Explicitly specify types to arguments of 'libc::syscall'

The 'libc::syscall' function uses varargs - as a result, its arguments
are completely untyped. THe user must ensure that it is called with the
proper types for the targeted syscall - otherwise, the calling
convention might cause arguments to be put into the wrong registers.

This commit explicitly casts the arguments to 'libc::syscall' to the
proper type for the 'getrandom' syscall. This ensures that the correct
types for the target platform will always be used, instead of relying on
the types used happening to match those required by the target platform.

src/linux_android.rs

@@ -15,15 +15,15 @@ pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
     static HAS_GETRANDOM: LazyBool = LazyBool::new();
     if HAS_GETRANDOM.unsync_init(is_getrandom_available) {
         sys_fill_exact(dest, |buf| unsafe {
-            libc::syscall(libc::SYS_getrandom, buf.as_mut_ptr(), buf.len(), 0) as libc::ssize_t
+            getrandom(buf.as_mut_ptr() as *mut libc::c_void, buf.len(), 0)
         })
     } else {
         use_file::getrandom_inner(dest)
     }
 }
 
 fn is_getrandom_available() -> bool {
-    let res = unsafe { libc::syscall(libc::SYS_getrandom, 0, 0, libc::GRND_NONBLOCK) };
+    let res = unsafe { getrandom(0 as *mut libc::c_void, 0, libc::GRND_NONBLOCK) };
     if res < 0 {
         match last_os_error().raw_os_error() {
             Some(libc::ENOSYS) => false, // No kernel support
@@ -34,3 +34,9 @@ fn is_getrandom_available() -> bool {
         true
     }
 }
+
+unsafe fn getrandom(buf: *mut libc::c_void,
+                    buflen: libc::size_t,
+                    flags: libc::c_uint) -> libc::ssize_t {
+    libc::syscall(libc::SYS_getrandom, buf, buflen, flags) as libc::ssize_t
+}