunk_extern { ret void }
+
 ; CHECK: attributes #0 = { noreturn }
 ; CHECK: attributes #1 = { nounwind }
 ; CHECK: attributes #2 = { readnone }
@@ -585,4 +588,5 @@ define void @f86() nosanitize_bounds
 ; CHECK: attributes #50 = { disable_sanitizer_instrumentation }
 ; CHECK: attributes #51 = { uwtable(sync) }
 ; CHECK: attributes #52 = { nosanitize_bounds }
+; CHECK: attributes [[FNRETTHUNKEXTERN]] = { fn_ret_thunk_extern }
 ; CHECK: attributes #[[NOBUILTIN]] = { nobuiltin }

llvm/test/CodeGen/X86/O0-pipeline.ll

@@ -71,8 +71,9 @@
 ; CHECK-NEXT:       Live DEBUG_VALUE analysis
 ; CHECK-NEXT:       X86 Speculative Execution Side Effect Suppression
 ; CHECK-NEXT:       X86 Indirect Thunks
+; CHECK-NEXT:       X86 Return Thunks
 ; CHECK-NEXT:       Check CFA info and insert CFI instructions if needed
-; CHECK-NEXT:       X86 Load Value Injection (LVI) Ret-Hardening  
+; CHECK-NEXT:       X86 Load Value Injection (LVI) Ret-Hardening
 ; CHECK-NEXT:       Pseudo Probe Inserter
 ; CHECK-NEXT:       Lazy Machine Block Frequency Analysis
 ; CHECK-NEXT:       Machine Optimization Remark Emitter

llvm/test/CodeGen/X86/attr-function-return.ll

@@ -0,0 +1,11 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc --mtriple=i386-linux-gnu %s -o - -verify-machineinstrs \
+; RUN:   | FileCheck %s
+; RUN: llc --mtriple=x86_64-linux-gnu %s -o - -verify-machineinstrs \
+; RUN:   | FileCheck %s
+define void @x() fn_ret_thunk_extern {
+; CHECK-LABEL: x:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    jmp __x86_return_thunk
+  ret void
+}

llvm/test/CodeGen/X86/attr-function-return.mir

@@ -0,0 +1,62 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc --mtriple=x86_64-linux-gnu -run-pass=x86-return-thunks \
+# RUN:   -verify-machineinstrs %s -o - | FileCheck %s
+--- |
+  ; ModuleID = 'y.ll'
+  source_filename = "y.ll"
+  target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+  define void @x() #0 {
+    ret void
+  }
+
+  attributes #0 = { fn_ret_thunk_extern }
+
+...
+---
+name:            x
+alignment:       16
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+hasWinCFI:       false
+failsVerification: false
+tracksDebugUserValues: true
+registers:       []
+liveins:         []
+frameInfo:
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       0
+  offsetAdjustment: 0
+  maxAlignment:    1
+  adjustsStack:    false
+  hasCalls:        false
+  stackProtector:  ''
+  maxCallFrameSize: 0
+  cvBytesOfCalleeSavedRegisters: 0
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+  hasTailCall:     false
+  localFrameSize:  0
+  savePoint:       ''
+  restorePoint:    ''
+fixedStack:      []
+stack:           []
+callSites:       []
+debugValueSubstitutions: []
+constants:       []
+machineFunctionInfo: {}
+body:             |
+  bb.0 (%ir-block.0):
+    ; CHECK-LABEL: name: x
+    ; CHECK: TAILJMPd &__x86_return_thunk, implicit $esp, implicit $ssp
+    RET64
+
+...

llvm/test/CodeGen/X86/opt-pipeline.ll

@@ -204,6 +204,7 @@
 ; CHECK-NEXT:       Live DEBUG_VALUE analysis
 ; CHECK-NEXT:       X86 Speculative Execution Side Effect Suppression
 ; CHECK-NEXT:       X86 Indirect Thunks
+; CHECK-NEXT:       X86 Return Thunks
 ; CHECK-NEXT:       Check CFA info and insert CFI instructions if needed
 ; CHECK-NEXT:       X86 Load Value Injection (LVI) Ret-Hardening
 ; CHECK-NEXT:       Pseudo Probe Inserter

llvm/test/Transforms/Inline/attributes.ll

@@ -600,6 +600,33 @@ define i32 @test_unsafe-fp-math3(i32 %i) "unsafe-fp-math"="true" {
 ; CHECK-NEXT: ret i32
 }
 
+; Test that fn_ret_thunk_extern has no CompatRule; inlining is permitted.
+; Test that fn_ret_thunk_extern has no MergeRule; fn_ret_thunk_extern is not
+; propagated or dropped on the caller after inlining.
+define i32 @thunk_extern_callee() fn_ret_thunk_extern {
+; CHECK: @thunk_extern_callee() [[FNRETTHUNK_EXTERN:#[0-9]+]]
+  ret i32 42
+}
+
+define i32 @thunk_keep_caller() {
+; CHECK: @thunk_keep_caller() {
+; CHECK-NEXT: ret i32 42
+  %1 = call i32 @thunk_extern_callee()
+  ret i32 %1
+}
+
+define i32 @thunk_keep_callee() {
+; CHECK: @thunk_keep_callee() {
+  ret i32 42
+}
+
+define i32 @thunk_extern_caller() fn_ret_thunk_extern {
+; CHECK: @thunk_extern_caller() [[FNRETTHUNK_EXTERN]]
+; CHECK-NEXT: ret i32 42
+  %1 = call i32 @thunk_keep_callee()
+  ret i32 %1
+}
+
 ; CHECK: attributes [[SLH]] = { speculative_load_hardening }
 ; CHECK: attributes [[FPMAD_FALSE]] = { "less-precise-fpmad"="false" }
 ; CHECK: attributes [[FPMAD_TRUE]] = { "less-precise-fpmad"="true" }
@@ -614,3 +641,4 @@ define i32 @test_unsafe-fp-math3(i32 %i) "unsafe-fp-math"="true" {
 ; CHECK: attributes [[NO_SIGNED_ZEROS_FPMATH_TRUE]] = { "no-signed-zeros-fp-math"="true" }
 ; CHECK: attributes [[UNSAFE_FPMATH_FALSE]] = { "unsafe-fp-math"="false" }
 ; CHECK: attributes [[UNSAFE_FPMATH_TRUE]] = { "unsafe-fp-math"="true" }
+; CHECK: attributes [[FNRETTHUNK_EXTERN]] = { fn_ret_thunk_extern }