Add Window7-specific implementation

Author: josephlr

src/lib.rs

@@ -323,6 +323,8 @@ cfg_if! {
         #[path = "solid.rs"] mod imp;
     } else if #[cfg(target_os = "espidf")] {
         #[path = "espidf.rs"] mod imp;
+    } else if #[cfg(all(windows, target_vendor = "win7"))] {
+        #[path = "windows7.rs"] mod imp;
     } else if #[cfg(windows)] {
         #[path = "windows.rs"] mod imp;
     } else if #[cfg(all(target_arch = "x86_64", target_env = "sgx"))] {

src/windows7.rs

@@ -0,0 +1,37 @@
+//! Implementation for Windows 7 and 8
+//!
+//! For targets where we cannot use ProcessPrng (added in Windows 10), we use
+//! RtlGenRandom. See windows.rs for a more detailed discussion of the Windows
+//! RNG APIs (and why we don't use BCryptGenRandom). On versions prior to
+//! Windows 10, this implementation works, while on Windows 10 and later, this
+//! is a thin wrapper around ProcessPrng.
+//!
+//! This implementation will not work on UWP targets, but those targets require
+//! Windows 10 regardless, so can use the ProcessPrng implementation.
+use crate::Error;
+use core::{ffi::c_void, mem::MaybeUninit};
+
+// This code is based on that produced by windows-bindgen with the APIs:
+//   Windows.Win32.Foundation.TRUE
+//   Windows.Win32.Security.Authentication.Identity.RtlGenRandom
+// but we avoid using windows-targets as it doesn't support older Windows.
+#[link(name = "advapi32")]
+extern "system" {
+    #[link_name = "SystemFunction036"]
+    fn RtlGenRandom(randombuffer: *mut c_void, randombufferlength: u32) -> BOOLEAN;
+}
+#[repr(transparent)]
+#[derive(PartialEq, Eq)]
+pub struct BOOLEAN(pub u8);
+pub const TRUE: BOOLEAN = BOOLEAN(1u8);
+
+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) {
+        let ret = unsafe { RtlGenRandom(chunk.as_mut_ptr().cast::<c_void>(), chunk.len() as u32) };
+        if ret != TRUE {
+            return Err(Error::WINDOWS_RTL_GEN_RANDOM);
+        }
+    }
+    Ok(())
+}