Tweak Hermit code

src/hermit.rs

@@ -1,5 +1,10 @@
 use crate::Error;
-use core::{convert::TryInto, mem::MaybeUninit, num::NonZeroU32};
+use core::{mem::MaybeUninit, num::NonZeroU32};
+
+/// Minimum return value which we should get from syscalls in practice,
+/// because Hermit uses positive `i32`s for error codes:
+/// https://github.com/hermitcore/libhermit-rs/blob/main/src/errno.rs
+const MIN_RET_CODE: isize = -(i32::MAX as isize);
 
 extern "C" {
     fn sys_read_entropy(buffer: *mut u8, length: usize, flags: u32) -> isize;
@@ -8,17 +13,14 @@ extern "C" {
 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 {
-            dest = dest.get_mut(res as usize..).ok_or(Error::UNEXPECTED)?;
+        // Positive `isize`s can be safely casted to `usize`
+        if res > 0 && (res as usize) <= dest.len() {
+            dest = &mut dest[res as usize..];
         } else {
-            // We should not get `res` equal to zero or smaller than `-i32::MAX`.
-            // If we get such unexpected value after all, we will return `Error::UNEXPECTED`.
-            let err = res
-                .checked_neg()
-                .and_then(|val| val.try_into().ok())
-                .and_then(NonZeroU32::new)
-                .map(Into::into)
-                .unwrap_or(Error::UNEXPECTED);
+            let err = match res {
+                MIN_RET_CODE..=-1 => NonZeroU32::new(-res as u32).unwrap().into(),
+                _ => Error::UNEXPECTED,
+            };
             return Err(err);
         }
     }