Add a build flavor to opt-out of BTCFI on OpenBSD.

To work-around #80059, we need to stop return address signing and
opt-out of BTCFI enforcement via enabling a platform linker option.

We don't want to completely undo the BTCFI work in the rare case that
we later figure out how to properly address the above issue, or allow
users who might want to benefit from BTCFI enforcement and won't use
Concurrency. To do this, condition the existing BTCFI flag enforcement
into a configuration option that defaults to off for now.

Because the new swift-driver needs to "know" whether the frontend is
configured to opt-out or not, and since the new driver communicates with
the frontend via the target info JSON to begin with, we add a field
that emits the build flavor to signal the right behavior.

CMakeLists.txt

@@ -575,6 +575,17 @@ set(SWIFT_DARWIN_XCRUN_TOOLCHAIN "XcodeDefault" CACHE STRING
 set(SWIFT_DARWIN_STDLIB_INSTALL_NAME_DIR "/usr/lib/swift" CACHE STRING
     "The directory of the install_name for standard library dylibs")
 
+#
+# User-configurable OpenBSD-specific options.
+#
+
+option(SWIFT_OPENBSD_BTCFI
+  "Emit branch target identification instructions and sign return addresses when available"
+  FALSE)
+if(SWIFT_OPENBSD_BTCFI)
+  add_definitions("-DSWIFT_OPENBSD_BTCFI")
+endif()
+
 # We don't want to use the same install_name_dir as the standard library which
 # will be installed in /usr/lib/swift. These private libraries should continue
 # to use @rpath for now.

include/swift/Basic/Platform.h

@@ -71,6 +71,10 @@ namespace swift {
   /// (eg. in /usr/lib/swift).
   bool tripleRequiresRPathForSwiftLibrariesInOS(const llvm::Triple &triple);
 
+  /// Returns true if the given triple represents a version of OpenBSD
+  /// that enforces BTCFI by default.
+  bool tripleBTCFIByDefaultInOpenBSD(const llvm::Triple &triple);
+
   /// Returns the platform name for a given target triple.
   ///
   /// For example, the iOS simulator has the name "iphonesimulator", while real

lib/Basic/Platform.cpp

@@ -140,6 +140,10 @@ bool swift::tripleRequiresRPathForSwiftLibrariesInOS(
   return false;
 }
 
+bool swift::tripleBTCFIByDefaultInOpenBSD(const llvm::Triple &triple) {
+  return triple.isOSOpenBSD() && triple.getArch() == llvm::Triple::aarch64;
+}
+
 DarwinPlatformKind swift::getDarwinPlatformKind(const llvm::Triple &triple) {
   if (triple.isiOS()) {
     if (triple.isTvOS()) {

lib/Basic/TargetInfo.cpp

@@ -171,6 +171,16 @@ void printTripleInfo(const CompilerInvocation &invocation,
     out << "    \"compatibilityLibraries\": [ ],\n";
   }
 
+  if (tripleBTCFIByDefaultInOpenBSD(triple)) {
+#if SWIFT_OPENBSD_BTCFI
+     out << "    \"openbsdBTCFIEnabled\": true,\n";
+#else
+     out << "    \"openbsdBTCFIEnabled\": false,\n";
+#endif
+  } else {
+     out << "    \"openbsdBTCFIEnabled\": false,\n";
+  }
+
   out << "    \"librariesRequireRPath\": "
       << (tripleRequiresRPathForSwiftLibrariesInOS(triple) ? "true" : "false")
       << "\n";

lib/Driver/ToolChains.cpp

@@ -205,12 +205,14 @@ void ToolChain::addCommonFrontendArgs(const OutputInfo &OI,
   }
 
   if (Triple.isOSOpenBSD() && Triple.getArch() == llvm::Triple::aarch64) {
+#ifdef SWIFT_OPENBSD_BTCFI
     arguments.push_back("-Xcc");
     arguments.push_back("-Xclang=-mbranch-target-enforce");
     arguments.push_back("-Xcc");
     arguments.push_back("-Xclang=-msign-return-address=non-leaf");
     arguments.push_back("-Xcc");
     arguments.push_back("-Xclang=-msign-return-address-key=a_key");
+#endif
   }
 
   if (inputArgs.getLastArg(options::OPT_experimental_serialize_debug_info)) {

lib/Driver/UnixToolChains.cpp

@@ -211,6 +211,15 @@ toolchains::GenericUnix::constructInvocation(const DynamicLinkJobAction &job,
 #endif
   }
 
+  if (tripleBTCFIByDefaultInOpenBSD(getTriple())) {
+#ifndef SWIFT_OPENBSD_BTCFI
+    Arguments.push_back("-Xlinker");
+    Arguments.push_back("-z");
+    Arguments.push_back("-Xlinker");
+    Arguments.push_back("nobtcfi");
+#endif
+  }
+
   // Configure the toolchain.
   if (const Arg *A = context.Args.getLastArg(options::OPT_tools_directory)) {
     StringRef toolchainPath(A->getValue());

utils/build-script-impl

@@ -124,6 +124,9 @@ KNOWN_SETTINGS=(
     darwin-toolchain-require-use-os-runtime       "0"               "When setting up a plist for a toolchain, require the users of the toolchain to link against the OS instead of the packaged toolchain runtime. 0 for false, 1 for true"
     darwin-xcrun-toolchain                        "default"         "the name of the toolchain to use on Darwin"
 
+    ## OpenBSD options
+    openbsd-btcfi                                 ""                "enables BTCFI when possible. May cause stability problems."
+
     ## Runtime options
     min-runtime-version                ""                "Used to specify the minimum host runtime version when building the compiler on non-Darwin platforms"
 
@@ -877,6 +880,12 @@ function set_build_options_for_host() {
     swift_cmake_options+=(
         -DCOVERAGE_DB="${COVERAGE_DB}"
     )
+
+    if [[ "${OPENBSD_BTCFI}" ]]; then
+        swift_cmake_options+=(
+            -DSWIFT_OPENBSD_BTCFI:BOOL=TRUE
+        )
+    fi
 }
 
 function configure_default_options() {
@@ -1383,7 +1392,9 @@ function swift_c_flags() {
             echo -n " -D_GNU_SOURCE -DHAVE_DECL_PROGRAM_INVOCATION_SHORT_NAME"
             ;;
         openbsd-aarch64)
-            echo -n " -Xclang=-msign-return-address=non-leaf -Xclang=-msign-return-address-key=a_key -Xclang=-mbranch-target-enforce"
+            if [[ "${OPENBSD_BTCFI}" ]]; then
+                echo -n " -Xclang=-msign-return-address=non-leaf -Xclang=-msign-return-address-key=a_key -Xclang=-mbranch-target-enforce"
+            fi
             ;;
     esac
 }