cpufeatures: add compile time fallback for unsupported targets and target features

cpufeatures/Cargo.toml

@@ -15,6 +15,9 @@ readme = "README.md"
 edition = "2024"
 rust-version = "1.85"
 
+[dependencies]
+cfg-if = "1"
+
 [target.'cfg(all(target_arch = "aarch64", target_vendor = "apple"))'.dependencies]
 libc = { version = "0.2.155", default-features = false }
 

cpufeatures/src/aarch64_apple.rs

@@ -0,0 +1,84 @@
+//! ARM64 CPU feature detection support.
+//!
+//! Unfortunately ARM instructions to detect CPU features cannot be called from
+//! unprivileged userspace code, so this implementation relies on OS-specific
+//! APIs for feature detection.
+
+use core::ffi::CStr;
+
+// Evaluate the given `$body` expression any of the supplied target features
+// are not enabled. Otherwise returns true.
+#[macro_export]
+#[doc(hidden)]
+macro_rules! __can_detect {
+    ($($tf:tt),+) => {
+        true
+    };
+}
+
+// Apple platform's runtime detection of target CPU features using `sysctlbyname`.
+#[macro_export]
+#[doc(hidden)]
+macro_rules! __detect {
+    ($($tf:tt),+) => {{
+        $($crate::check!($tf) & )+ true
+    }};
+}
+
+// Apple OS (macOS, iOS, watchOS, and tvOS) `check!` macro.
+//
+// NOTE: several of these instructions (e.g. `aes`, `sha2`) can be assumed to
+// be present on all Apple ARM64 hardware.
+//
+// Newer CPU instructions now have nodes within sysctl's `hw.optional`
+// namespace, however the ones that do not can safely be assumed to be
+// present on all Apple ARM64 devices, now and for the foreseeable future.
+//
+// See discussion on this issue for more information:
+// <https://github.com/RustCrypto/utils/issues/378>
+#[macro_export]
+#[doc(hidden)]
+macro_rules! check {
+    ("aes") => {
+        true
+    };
+    ("dit") => {
+        // https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms#Enable-DIT-for-constant-time-cryptographic-operations
+        unsafe {
+            $crate::aarch64_apple::sysctlbyname(c"hw.optional.arm.FEAT_DIT")
+        }
+    };
+    ("sha2") => {
+        true
+    };
+    ("sha3") => {
+        unsafe {
+            // `sha3` target feature implies SHA-512 as well
+            $crate::aarch64_apple::sysctlbyname(c"hw.optional.armv8_2_sha512")
+                && $crate::aarch64_apple::sysctlbyname(c"hw.optional.armv8_2_sha3")
+        }
+    };
+    ("sm4") => {
+        false
+    };
+}
+
+/// Apple helper function for calling `sysctlbyname`.
+pub unsafe fn sysctlbyname(name: &CStr) -> bool {
+    let mut value: u32 = 0;
+    let mut size = core::mem::size_of::<u32>();
+
+    let rc = unsafe {
+        libc::sysctlbyname(
+            name.as_ptr(),
+            &mut value as *mut _ as *mut libc::c_void,
+            &mut size,
+            core::ptr::null_mut(),
+            0,
+        )
+    };
+
+    assert_eq!(size, 4, "unexpected sysctlbyname(3) result size");
+    assert_eq!(rc, 0, "sysctlbyname returned error code: {}", rc);
+    value != 0
+}