From 530b5da49b822fd65214ecbe85455114e0ce725f Mon Sep 17 00:00:00 2001
From: Jubilee Young <workingjubilee@gmail.com>
Date: Wed, 6 Jul 2022 19:09:33 -0700
Subject: [PATCH] Also stop emitting BTI prologues for naked functions

Same idea but for AArch64.
---
 compiler/rustc_codegen_llvm/src/attributes.rs |  2 ++
 .../aarch64-naked-fn-no-bti-prolog.rs         | 21 +++++++++++++++++++
 2 files changed, 23 insertions(+)
 create mode 100644 src/test/assembly/aarch64-naked-fn-no-bti-prolog.rs

diff --git a/compiler/rustc_codegen_llvm/src/attributes.rs b/compiler/rustc_codegen_llvm/src/attributes.rs
index 8c72f7dc06f25..c51c8391fa5a4 100644
--- a/compiler/rustc_codegen_llvm/src/attributes.rs
+++ b/compiler/rustc_codegen_llvm/src/attributes.rs
@@ -303,6 +303,8 @@ pub fn from_fn_attrs<'ll, 'tcx>(
         // And it is a module-level attribute, so the alternative is pulling naked functions into new LLVM modules.
         // Otherwise LLVM's "naked" functions come with endbr prefixes per https://github.com/rust-lang/rust/issues/98768
         to_add.push(AttributeKind::NoCfCheck.create_attr(cx.llcx));
+        // Need this for AArch64.
+        to_add.push(llvm::CreateAttrStringValue(cx.llcx, "branch-target-enforcement", "false"));
     }
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::ALLOCATOR) {
         // apply to return place instead of function (unlike all other attributes applied in this function)
diff --git a/src/test/assembly/aarch64-naked-fn-no-bti-prolog.rs b/src/test/assembly/aarch64-naked-fn-no-bti-prolog.rs
new file mode 100644
index 0000000000000..79b0bb2d7eead
--- /dev/null
+++ b/src/test/assembly/aarch64-naked-fn-no-bti-prolog.rs
@@ -0,0 +1,21 @@
+// compile-flags: -C no-prepopulate-passes -Zbranch-protection=bti
+// assembly-output: emit-asm
+// needs-asm-support
+// only-aarch64
+
+#![crate_type = "lib"]
+#![feature(naked_functions)]
+use std::arch::asm;
+
+// The problem at hand: Rust has adopted a fairly strict meaning for "naked functions",
+// meaning "no prologue whatsoever, no, really, not one instruction."
+// Unfortunately, aarch64's "branch target identification" works via hints at landing sites.
+// LLVM implements this via making sure of that, even for functions with the naked attribute.
+// So, we must emit an appropriate instruction instead!
+#[no_mangle]
+#[naked]
+pub unsafe extern "C" fn _hlt() -> ! {
+    // CHECK-NOT: hint #34
+    // CHECK: hlt #0x1
+    asm!("hlt #1", options(noreturn))
+}