1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139

// 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,
    sync::atomic::{AtomicUsize, Ordering::Relaxed},
};

#[cfg(any(
    target_os = "dragonfly",
    target_os = "emscripten",
    target_os = "haiku",
    target_os = "macos",
    target_os = "solaris",
    target_os = "illumos"
))]
const FILE_PATH: &str = "/dev/random\0";
#[cfg(any(target_os = "android", target_os = "linux", target_os = "redox"))]
const FILE_PATH: &str = "/dev/urandom\0";

pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
    let fd = get_rng_fd()?;
    let read = |buf: &mut [u8]| unsafe { libc::read(fd, buf.as_mut_ptr() as *mut _, buf.len()) };

    if cfg!(target_os = "emscripten") {
        // `Crypto.getRandomValues` documents `dest` should be at most 65536 bytes.
        for chunk in dest.chunks_mut(65536) {
            sys_fill_exact(chunk, read)?;
        }
    } else {
        sys_fill_exact(dest, read)?;
    }
    Ok(())
}

// 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()
    }
}