unconditionally allow shadow call-stack for AArch64 whenever fixed-x18 is applied

Author: dingxiangfei2009

compiler/rustc_session/src/session.rs

@@ -1187,7 +1187,12 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
 
     // Sanitizers can only be used on platforms that we know have working sanitizer codegen.
     let supported_sanitizers = sess.target.options.supported_sanitizers;
-    let unsupported_sanitizers = sess.opts.unstable_opts.sanitizer - supported_sanitizers;
+    let mut unsupported_sanitizers = sess.opts.unstable_opts.sanitizer - supported_sanitizers;
+    // Niche: if `fixed-x18`, or effectively switching on `reserved-x18` flag, is enabled
+    // we should allow Shadow Call Stack sanitizer.
+    if sess.opts.unstable_opts.fixed_x18 && sess.target.arch == "aarch64" {
+        unsupported_sanitizers -= SanitizerSet::SHADOWCALLSTACK;
+    }
     match unsupported_sanitizers.into_iter().count() {
         0 => {}
         1 => {

compiler/rustc_target/src/spec/targets/aarch64_unknown_none.rs

@@ -21,7 +21,9 @@ pub fn target() -> Target {
             &["--fix-cortex-a53-843419"],
         ),
         features: "+v8a,+strict-align,+neon,+fp-armv8".into(),
-        supported_sanitizers: SanitizerSet::KCFI | SanitizerSet::KERNELADDRESS,
+        supported_sanitizers: SanitizerSet::KCFI
+            | SanitizerSet::KERNELADDRESS
+            | SanitizerSet::SHADOWCALLSTACK,
         relocation_model: RelocModel::Static,
         disable_redzone: true,
         max_atomic_width: Some(128),