WIP: Use BTI c in aarch64_outline_atomic.rs

.github/workflows/main.yaml

@@ -50,6 +50,26 @@ jobs:
           os: macos-15
         - target: aarch64-unknown-linux-gnu
           os: ubuntu-24.04-arm
+        - target: aarch64-unknown-linux-gnu
+          os: ubuntu-24.04-arm
+          flags: -Z branch-protection=bti -C link-arg=-Wl,-z,force-bti
+        - target: aarch64-unknown-linux-gnu
+          os: ubuntu-24.04-arm
+          flags: -Z branch-protection=pac-ret,bti -C link-arg=-Wl,-z,force-bti
+        - target: aarch64-unknown-linux-gnu
+          os: ubuntu-24.04-arm
+          flags: -Z branch-protection=pac-ret,bti,b-key -C link-arg=-Wl,-z,force-bti
+        - target: aarch64-unknown-linux-gnu
+          os: ubuntu-24.04 # QEMU
+        - target: aarch64-unknown-linux-gnu
+          os: ubuntu-24.04 # QEMU
+          flags: -Z branch-protection=bti -C link-arg=-Wl,-z,force-bti
+        - target: aarch64-unknown-linux-gnu
+          os: ubuntu-24.04 # QEMU
+          flags: -Z branch-protection=pac-ret,bti -C link-arg=-Wl,-z,force-bti
+        - target: aarch64-unknown-linux-gnu
+          os: ubuntu-24.04 # QEMU
+          flags: -Z branch-protection=pac-ret,bti,b-key -C link-arg=-Wl,-z,force-bti
         - target: aarch64-pc-windows-msvc
           os: windows-11-arm
         - target: arm-unknown-linux-gnueabi
@@ -174,11 +194,15 @@ jobs:
       if: matrix.os != 'ubuntu-24.04'
       shell: bash
       run: ./ci/run.sh ${{ matrix.target }}
+      env:
+        RUSTFLAGS: ${{ matrix.flags }} ${{ env.RUSTFLAGS }}
 
     # Otherwise we use our docker containers to run builds
     - name: Run in Docker
       if: matrix.os == 'ubuntu-24.04'
       run: ./ci/run-docker.sh ${{ matrix.target }}
+      env:
+        RUSTFLAGS: ${{ matrix.flags }} ${{ env.RUSTFLAGS }}
 
     - name: Print test logs if available
       if: always()

compiler-builtins/build.rs

@@ -21,6 +21,8 @@ fn main() {
     println!("cargo::rustc-check-cfg=cfg(kernel_user_helpers)");
     println!("cargo::rustc-check-cfg=cfg(feature, values(\"mem-unaligned\"))");
 
+    branch_protection_cfg();
+
     // Emscripten's runtime includes all the builtins
     if target.os == "emscripten" {
         return;
@@ -180,6 +182,25 @@ fn configure_check_cfg() {
     // FIXME: these come from libm and should be changed there
     println!("cargo::rustc-check-cfg=cfg(feature, values(\"checked\"))");
     println!("cargo::rustc-check-cfg=cfg(assert_no_panic)");
+
+    println!("cargo::rustc-check-cfg=cfg(branch_protection, values(\"pac-ret\",\"bti\"))");
+}
+
+fn branch_protection_cfg() {
+    //   flags: -Z branch-protection=pac-ret,bti,b-key
+    if let Some(rustflags) = env::var_os("CARGO_ENCODED_RUSTFLAGS") {
+        for mut flag in rustflags.to_string_lossy().split('\x1f') {
+            if flag.starts_with("-Z") {
+                flag = &flag["-Z".len()..];
+            }
+            if let Some(branch_protection) = flag.strip_prefix("branch-protection=") {
+                for x in branch_protection.split(",") {
+                    println!("cargo:rustc-cfg=branch_protection=\"{x}\"")
+                }
+                return;
+            }
+        }
+    }
 }
 
 #[cfg(feature = "c")]

compiler-builtins/src/aarch64_outline_atomics.rs

@@ -181,6 +181,7 @@ macro_rules! compare_and_swap {
             ) -> int_ty!($bytes) {
                 // We can't use `AtomicI8::compare_and_swap`; we *are* compare_and_swap.
                 core::arch::naked_asm! {
+                    "hint #34", // BTI c
                     // CAS    s(0), s(1), [x2]; if LSE supported.
                     try_lse_op!("cas", $ordering, $bytes, 0, 1, [x2]),
                     // UXT    s(tmp0), s(0)
@@ -213,6 +214,7 @@ macro_rules! compare_and_swap_i128 {
                 expected: i128, desired: i128, ptr: *mut i128
             ) -> i128 {
                 core::arch::naked_asm! {
+                    "hint #34", // BTI c
                     // CASP   x0, x1, x2, x3, [x4]; if LSE supported.
                     try_lse_op!("cas", $ordering, 16, 0, 1, 2, 3, [x4]),
                     "mov    x16, x0",
@@ -245,6 +247,7 @@ macro_rules! swap {
                 left: int_ty!($bytes), right_ptr: *mut int_ty!($bytes)
             ) -> int_ty!($bytes) {
                 core::arch::naked_asm! {
+                    "hint #34", // BTI c
                     // SWP    s(0), s(0), [x1]; if LSE supported.
                     try_lse_op!("swp", $ordering, $bytes, 0, 0, [x1]),
                     // mov    s(tmp0), s(0)
@@ -273,6 +276,7 @@ macro_rules! fetch_op {
                 val: int_ty!($bytes), ptr: *mut int_ty!($bytes)
             ) -> int_ty!($bytes) {
                 core::arch::naked_asm! {
+                    "hint #34", // BTI c
                     // LSEOP  s(0), s(0), [x1]; if LSE supported.
                     try_lse_op!($lse_op, $ordering, $bytes, 0, 0, [x1]),
                     // mov    s(tmp0), s(0)
@@ -397,3 +401,28 @@ foreach_ldadd!(add);
 foreach_ldclr!(and);
 foreach_ldeor!(xor);
 foreach_ldset!(or);
+
+core::arch::global_asm!(
+    "
+    .pushsection .note.gnu.property, \"a\"
+    .balign 8
+    .word 4
+    .word 0x10
+    .word 0x5 // NT_GNU_PROPERTY_TYPE_0
+    .asciz \"GNU\"
+    .word 0xc0000000 // GNU_PROPERTY_AARCH64_FEATURE_1_AND
+    .word 4
+    ",
+    #[cfg(all(branch_protection = "bti", branch_protection = "pac-ret"))]
+    ".word 3",
+    #[cfg(all(not(branch_protection = "bti"), branch_protection = "pac-ret"))]
+    ".word 2",
+    #[cfg(all(branch_protection = "bti", not(branch_protection = "pac-ret")))]
+    ".word 1",
+    #[cfg(all(not(branch_protection = "bti"), not(branch_protection = "pac-ret")))]
+    ".word 0",
+    "
+    .word 0
+    .popsection
+    "
+);