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//! Implementation for Windows
use crate::Error;
use core::{ffi::c_void, mem::MaybeUninit, num::NonZeroU32, ptr};
const BCRYPT_USE_SYSTEM_PREFERRED_RNG: u32 = 0x00000002;
#[link(name = "bcrypt")]
extern "system" {
fn BCryptGenRandom(
hAlgorithm: *mut c_void,
pBuffer: *mut u8,
cbBuffer: u32,
dwFlags: u32,
) -> u32;
}
// Forbidden when targetting UWP
#[cfg(not(target_vendor = "uwp"))]
#[link(name = "advapi32")]
extern "system" {
#[link_name = "SystemFunction036"]
fn RtlGenRandom(RandomBuffer: *mut c_void, RandomBufferLength: u32) -> u8;
}
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 {
// Failed. Try RtlGenRandom as a fallback.
#[cfg(not(target_vendor = "uwp"))]
{
let ret =
unsafe { RtlGenRandom(chunk.as_mut_ptr() as *mut c_void, chunk.len() as u32) };
if ret != 0 {
continue;
}
}
// 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));
}
}
Ok(())
}