[Driver] Enable IBT by default on OpenBSD/amd64 (llvm#125395)

Author: brad0

clang/lib/Driver/ToolChains/Clang.cpp

@@ -7064,6 +7064,11 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     if (Arg *SA = Args.getLastArg(options::OPT_mcf_branch_label_scheme_EQ))
       CmdArgs.push_back(Args.MakeArgString(Twine("-mcf-branch-label-scheme=") +
                                            SA->getValue()));
+  } else if (Triple.isOSOpenBSD() && Triple.getArch() == llvm::Triple::x86_64) {
+    // Emit IBT endbr64 instructions by default
+    CmdArgs.push_back("-fcf-protection=branch");
+    // jump-table can generate indirect jumps, which are not permitted
+    CmdArgs.push_back("-fno-jump-tables");
   }
 
   if (Arg *A = Args.getLastArg(options::OPT_mfunction_return_EQ))

clang/test/Driver/openbsd.c

@@ -141,3 +141,8 @@
 // RUN: %clang --target=aarch64-unknown-openbsd -### -c %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=CHECK-AARCH64-BTI-PAC %s
 // CHECK-AARCH64-BTI-PAC: "-msign-return-address=non-leaf" "-msign-return-address-key=a_key" "-mbranch-target-enforce"
+
+// Check 64-bit X86 for IBT flags
+// RUN: %clang --target=amd64-unknown-openbsd -### -c %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-AMD64-IBT %s
+// CHECK-AMD64-IBT: "-fcf-protection=branch" "-fno-jump-tables"