getrandom/
use_file.rs

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1// Copyright 2018 Developers of the Rand project.
2//
3// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
4// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
5// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
6// option. This file may not be copied, modified, or distributed
7// except according to those terms.
8
9//! Implementations that just need to read from a file
10use crate::{
11    util::LazyUsize,
12    util_libc::{open_readonly, sys_fill_exact},
13    Error,
14};
15use core::{
16    cell::UnsafeCell,
17    mem::MaybeUninit,
18    sync::atomic::{AtomicUsize, Ordering::Relaxed},
19};
20
21// We prefer using /dev/urandom and only use /dev/random if the OS
22// documentation indicates that /dev/urandom is insecure.
23// On Solaris/Illumos, see src/solaris_illumos.rs
24// On Dragonfly, Haiku, macOS, and QNX Neutrino the devices are identical.
25#[cfg(any(target_os = "solaris", target_os = "illumos"))]
26const FILE_PATH: &str = "/dev/random\0";
27#[cfg(any(
28    target_os = "aix",
29    target_os = "android",
30    target_os = "linux",
31    target_os = "redox",
32    target_os = "dragonfly",
33    target_os = "haiku",
34    target_os = "macos",
35    target_os = "nto",
36))]
37const FILE_PATH: &str = "/dev/urandom\0";
38
39pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
40    let fd = get_rng_fd()?;
41    sys_fill_exact(dest, |buf| unsafe {
42        libc::read(fd, buf.as_mut_ptr() as *mut libc::c_void, buf.len())
43    })
44}
45
46// Returns the file descriptor for the device file used to retrieve random
47// bytes. The file will be opened exactly once. All subsequent calls will
48// return the same file descriptor. This file descriptor is never closed.
49fn get_rng_fd() -> Result<libc::c_int, Error> {
50    static FD: AtomicUsize = AtomicUsize::new(LazyUsize::UNINIT);
51    fn get_fd() -> Option<libc::c_int> {
52        match FD.load(Relaxed) {
53            LazyUsize::UNINIT => None,
54            val => Some(val as libc::c_int),
55        }
56    }
57
58    // Use double-checked locking to avoid acquiring the lock if possible.
59    if let Some(fd) = get_fd() {
60        return Ok(fd);
61    }
62
63    // SAFETY: We use the mutex only in this method, and we always unlock it
64    // before returning, making sure we don't violate the pthread_mutex_t API.
65    static MUTEX: Mutex = Mutex::new();
66    unsafe { MUTEX.lock() };
67    let _guard = DropGuard(|| unsafe { MUTEX.unlock() });
68
69    if let Some(fd) = get_fd() {
70        return Ok(fd);
71    }
72
73    // On Linux, /dev/urandom might return insecure values.
74    #[cfg(any(target_os = "android", target_os = "linux"))]
75    wait_until_rng_ready()?;
76
77    let fd = unsafe { open_readonly(FILE_PATH)? };
78    // The fd always fits in a usize without conflicting with UNINIT.
79    debug_assert!(fd >= 0 && (fd as usize) < LazyUsize::UNINIT);
80    FD.store(fd as usize, Relaxed);
81
82    Ok(fd)
83}
84
85// Succeeds once /dev/urandom is safe to read from
86#[cfg(any(target_os = "android", target_os = "linux"))]
87fn wait_until_rng_ready() -> Result<(), Error> {
88    // Poll /dev/random to make sure it is ok to read from /dev/urandom.
89    let fd = unsafe { open_readonly("/dev/random\0")? };
90    let mut pfd = libc::pollfd {
91        fd,
92        events: libc::POLLIN,
93        revents: 0,
94    };
95    let _guard = DropGuard(|| unsafe {
96        libc::close(fd);
97    });
98
99    loop {
100        // A negative timeout means an infinite timeout.
101        let res = unsafe { libc::poll(&mut pfd, 1, -1) };
102        if res >= 0 {
103            debug_assert_eq!(res, 1); // We only used one fd, and cannot timeout.
104            return Ok(());
105        }
106        let err = crate::util_libc::last_os_error();
107        match err.raw_os_error() {
108            Some(libc::EINTR) | Some(libc::EAGAIN) => continue,
109            _ => return Err(err),
110        }
111    }
112}
113
114struct Mutex(UnsafeCell<libc::pthread_mutex_t>);
115
116impl Mutex {
117    const fn new() -> Self {
118        Self(UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER))
119    }
120    unsafe fn lock(&self) {
121        let r = libc::pthread_mutex_lock(self.0.get());
122        debug_assert_eq!(r, 0);
123    }
124    unsafe fn unlock(&self) {
125        let r = libc::pthread_mutex_unlock(self.0.get());
126        debug_assert_eq!(r, 0);
127    }
128}
129
130unsafe impl Sync for Mutex {}
131
132struct DropGuard<F: FnMut()>(F);
133
134impl<F: FnMut()> Drop for DropGuard<F> {
135    fn drop(&mut self) {
136        self.0()
137    }
138}
getrandom/use_file.rs

getrandom/
use_file.rs
