getrandom/
use_file.rs

1//! Implementations that just need to read from a file
2use crate::{
3    util_libc::{open_readonly, sys_fill_exact},
4    Error,
5};
6use core::{
7    cell::UnsafeCell,
8    mem::MaybeUninit,
9    sync::atomic::{AtomicUsize, Ordering::Relaxed},
10};
11
12/// For all platforms, we use `/dev/urandom` rather than `/dev/random`.
13/// For more information see the linked man pages in lib.rs.
14///   - On Linux, "/dev/urandom is preferred and sufficient in all use cases".
15///   - On Redox, only /dev/urandom is provided.
16///   - On AIX, /dev/urandom will "provide cryptographically secure output".
17///   - On Haiku and QNX Neutrino they are identical.
18const FILE_PATH: &str = "/dev/urandom\0";
19const FD_UNINIT: usize = usize::max_value();
20
21pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
22    let fd = get_rng_fd()?;
23    sys_fill_exact(dest, |buf| unsafe {
24        libc::read(fd, buf.as_mut_ptr() as *mut libc::c_void, buf.len())
25    })
26}
27
28// Returns the file descriptor for the device file used to retrieve random
29// bytes. The file will be opened exactly once. All subsequent calls will
30// return the same file descriptor. This file descriptor is never closed.
31fn get_rng_fd() -> Result<libc::c_int, Error> {
32    static FD: AtomicUsize = AtomicUsize::new(FD_UNINIT);
33    fn get_fd() -> Option<libc::c_int> {
34        match FD.load(Relaxed) {
35            FD_UNINIT => None,
36            val => Some(val as libc::c_int),
37        }
38    }
39
40    // Use double-checked locking to avoid acquiring the lock if possible.
41    if let Some(fd) = get_fd() {
42        return Ok(fd);
43    }
44
45    // SAFETY: We use the mutex only in this method, and we always unlock it
46    // before returning, making sure we don't violate the pthread_mutex_t API.
47    static MUTEX: Mutex = Mutex::new();
48    unsafe { MUTEX.lock() };
49    let _guard = DropGuard(|| unsafe { MUTEX.unlock() });
50
51    if let Some(fd) = get_fd() {
52        return Ok(fd);
53    }
54
55    // On Linux, /dev/urandom might return insecure values.
56    #[cfg(any(target_os = "android", target_os = "linux"))]
57    wait_until_rng_ready()?;
58
59    let fd = unsafe { open_readonly(FILE_PATH)? };
60    // The fd always fits in a usize without conflicting with FD_UNINIT.
61    debug_assert!(fd >= 0 && (fd as usize) < FD_UNINIT);
62    FD.store(fd as usize, Relaxed);
63
64    Ok(fd)
65}
66
67// Succeeds once /dev/urandom is safe to read from
68#[cfg(any(target_os = "android", target_os = "linux"))]
69fn wait_until_rng_ready() -> Result<(), Error> {
70    // Poll /dev/random to make sure it is ok to read from /dev/urandom.
71    let fd = unsafe { open_readonly("/dev/random\0")? };
72    let mut pfd = libc::pollfd {
73        fd,
74        events: libc::POLLIN,
75        revents: 0,
76    };
77    let _guard = DropGuard(|| unsafe {
78        libc::close(fd);
79    });
80
81    loop {
82        // A negative timeout means an infinite timeout.
83        let res = unsafe { libc::poll(&mut pfd, 1, -1) };
84        if res >= 0 {
85            debug_assert_eq!(res, 1); // We only used one fd, and cannot timeout.
86            return Ok(());
87        }
88        let err = crate::util_libc::last_os_error();
89        match err.raw_os_error() {
90            Some(libc::EINTR) | Some(libc::EAGAIN) => continue,
91            _ => return Err(err),
92        }
93    }
94}
95
96struct Mutex(UnsafeCell<libc::pthread_mutex_t>);
97
98impl Mutex {
99    const fn new() -> Self {
100        Self(UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER))
101    }
102    unsafe fn lock(&self) {
103        let r = libc::pthread_mutex_lock(self.0.get());
104        debug_assert_eq!(r, 0);
105    }
106    unsafe fn unlock(&self) {
107        let r = libc::pthread_mutex_unlock(self.0.get());
108        debug_assert_eq!(r, 0);
109    }
110}
111
112unsafe impl Sync for Mutex {}
113
114struct DropGuard<F: FnMut()>(F);
115
116impl<F: FnMut()> Drop for DropGuard<F> {
117    fn drop(&mut self) {
118        self.0()
119    }
120}
getrandom/use_file.rs

getrandom/
use_file.rs
