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// Copyright 2018 Developers of the Rand project.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//! Implementations that just need to read from a file
use crate::{
util::LazyUsize,
util_libc::{open_readonly, sys_fill_exact},
Error,
};
use core::{
cell::UnsafeCell,
mem::MaybeUninit,
sync::atomic::{AtomicUsize, Ordering::Relaxed},
};
// We prefer using /dev/urandom and only use /dev/random if the OS
// documentation indicates that /dev/urandom is insecure.
// On Solaris/Illumos, see src/solaris_illumos.rs
// On Dragonfly, Haiku, macOS, and QNX Neutrino the devices are identical.
#[cfg(any(target_os = "solaris", target_os = "illumos"))]
const FILE_PATH: &str = "/dev/random\0";
#[cfg(any(
target_os = "aix",
target_os = "android",
target_os = "linux",
target_os = "redox",
target_os = "dragonfly",
target_os = "haiku",
target_os = "macos",
target_os = "nto",
))]
const FILE_PATH: &str = "/dev/urandom\0";
pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
let fd = get_rng_fd()?;
sys_fill_exact(dest, |buf| unsafe {
libc::read(fd, buf.as_mut_ptr() as *mut libc::c_void, buf.len())
})
}
// Returns the file descriptor for the device file used to retrieve random
// bytes. The file will be opened exactly once. All subsequent calls will
// return the same file descriptor. This file descriptor is never closed.
fn get_rng_fd() -> Result<libc::c_int, Error> {
static FD: AtomicUsize = AtomicUsize::new(LazyUsize::UNINIT);
fn get_fd() -> Option<libc::c_int> {
match FD.load(Relaxed) {
LazyUsize::UNINIT => None,
val => Some(val as libc::c_int),
}
}
// Use double-checked locking to avoid acquiring the lock if possible.
if let Some(fd) = get_fd() {
return Ok(fd);
}
// SAFETY: We use the mutex only in this method, and we always unlock it
// before returning, making sure we don't violate the pthread_mutex_t API.
static MUTEX: Mutex = Mutex::new();
unsafe { MUTEX.lock() };
let _guard = DropGuard(|| unsafe { MUTEX.unlock() });
if let Some(fd) = get_fd() {
return Ok(fd);
}
// On Linux, /dev/urandom might return insecure values.
#[cfg(any(target_os = "android", target_os = "linux"))]
wait_until_rng_ready()?;
let fd = unsafe { open_readonly(FILE_PATH)? };
// The fd always fits in a usize without conflicting with UNINIT.
debug_assert!(fd >= 0 && (fd as usize) < LazyUsize::UNINIT);
FD.store(fd as usize, Relaxed);
Ok(fd)
}
// Succeeds once /dev/urandom is safe to read from
#[cfg(any(target_os = "android", target_os = "linux"))]
fn wait_until_rng_ready() -> Result<(), Error> {
// Poll /dev/random to make sure it is ok to read from /dev/urandom.
let fd = unsafe { open_readonly("/dev/random\0")? };
let mut pfd = libc::pollfd {
fd,
events: libc::POLLIN,
revents: 0,
};
let _guard = DropGuard(|| unsafe {
libc::close(fd);
});
loop {
// A negative timeout means an infinite timeout.
let res = unsafe { libc::poll(&mut pfd, 1, -1) };
if res >= 0 {
debug_assert_eq!(res, 1); // We only used one fd, and cannot timeout.
return Ok(());
}
let err = crate::util_libc::last_os_error();
match err.raw_os_error() {
Some(libc::EINTR) | Some(libc::EAGAIN) => continue,
_ => return Err(err),
}
}
}
struct Mutex(UnsafeCell<libc::pthread_mutex_t>);
impl Mutex {
const fn new() -> Self {
Self(UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER))
}
unsafe fn lock(&self) {
let r = libc::pthread_mutex_lock(self.0.get());
debug_assert_eq!(r, 0);
}
unsafe fn unlock(&self) {
let r = libc::pthread_mutex_unlock(self.0.get());
debug_assert_eq!(r, 0);
}
}
unsafe impl Sync for Mutex {}
struct DropGuard<F: FnMut()>(F);
impl<F: FnMut()> Drop for DropGuard<F> {
fn drop(&mut self) {
self.0()
}
}
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