aarch64-apple: Clarify feature detection.

Use the `libc` crate so we don't have to maintain the binding for
`sysctlbyname`, and so `libc` will take care of the linking to
libc as needed..

Wrap `sysctlbyname` in a safe wrapper. Separate the branches for
the `sysctlbyname` results so we can see them individually in code
coverage.

Author: briansmith

Cargo.toml

@@ -162,7 +162,7 @@ untrusted = { version = "0.9" }
 [target.'cfg(any(target_arch = "aarch64", target_arch = "arm", target_arch = "x86",target_arch = "x86_64"))'.dependencies]
 spin = { version = "0.9.8", default-features = false, features = ["once"] }
 
-[target.'cfg(all(any(target_os = "android", target_os = "linux"), any(target_arch = "aarch64", target_arch = "arm")))'.dependencies]
+[target.'cfg(all(any(target_os = "android", target_os = "linux", any(target_os = "ios", target_os = "macos", target_os = "tvos")), any(target_arch = "aarch64", target_arch = "arm")))'.dependencies]
 libc = { version = "0.2.148", default-features = false }
 
 [target.'cfg(all(target_arch = "aarch64", target_os = "windows"))'.dependencies]

src/cpu/arm.rs

@@ -185,30 +185,42 @@ fn detect_features() -> u32 {
     any(target_os = "ios", target_os = "macos", target_os = "tvos")
 ))]
 fn detect_features() -> u32 {
-    let mut features = ARMCAP_STATIC;
-
-    extern "C" {
-        fn sysctlbyname(
-            name: *const u8,
-            oldp: *mut core::ffi::c_void,
-            oldlenp: *mut crate::c::size_t,
-            newp: *mut core::ffi::c_void,
-            newlen: crate::c::size_t,
-        ) -> core::ffi::c_int;
+    use core::ffi::CStr;
+
+    fn detect_feature(name: &CStr) -> bool {
+        use core::mem;
+
+        let mut value: libc::c_int = 0;
+        let mut len = mem::size_of_val(&value);
+        let value_ptr = polyfill::ptr::from_mut::from_mut(&mut value).cast::<c_void>();
+        let rc = unsafe {
+            libc::sysctlbyname(
+                FEATURE_NAME.as_ptr(),
+                value_ptr,
+                &mut len,
+                core::ptr::null_mut(),
+                0,
+            )
+        };
+        // All the conditions are separated so we can observe them in code coverage.
+        if rc == 0 {
+            return false;
+        }
+        debug_assert_eq!(len, mem::size_of_val(&value));
+        if len != mem::size_of_val(&value) {
+            return false;
+        }
+        if value == 0 {
+            return false;
+        }
+        true
     }
 
-    let mut value: core::ffi::c_int = 0;
-    let mut len = core::mem::size_of_val(&value);
-    let rc = unsafe {
-        sysctlbyname(
-            "hw.optional.armv8_2_sha512\0".as_ptr(),
-            &mut value as *mut _ as _,
-            &mut len,
-            core::ptr::null_mut(),
-            0,
-        )
-    };
-    if rc == 0 && len == core::mem::size_of_val(&value) && value != 0 {
+    let mut features = ARMCAP_STATIC;
+
+    const SHA512_NAME: CStr =
+        polyfill::unwrap_const(CStr::from_bytes_until_nul("hw.optional.armv8_2_sha512\0"));
+    if detect_feature(SHA512_NAME) {
         features |= SHA512.mask;
     }
 

src/polyfill.rs

@@ -51,6 +51,8 @@ mod array_split_map;
 #[cfg(feature = "alloc")]
 mod leading_zeros_skipped;
 
+pub mod ptr;
+
 #[cfg(test)]
 mod test;
 

src/polyfill/ptr.rs

@@ -0,0 +1,20 @@
+// Copyright 2024 Brian Smith.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
+// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+// TODO(MSRV 1.76): Replace with `core::ptr::from_mut`.
+#[allow(dead_code)]
+#[inline(always)]
+pub fn from_mut<T: ?Sized>(r: &mut T) -> *mut T {
+    <*mut T>::from(r)
+}