121 lines
3.7 KiB
Rust
121 lines
3.7 KiB
Rust
//! Implementations that just need to read from a file
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use crate::{
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util_libc::{open_readonly, sys_fill_exact},
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Error,
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};
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use core::{
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cell::UnsafeCell,
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mem::MaybeUninit,
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sync::atomic::{AtomicUsize, Ordering::Relaxed},
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};
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/// For all platforms, we use `/dev/urandom` rather than `/dev/random`.
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/// For more information see the linked man pages in lib.rs.
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/// - On Linux, "/dev/urandom is preferred and sufficient in all use cases".
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/// - On Redox, only /dev/urandom is provided.
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/// - On AIX, /dev/urandom will "provide cryptographically secure output".
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/// - On Haiku and QNX Neutrino they are identical.
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const FILE_PATH: &str = "/dev/urandom\0";
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const FD_UNINIT: usize = usize::max_value();
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pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
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let fd = get_rng_fd()?;
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sys_fill_exact(dest, |buf| unsafe {
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libc::read(fd, buf.as_mut_ptr() as *mut libc::c_void, buf.len())
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})
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}
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// Returns the file descriptor for the device file used to retrieve random
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// bytes. The file will be opened exactly once. All subsequent calls will
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// return the same file descriptor. This file descriptor is never closed.
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fn get_rng_fd() -> Result<libc::c_int, Error> {
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static FD: AtomicUsize = AtomicUsize::new(FD_UNINIT);
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fn get_fd() -> Option<libc::c_int> {
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match FD.load(Relaxed) {
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FD_UNINIT => None,
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val => Some(val as libc::c_int),
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}
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}
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// Use double-checked locking to avoid acquiring the lock if possible.
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if let Some(fd) = get_fd() {
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return Ok(fd);
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}
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// SAFETY: We use the mutex only in this method, and we always unlock it
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// before returning, making sure we don't violate the pthread_mutex_t API.
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static MUTEX: Mutex = Mutex::new();
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unsafe { MUTEX.lock() };
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let _guard = DropGuard(|| unsafe { MUTEX.unlock() });
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if let Some(fd) = get_fd() {
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return Ok(fd);
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}
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// On Linux, /dev/urandom might return insecure values.
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#[cfg(any(target_os = "android", target_os = "linux"))]
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wait_until_rng_ready()?;
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let fd = unsafe { open_readonly(FILE_PATH)? };
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// The fd always fits in a usize without conflicting with FD_UNINIT.
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debug_assert!(fd >= 0 && (fd as usize) < FD_UNINIT);
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FD.store(fd as usize, Relaxed);
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Ok(fd)
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}
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// Succeeds once /dev/urandom is safe to read from
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#[cfg(any(target_os = "android", target_os = "linux"))]
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fn wait_until_rng_ready() -> Result<(), Error> {
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// Poll /dev/random to make sure it is ok to read from /dev/urandom.
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let fd = unsafe { open_readonly("/dev/random\0")? };
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let mut pfd = libc::pollfd {
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fd,
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events: libc::POLLIN,
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revents: 0,
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};
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let _guard = DropGuard(|| unsafe {
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libc::close(fd);
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});
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loop {
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// A negative timeout means an infinite timeout.
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let res = unsafe { libc::poll(&mut pfd, 1, -1) };
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if res >= 0 {
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debug_assert_eq!(res, 1); // We only used one fd, and cannot timeout.
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return Ok(());
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}
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let err = crate::util_libc::last_os_error();
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match err.raw_os_error() {
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Some(libc::EINTR) | Some(libc::EAGAIN) => continue,
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_ => return Err(err),
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}
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}
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}
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struct Mutex(UnsafeCell<libc::pthread_mutex_t>);
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impl Mutex {
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const fn new() -> Self {
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Self(UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER))
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}
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unsafe fn lock(&self) {
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let r = libc::pthread_mutex_lock(self.0.get());
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debug_assert_eq!(r, 0);
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}
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unsafe fn unlock(&self) {
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let r = libc::pthread_mutex_unlock(self.0.get());
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debug_assert_eq!(r, 0);
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}
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}
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unsafe impl Sync for Mutex {}
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struct DropGuard<F: FnMut()>(F);
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impl<F: FnMut()> Drop for DropGuard<F> {
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fn drop(&mut self) {
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self.0()
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}
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}
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