use_file.rs

//! Implementations that just need to read from a file
use crate::{
    util_libc::{open_readonly, sys_fill_exact},
    Error,
};
use core::{
    cell::UnsafeCell,
    ffi::c_void,
    mem::MaybeUninit,
    sync::atomic::{AtomicUsize, Ordering::Relaxed},
};

/// For all platforms, we use `/dev/urandom` rather than `/dev/random`.
/// For more information see the linked man pages in lib.rs.
///   - On Linux, "/dev/urandom is preferred and sufficient in all use cases".
///   - On Redox, only /dev/urandom is provided.
///   - On AIX, /dev/urandom will "provide cryptographically secure output".
///   - On Haiku and QNX Neutrino they are identical.
const FILE_PATH: &[u8] = b"/dev/urandom\0";
const FD_UNINIT: usize = usize::max_value();

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

// 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(FD_UNINIT);
    fn get_fd() -> Option<libc::c_int> {
        match FD.load(Relaxed) {
            FD_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 = open_readonly(FILE_PATH)?;
    // The fd always fits in a usize without conflicting with FD_UNINIT.
    debug_assert!(fd >= 0 && (fd as usize) < FD_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> {
    // Read a byte from /dev/random to make sure it is ok to read from
    // /dev/urandom. reading a byte instead of polling is more compatible with
    // sandboxes that disallow `poll()` but which allow reading /dev/random,
    // e.g. sandboxes that assume that `poll()` is for network I/O. This way,
    // fewer applications will have to insert pre-sandbox-initialization logic.
    // Often (blocking) file I/O is not allowed in such early phases of an
    // application for performance and/or security reasons.
    //
    // It is hard to write a sandbox policy to support `libc::poll()` because
    // it may be invoked as `SYS_POLL`, `SYS_PPOLL`, or even `SYS_SELECT`,
    // depending on the libc implementation (e.g. glibc vs musl), libc version,
    // potentially the kernel version at runtime, and/or the target
    // architecture.
    //
    // BoringSSL and libstd don't try to protect against insecure output from
    // `/dev/urandom'; they don't open `/dev/random` at all.
    //
    // OpenSSL uses `libc::select()` unless the `dev/random` file descriptor
    // is too large; if it is too large then it does what we do here.

    let fd = open_readonly(b"/dev/random\0")?;
    let _guard = DropGuard(|| unsafe {
        libc::close(fd);
    });
    let mut dummy: [MaybeUninit<u8>; 1] = [MaybeUninit::uninit()];
    sys_fill_exact(&mut dummy, |buf| unsafe {
        libc::read(fd, buf.as_mut_ptr().cast::<c_void>(), buf.len())
    })
}

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()
    }
}