use_file: Remove use of spin-locks

Don't spin when polling /dev/random. We also remove the use of spin
locks when opening the persistent fd for platforms that require it.

For both these cases, we can just use the pthread lock/unlock methods
in libc. This includes adding Mutex and DropGuard abstractions.

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/use_file.rs

@@ -7,8 +7,11 @@
 // except according to those terms.
 
 //! Implementations that just need to read from a file
-use crate::util_libc::{last_os_error, open_readonly, sys_fill_exact, LazyFd};
+use crate::util::LazyUsize;
+use crate::util_libc::{open_readonly, sys_fill_exact};
 use crate::Error;
+use core::cell::UnsafeCell;
+use core::sync::atomic::{AtomicUsize, Ordering::Relaxed};
 
 #[cfg(target_os = "redox")]
 const FILE_PATH: &str = "rand:\0";
@@ -21,10 +24,11 @@ const FILE_PATH: &str = "rand:\0";
     target_os = "illumos"
 ))]
 const FILE_PATH: &str = "/dev/random\0";
+#[cfg(any(target_os = "android", target_os = "linux"))]
+const FILE_PATH: &str = "/dev/urandom\0";
 
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
-    static FD: LazyFd = LazyFd::new();
-    let fd = FD.init(init_file).ok_or_else(last_os_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") {
@@ -38,36 +42,96 @@ pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
     Ok(())
 }
 
-cfg_if! {
-    if #[cfg(any(target_os = "android", target_os = "linux"))] {
-        fn init_file() -> Option<libc::c_int> {
-            // Poll /dev/random to make sure it is ok to read from /dev/urandom.
-            let mut pfd = libc::pollfd {
-                fd: unsafe { open_readonly("/dev/random\0")? },
-                events: libc::POLLIN,
-                revents: 0,
-            };
-
-            let ret = loop {
-                // A negative timeout means an infinite timeout.
-                let res = unsafe { libc::poll(&mut pfd, 1, -1) };
-                if res == 1 {
-                    break unsafe { open_readonly("/dev/urandom\0") };
-                } else if res < 0 {
-                    let e = last_os_error().raw_os_error();
-                    if e == Some(libc::EINTR) || e == Some(libc::EAGAIN) {
-                        continue;
-                    }
-                }
-                // We either hard failed, or poll() returned the wrong pfd.
-                break None;
-            };
-            unsafe { libc::close(pfd.fd) };
-            ret
+// Returns the file descriptor for the device file used to retrieve random
+// bytes. The file will be opened exactly once. All successful 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),
         }
-    } else {
-        fn init_file() -> Option<libc::c_int> {
-            unsafe { open_readonly(FILE_PATH) }
+    }
+
+    // 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 {
+            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()
     }
 }