1//! Implementations that just need to read from a file
2use crate::Error;
3use core::{
4 ffi::c_void,
5 mem::MaybeUninit,
6 sync::atomic::{AtomicI32, Ordering},
7};
8
9#[cfg(not(any(target_os = "android", target_os = "linux")))]
10pub use crate::util::{inner_u32, inner_u64};
11
12#[path = "../util_libc.rs"]
13pub(super) mod util_libc;
14
15/// For all platforms, we use `/dev/urandom` rather than `/dev/random`.
16/// For more information see the linked man pages in lib.rs.
17/// - On Linux, "/dev/urandom is preferred and sufficient in all use cases".
18/// - On Redox, only /dev/urandom is provided.
19/// - On AIX, /dev/urandom will "provide cryptographically secure output".
20/// - On Haiku and QNX Neutrino they are identical.
21const FILE_PATH: &[u8] = b"/dev/urandom\0";
22
23// File descriptor is a "nonnegative integer", so we can safely use negative sentinel values.
24const FD_UNINIT: libc::c_int = -1;
25const FD_ONGOING_INIT: libc::c_int = -2;
26
27// In theory `libc::c_int` could be something other than `i32`, but for the
28// targets we currently support that use `use_file`, it is always `i32`.
29// If/when we add support for a target where that isn't the case, we may
30// need to use a different atomic type or make other accomodations. The
31// compiler will let us know if/when that is the case, because the
32// `FD.store(fd)` would fail to compile.
33//
34// The opening of the file, by libc/libstd/etc. may write some unknown
35// state into in-process memory. (Such state may include some sanitizer
36// bookkeeping, or we might be operating in a unikernal-like environment
37// where all the "kernel" file descriptor bookkeeping is done in our
38// process.) `get_fd_locked` stores into FD using `Ordering::Release` to
39// ensure any such state is synchronized. `get_fd` loads from `FD` with
40// `Ordering::Acquire` to synchronize with it.
41static FD: AtomicI32 = AtomicI32::new(FD_UNINIT);
42
43pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
44 let mut fd: i32 = FD.load(order:Ordering::Acquire);
45 if fd == FD_UNINIT || fd == FD_ONGOING_INIT {
46 fd = open_or_wait()?;
47 }
48 util_libc::sys_fill_exact(buf:dest, |buf: &mut [MaybeUninit]| unsafe {
49 libc::read(fd, buf.as_mut_ptr().cast::<c_void>(), count:buf.len())
50 })
51}
52
53/// Open a file in read-only mode.
54///
55/// # Panics
56/// If `path` does not contain any zeros.
57// TODO: Move `path` to `CStr` and use `CStr::from_bytes_until_nul` (MSRV 1.69)
58// or C-string literals (MSRV 1.77) for statics
59fn open_readonly(path: &[u8]) -> Result<libc::c_int, Error> {
60 assert!(path.iter().any(|&b| b == 0));
61 loop {
62 let fd: i32 = unsafe {
63 libc::open(
64 path.as_ptr().cast::<libc::c_char>(),
65 oflag:libc::O_RDONLY | libc::O_CLOEXEC,
66 )
67 };
68 if fd >= 0 {
69 return Ok(fd);
70 }
71 let err: Error = util_libc::last_os_error();
72 // We should try again if open() was interrupted.
73 if err.raw_os_error() != Some(libc::EINTR) {
74 return Err(err);
75 }
76 }
77}
78
79#[cold]
80fn open_or_wait() -> Result<libc::c_int, Error> {
81 loop {
82 match FD.load(Ordering::Acquire) {
83 FD_UNINIT => {
84 let res = FD.compare_exchange_weak(
85 FD_UNINIT,
86 FD_ONGOING_INIT,
87 Ordering::AcqRel,
88 Ordering::Relaxed,
89 );
90 if res.is_ok() {
91 break;
92 }
93 }
94 FD_ONGOING_INIT => sync::wait(),
95 fd => return Ok(fd),
96 }
97 }
98
99 let res = open_fd();
100 let val = match res {
101 Ok(fd) => fd,
102 Err(_) => FD_UNINIT,
103 };
104 FD.store(val, Ordering::Release);
105
106 // On non-Linux targets `wait` is just 1 ms sleep,
107 // so we don't need any explicit wake up in addition
108 // to updating value of `FD`.
109 #[cfg(any(target_os = "android", target_os = "linux"))]
110 sync::wake();
111
112 res
113}
114
115fn open_fd() -> Result<libc::c_int, Error> {
116 #[cfg(any(target_os = "android", target_os = "linux"))]
117 sync::wait_until_rng_ready()?;
118 let fd: i32 = open_readonly(FILE_PATH)?;
119 debug_assert!(fd >= 0);
120 Ok(fd)
121}
122
123#[cfg(not(any(target_os = "android", target_os = "linux")))]
124mod sync {
125 /// Sleep 1 ms before checking `FD` again.
126 ///
127 /// On non-Linux targets the critical section only opens file,
128 /// which should not block, so in the unlikely contended case,
129 /// we can sleep-wait for the opening operation to finish.
130 pub(super) fn wait() {
131 let rqtp = libc::timespec {
132 tv_sec: 0,
133 tv_nsec: 1_000_000,
134 };
135 let mut rmtp = libc::timespec {
136 tv_sec: 0,
137 tv_nsec: 0,
138 };
139 // We do not care if sleep gets interrupted, so the return value is ignored
140 unsafe {
141 libc::nanosleep(&rqtp, &mut rmtp);
142 }
143 }
144}
145
146#[cfg(any(target_os = "android", target_os = "linux"))]
147mod sync {
148 use super::{open_readonly, util_libc::last_os_error, Error, FD, FD_ONGOING_INIT};
149
150 /// Wait for atomic `FD` to change value from `FD_ONGOING_INIT` to something else.
151 ///
152 /// Futex syscall with `FUTEX_WAIT` op puts the current thread to sleep
153 /// until futex syscall with `FUTEX_WAKE` op gets executed for `FD`.
154 ///
155 /// For more information read: https://www.man7.org/linux/man-pages/man2/futex.2.html
156 pub(super) fn wait() {
157 let op = libc::FUTEX_WAIT | libc::FUTEX_PRIVATE_FLAG;
158 let timeout_ptr = core::ptr::null::<libc::timespec>();
159 let ret = unsafe { libc::syscall(libc::SYS_futex, &FD, op, FD_ONGOING_INIT, timeout_ptr) };
160 // FUTEX_WAIT should return either 0 or EAGAIN error
161 debug_assert!({
162 match ret {
163 0 => true,
164 -1 => last_os_error().raw_os_error() == Some(libc::EAGAIN),
165 _ => false,
166 }
167 });
168 }
169
170 /// Wake up all threads which wait for value of atomic `FD` to change.
171 pub(super) fn wake() {
172 let op = libc::FUTEX_WAKE | libc::FUTEX_PRIVATE_FLAG;
173 let ret = unsafe { libc::syscall(libc::SYS_futex, &FD, op, libc::INT_MAX) };
174 debug_assert!(ret >= 0);
175 }
176
177 // Polls /dev/random to make sure it is ok to read from /dev/urandom.
178 //
179 // Polling avoids draining the estimated entropy from /dev/random;
180 // short-lived processes reading even a single byte from /dev/random could
181 // be problematic if they are being executed faster than entropy is being
182 // collected.
183 //
184 // OTOH, reading a byte instead of polling is more compatible with
185 // sandboxes that disallow `poll()` but which allow reading /dev/random,
186 // e.g. sandboxes that assume that `poll()` is for network I/O. This way,
187 // fewer applications will have to insert pre-sandbox-initialization logic.
188 // Often (blocking) file I/O is not allowed in such early phases of an
189 // application for performance and/or security reasons.
190 //
191 // It is hard to write a sandbox policy to support `libc::poll()` because
192 // it may invoke the `poll`, `ppoll`, `ppoll_time64` (since Linux 5.1, with
193 // newer versions of glibc), and/or (rarely, and probably only on ancient
194 // systems) `select`. depending on the libc implementation (e.g. glibc vs
195 // musl), libc version, potentially the kernel version at runtime, and/or
196 // the target architecture.
197 //
198 // BoringSSL and libstd don't try to protect against insecure output from
199 // `/dev/urandom'; they don't open `/dev/random` at all.
200 //
201 // OpenSSL uses `libc::select()` unless the `dev/random` file descriptor
202 // is too large; if it is too large then it does what we do here.
203 //
204 // libsodium uses `libc::poll` similarly to this.
205 pub(super) fn wait_until_rng_ready() -> Result<(), Error> {
206 let fd = open_readonly(b"/dev/random\0")?;
207 let mut pfd = libc::pollfd {
208 fd,
209 events: libc::POLLIN,
210 revents: 0,
211 };
212
213 let res = loop {
214 // A negative timeout means an infinite timeout.
215 let res = unsafe { libc::poll(&mut pfd, 1, -1) };
216 if res >= 0 {
217 // We only used one fd, and cannot timeout.
218 debug_assert_eq!(res, 1);
219 break Ok(());
220 }
221 let err = last_os_error();
222 // Assuming that `poll` is called correctly,
223 // on Linux it can return only EINTR and ENOMEM errors.
224 match err.raw_os_error() {
225 Some(libc::EINTR) => continue,
226 _ => break Err(err),
227 }
228 };
229 unsafe { libc::close(fd) };
230 res
231 }
232}
233