KCFI: Add KCFI arity indicator support

Adds KCFI arity indicator support to the Rust compiler (see rust-lang/rust#138311,
https://github.com/llvm/llvm-project/pull/121070, and
https://lore.kernel.org/lkml/CANiq72=3ghFxy8E=AU9p+0imFxKr5iU3sd0hVUXed5BA+KjdNQ@mail.gmail.com/).

3 files changed, 22 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_llvm/messages.ftl b/compiler/rustc_codegen_llvm/messages.ftl
index 17f2e7ca9f7..41391b096cc 100644
--- a/compiler/rustc_codegen_llvm/messages.ftl
+++ b/compiler/rustc_codegen_llvm/messages.ftl
@@ -56,6 +56,8 @@ codegen_llvm_prepare_thin_lto_module_with_llvm_err = failed to prepare thin LTO
 codegen_llvm_run_passes = failed to run LLVM passes
 codegen_llvm_run_passes_with_llvm_err = failed to run LLVM passes: {$llvm_err}
 
+codegen_llvm_sanitizer_kcfi_arity_requires_llvm_21_0_0 = `-Zsanitizer-kcfi-arity` requires LLVM 21.0.0 or later.
+
 codegen_llvm_sanitizer_memtag_requires_mte =
     `-Zsanitizer=memtag` requires `-Ctarget-feature=+mte`
 
diff --git a/compiler/rustc_codegen_llvm/src/context.rs b/compiler/rustc_codegen_llvm/src/context.rs
index f7b096ff976..ad98e506690 100644
--- a/compiler/rustc_codegen_llvm/src/context.rs
+++ b/compiler/rustc_codegen_llvm/src/context.rs
@@ -327,6 +327,22 @@ pub(crate) unsafe fn create_module<'ll>(
                 pfe.prefix().into(),
             );
         }
+
+        // Add "kcfi-arity" module flag if KCFI arity indicator is enabled. (See
+        // https://github.com/llvm/llvm-project/pull/117121.)
+        if sess.is_sanitizer_kcfi_arity_enabled() {
+            // KCFI arity indicator requires LLVM 21.0.0 or later.
+            if llvm_version < (21, 0, 0) {
+                tcx.dcx().emit_err(crate::errors::SanitizerKcfiArityRequiresLLVM2100);
+            }
+
+            llvm::add_module_flag_u32(
+                llmod,
+                llvm::ModuleFlagMergeBehavior::Override,
+                "kcfi-arity",
+                1,
+            );
+        }
     }
 
     // Control Flow Guard is currently only supported by MSVC and LLVM on Windows.
diff --git a/compiler/rustc_codegen_llvm/src/errors.rs b/compiler/rustc_codegen_llvm/src/errors.rs
index 4c5a78ca74f..ecf108f988f 100644
--- a/compiler/rustc_codegen_llvm/src/errors.rs
+++ b/compiler/rustc_codegen_llvm/src/errors.rs
@@ -217,3 +217,7 @@ pub(crate) struct MismatchedDataLayout<'a> {
 pub(crate) struct FixedX18InvalidArch<'a> {
     pub arch: &'a str,
 }
+
+#[derive(Diagnostic)]
+#[diag(codegen_llvm_sanitizer_kcfi_arity_requires_llvm_21_0_0)]
+pub(crate) struct SanitizerKcfiArityRequiresLLVM2100;