Hermit: use sys_read_entropy syscall (#333)

* Hermit: use sys_read_entropy syscall

* Add doc link

src/hermit.rs

@@ -0,0 +1,21 @@
+use crate::Error;
+use core::{cmp::min, mem::MaybeUninit, num::NonZeroU32};
+
+extern "C" {
+    fn sys_read_entropy(buffer: *mut u8, length: usize, flags: u32) -> isize;
+}
+
+pub fn getrandom_inner(mut dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
+    while !dest.is_empty() {
+        let res = unsafe { sys_read_entropy(dest.as_mut_ptr() as *mut u8, dest.len(), 0) };
+        if res < 0 {
+            // SAFETY: all Hermit error codes use i32 under the hood:
+            // https://github.com/hermitcore/libhermit-rs/blob/master/src/errno.rs
+            let code = unsafe { NonZeroU32::new_unchecked((-res) as u32) };
+            return Err(code.into());
+        }
+        let len = min(res as usize, dest.len());
+        dest = &mut dest[len..];
+    }
+    Ok(())
+}

src/lib.rs

@@ -24,7 +24,7 @@
 //! | Fuchsia OS        | `*‑fuchsia`        | [`cprng_draw`]
 //! | Redox             | `*‑redox`          | `/dev/urandom`
 //! | Haiku             | `*‑haiku`          | `/dev/urandom` (identical to `/dev/random`)
-//! | Hermit            | `x86_64-*-hermit`  | [`RDRAND`]
+//! | Hermit            | `*-hermit`         | [`sys_read_entropy`]
 //! | SGX               | `x86_64‑*‑sgx`     | [`RDRAND`]
 //! | VxWorks           | `*‑wrs‑vxworks‑*`  | `randABytes` after checking entropy pool initialization with `randSecure`
 //! | ESP-IDF           | `*‑espidf`         | [`esp_fill_random`]
@@ -179,6 +179,7 @@
 //! [`module`]: https://rustwasm.github.io/wasm-bindgen/reference/attributes/on-js-imports/module.html
 //! [CommonJS modules]: https://nodejs.org/api/modules.html
 //! [ES modules]: https://nodejs.org/api/esm.html
+//! [`sys_read_entropy`]: https://hermitcore.github.io/libhermit-rs/hermit/fn.sys_read_entropy.html
 
 #![doc(
     html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk.png",
@@ -245,8 +246,8 @@ cfg_if! {
         #[path = "openbsd.rs"] mod imp;
     } else if #[cfg(all(target_arch = "wasm32", target_os = "wasi"))] {
         #[path = "wasi.rs"] mod imp;
-    } else if #[cfg(all(target_arch = "x86_64", target_os = "hermit"))] {
-        #[path = "rdrand.rs"] mod imp;
+    } else if #[cfg(target_os = "hermit")] {
+        #[path = "hermit.rs"] mod imp;
     } else if #[cfg(target_os = "vxworks")] {
         mod util_libc;
         #[path = "vxworks.rs"] mod imp;