windows: move BCryptGenRandom to helper function

This _is not_ a functional change. It just makes adding additional
fallback implementations easier in the future.

Note that the try_into().unwrap() is optimized away: https://godbolt.org/z/nbThsqsTE

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/windows.rs

@@ -7,7 +7,7 @@
 // except according to those terms.
 
 use crate::Error;
-use core::{ffi::c_void, mem::MaybeUninit, num::NonZeroU32, ptr};
+use core::{convert::TryInto, ffi::c_void, mem::MaybeUninit, num::NonZeroU32, ptr};
 
 const BCRYPT_USE_SYSTEM_PREFERRED_RNG: u32 = 0x00000002;
 
@@ -21,29 +21,41 @@ extern "system" {
     ) -> u32;
 }
 
+// BCryptGenRandom was introduced in Windows Vista. However, CNG Algorithm
+// Pseudo-handles (specifically BCRYPT_RNG_ALG_HANDLE) weren't introduced
+// until Windows 10, so we cannot use them yet. Note that on older systems
+// these Pseudo-handles are interpreted as pointers, causing crashes if used.
+fn bcrypt_random(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
+    // Will always succeed given the chunking in getrandom_inner().
+    let len: u32 = dest.len().try_into().unwrap();
+    // SAFETY: dest is valid, writable buffer of length len
+    let ret = unsafe {
+        BCryptGenRandom(
+            ptr::null_mut(),
+            dest.as_mut_ptr() as *mut u8,
+            len,
+            BCRYPT_USE_SYSTEM_PREFERRED_RNG,
+        )
+    };
+
+    // NTSTATUS codes use the two highest bits for severity status.
+    if ret >> 30 != 0b11 {
+        return Ok(());
+    }
+    // We zeroize the highest bit, so the error code will reside
+    // inside the range designated for OS codes.
+    let code = ret ^ (1 << 31);
+    // SAFETY: the second highest bit is always equal to one,
+    // so it's impossible to get zero. Unfortunately the type
+    // system does not have a way to express this yet.
+    let code = unsafe { NonZeroU32::new_unchecked(code) };
+    Err(Error::from(code))
+}
+
 pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     // Prevent overflow of u32
     for chunk in dest.chunks_mut(u32::max_value() as usize) {
-        // BCryptGenRandom was introduced in Windows Vista
-        let ret = unsafe {
-            BCryptGenRandom(
-                ptr::null_mut(),
-                chunk.as_mut_ptr() as *mut u8,
-                chunk.len() as u32,
-                BCRYPT_USE_SYSTEM_PREFERRED_RNG,
-            )
-        };
-        // NTSTATUS codes use the two highest bits for severity status.
-        if ret >> 30 == 0b11 {
-            // We zeroize the highest bit, so the error code will reside
-            // inside the range designated for OS codes.
-            let code = ret ^ (1 << 31);
-            // SAFETY: the second highest bit is always equal to one,
-            // so it's impossible to get zero. Unfortunately the type
-            // system does not have a way to express this yet.
-            let code = unsafe { NonZeroU32::new_unchecked(code) };
-            return Err(Error::from(code));
-        }
+        bcrypt_random(chunk)?;
     }
     Ok(())
 }