Improve robustness of the Hermit backend and sys_fill_exact

Author: newpavlov

src/error.rs

@@ -35,6 +35,8 @@ impl Error {
     pub const UNSUPPORTED: Error = internal_error(0);
     /// The platform-specific `errno` returned a non-positive value.
     pub const ERRNO_NOT_POSITIVE: Error = internal_error(1);
+    /// Encountered an unexpected situation which should not happen in practice.
+    pub const UNEXPECTED: Error = internal_error(2);
     /// Call to iOS [`SecRandomCopyBytes`](https://developer.apple.com/documentation/security/1399291-secrandomcopybytes) failed.
     pub const IOS_SEC_RANDOM: Error = internal_error(3);
     /// Call to Windows [`RtlGenRandom`](https://docs.microsoft.com/en-us/windows/win32/api/ntsecapi/nf-ntsecapi-rtlgenrandom) failed.

src/hermit.rs

@@ -1,5 +1,5 @@
 use crate::Error;
-use core::{cmp::min, mem::MaybeUninit, num::NonZeroU32};
+use core::{convert::TryInto, mem::MaybeUninit, num::NonZeroU32};
 
 extern "C" {
     fn sys_read_entropy(buffer: *mut u8, length: usize, flags: u32) -> isize;
@@ -8,14 +8,19 @@ 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 {
-            // 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());
+        if res > 0 {
+            dest = dest.get_mut(res as usize..).ok_or(Error::UNEXPECTED)?;
+        } 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)
+                .try_into()
+                .ok()
+                .and_then(NonZeroU32::new)
+                .map(Into::into)
+                .unwrap_or(Error::UNEXPECTED);
+            return Err(err);
         }
-        let len = min(res as usize, dest.len());
-        dest = &mut dest[len..];
     }
     Ok(())
 }

src/util_libc.rs

@@ -8,7 +8,6 @@
 #![allow(dead_code)]
 use crate::Error;
 use core::{
-    cmp::min,
     mem::MaybeUninit,
     num::NonZeroU32,
     ptr::NonNull,
@@ -68,19 +67,20 @@ pub fn sys_fill_exact(
     mut buf: &mut [MaybeUninit<u8>],
     sys_fill: impl Fn(&mut [MaybeUninit<u8>]) -> libc::ssize_t,
 ) -> Result<(), Error> {
+    use core::cmp::Ordering::*;
+
     while !buf.is_empty() {
         let res = sys_fill(buf);
-        if res < 0 {
-            let err = last_os_error();
-            // We should try again if the call was interrupted.
-            if err.raw_os_error() != Some(libc::EINTR) {
-                return Err(err);
+        match res.cmp(&0) {
+            Greater => buf = buf.get_mut(res as usize..).ok_or(Error::UNEXPECTED)?,
+            Less => {
+                let err = last_os_error();
+                // We should try again if the call was interrupted.
+                if err.raw_os_error() != Some(libc::EINTR) {
+                    return Err(err);
+                }
             }
-        } else {
-            // We don't check for EOF (ret = 0) as the data we are reading
-            // should be an infinite stream of random bytes.
-            let len = min(res as usize, buf.len());
-            buf = &mut buf[len..];
+            Equal => return Err(Error::UNEXPECTED),
         }
     }
     Ok(())