Improve memory safety of Weak::new().

Do null terminator checking more completely and at compile time.

src/netbsd.rs

@@ -29,7 +29,8 @@ type GetRandomFn = unsafe extern "C" fn(*mut u8, libc::size_t, libc::c_uint) ->
 
 pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     // getrandom(2) was introduced in NetBSD 10.0
-    static GETRANDOM: Weak = unsafe { Weak::new("getrandom\0") };
+    static GETRANDOM_NAME: cstr::Ref = cstr::unwrap_const_from_bytes_with_nul(b"getrandom\0");
+    static GETRANDOM: Weak = Weak::new(GETRANDOM_NAME);
     if let Some(fptr) = GETRANDOM.ptr() {
         let func: GetRandomFn = unsafe { core::mem::transmute(fptr) };
         return sys_fill_exact(dest, |buf| unsafe {

src/util_libc.rs

@@ -85,7 +85,7 @@ pub fn sys_fill_exact(
 // https://github.com/rust-lang/rust/blob/1.61.0/library/std/src/sys/unix/weak.rs#L84
 // except that the caller must manually cast self.ptr() to a function pointer.
 pub struct Weak {
-    name: &'static str,
+    name: cstr::Ref,
     addr: AtomicPtr<c_void>,
 }
 
@@ -98,9 +98,8 @@ impl Weak {
     // TODO: Replace with core::ptr::invalid_mut(1) when that is stable.
     const UNINIT: *mut c_void = 1 as *mut c_void;
 
-    // Construct a binding to a C function with a given name. This function is
-    // unsafe because `name` _must_ be null terminated.
-    pub const unsafe fn new(name: &'static str) -> Self {
+    // Construct a binding to a C function with a given name.
+    pub const fn new(name: cstr::Ref) -> Self {
         Self {
             name,
             addr: AtomicPtr::new(Self::UNINIT),
@@ -120,8 +119,7 @@ impl Weak {
         // the use of non-Relaxed operations is probably unnecessary.
         match self.addr.load(Ordering::Relaxed) {
             Self::UNINIT => {
-                // XXX/FIXME: Unchecked UTF-8-to-c_char cast.
-                let symbol = self.name.as_ptr().cast::<libc::c_char>();
+                let symbol = self.name.as_ptr();
                 let addr = unsafe { libc::dlsym(libc::RTLD_DEFAULT, symbol) };
                 // Synchronizes with the Acquire fence below
                 self.addr.store(addr, Ordering::Release);