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/).

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`
 

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.

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;

compiler/rustc_interface/src/tests.rs

@@ -853,6 +853,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(sanitizer_cfi_generalize_pointers, Some(true));
     tracked!(sanitizer_cfi_normalize_integers, Some(true));
     tracked!(sanitizer_dataflow_abilist, vec![String::from("/rustc/abc")]);
+    tracked!(sanitizer_kcfi_arity, Some(true));
     tracked!(sanitizer_memory_track_origins, 2);
     tracked!(sanitizer_recover, SanitizerSet::ADDRESS);
     tracked!(saturating_float_casts, Some(true));

compiler/rustc_session/messages.ftl

@@ -94,6 +94,8 @@ session_sanitizer_cfi_requires_lto = `-Zsanitizer=cfi` requires `-Clto` or `-Cli
 
 session_sanitizer_cfi_requires_single_codegen_unit = `-Zsanitizer=cfi` with `-Clto` requires `-Ccodegen-units=1`
 
+session_sanitizer_kcfi_arity_requires_kcfi = `-Zsanitizer-kcfi-arity` requires `-Zsanitizer=kcfi`
+
 session_sanitizer_kcfi_requires_panic_abort = `-Z sanitizer=kcfi` requires `-C panic=abort`
 
 session_sanitizer_not_supported = {$us} sanitizer is not supported for this target

compiler/rustc_session/src/errors.rs

@@ -147,6 +147,10 @@ pub(crate) struct SanitizerCfiGeneralizePointersRequiresCfi;
 #[diag(session_sanitizer_cfi_normalize_integers_requires_cfi)]
 pub(crate) struct SanitizerCfiNormalizeIntegersRequiresCfi;
 
+#[derive(Diagnostic)]
+#[diag(session_sanitizer_kcfi_arity_requires_kcfi)]
+pub(crate) struct SanitizerKcfiArityRequiresKcfi;
+
 #[derive(Diagnostic)]
 #[diag(session_sanitizer_kcfi_requires_panic_abort)]
 pub(crate) struct SanitizerKcfiRequiresPanicAbort;

compiler/rustc_session/src/options.rs

@@ -2443,6 +2443,8 @@ written to standard error output)"),
         "enable normalizing integer types (default: no)"),
     sanitizer_dataflow_abilist: Vec<String> = (Vec::new(), parse_comma_list, [TRACKED],
         "additional ABI list files that control how shadow parameters are passed (comma separated)"),
+    sanitizer_kcfi_arity: Option<bool> = (None, parse_opt_bool, [TRACKED],
+        "enable KCFI arity indicator (default: no)"),
     sanitizer_memory_track_origins: usize = (0, parse_sanitizer_memory_track_origins, [TRACKED],
         "enable origins tracking in MemorySanitizer"),
     sanitizer_recover: SanitizerSet = (SanitizerSet::empty(), parse_sanitizers, [TRACKED],

compiler/rustc_session/src/session.rs

@@ -381,6 +381,10 @@ impl Session {
         self.opts.unstable_opts.sanitizer_cfi_normalize_integers == Some(true)
     }
 
+    pub fn is_sanitizer_kcfi_arity_enabled(&self) -> bool {
+        self.opts.unstable_opts.sanitizer_kcfi_arity == Some(true)
+    }
+
     pub fn is_sanitizer_kcfi_enabled(&self) -> bool {
         self.opts.unstable_opts.sanitizer.contains(SanitizerSet::KCFI)
     }
@@ -1211,6 +1215,11 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
         }
     }
 
+    // KCFI arity indicator requires KCFI.
+    if sess.is_sanitizer_kcfi_arity_enabled() && !sess.is_sanitizer_kcfi_enabled() {
+        sess.dcx().emit_err(errors::SanitizerKcfiArityRequiresKcfi);
+    }
+
     // LLVM CFI pointer generalization requires CFI or KCFI.
     if sess.is_sanitizer_cfi_generalize_pointers_enabled() {
         if !(sess.is_sanitizer_cfi_enabled() || sess.is_sanitizer_kcfi_enabled()) {