use_file: Remove use of spin-locks

Remove the general purpose spin-lock from getrandom, and 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. We also do some minor cleanup to better make use of Result
types and DropGuards.

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

src/use_file.rs

@@ -7,8 +7,10 @@
 // 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::{last_os_error, open_readonly, sys_fill_exact};
 use crate::Error;
+use core::sync::atomic::{AtomicUsize, Ordering::Relaxed};
 
 #[cfg(target_os = "redox")]
 const FILE_PATH: &str = "rand:\0";
@@ -21,10 +23,11 @@ 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 +41,83 @@ 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,
-            };
+// Returns the field descriptor for the device file used to retrieve random
+// numbers. 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),
+        }
+    }
+
+    // Use double-checked locking to avoid acquiring the lock if possible.
+    if let Some(fd) = get_fd() {
+        return Ok(fd);
+    }
+
+    // SAFETY: Only a raw pointer is created from MUTEX to pass to libc.
+    static mut MUTEX: libc::pthread_mutex_t = libc::PTHREAD_MUTEX_INITIALIZER;
+    let r = unsafe { libc::pthread_mutex_lock(&mut MUTEX as *mut _) };
+    debug_assert_eq!(r, 0);
+    let _guard = DropGuard(|| {
+        let r = unsafe { libc::pthread_mutex_unlock(&mut MUTEX as *mut _) };
+        debug_assert_eq!(r, 0);
+    });
+
+    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);
 
-            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
+    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 == 1 {
+            return Ok(());
         }
-    } else {
-        fn init_file() -> Option<libc::c_int> {
-            unsafe { open_readonly(FILE_PATH) }
+        let err = last_os_error();
+        if res >= 0 {
+            return Err(err); // poll() returned the wrong pfd
+        }
+        match err.raw_os_error() {
+            Some(libc::EINTR) | Some(libc::EAGAIN) => continue,
+            _ => return Err(err), // poll() hard failed
         }
     }
 }
+
+struct DropGuard<F: FnMut()>(F);
+
+impl<F: FnMut()> Drop for DropGuard<F> {
+    fn drop(&mut self) {
+        self.0()
+    }
+}

src/util.rs

@@ -35,8 +35,6 @@ impl LazyUsize {
 
     // The initialization is not completed.
     pub const UNINIT: usize = usize::max_value();
-    // The initialization is currently running.
-    pub const ACTIVE: usize = usize::max_value() - 1;
 
     // Runs the init() function at least once, returning the value of some run
     // of init(). Multiple callers can run their init() functions in parallel.
@@ -50,36 +48,6 @@ impl LazyUsize {
         }
         val
     }
-
-    // Synchronously runs the init() function. Only one caller will have their
-    // init() function running at a time, and exactly one successful call will
-    // be run. init() returning UNINIT or ACTIVE will be considered a failure,
-    // and future calls to sync_init will rerun their init() function.
-    pub fn sync_init(&self, init: impl FnOnce() -> usize, mut wait: impl FnMut()) -> usize {
-        // Common and fast path with no contention. Don't wast time on CAS.
-        match self.0.load(Relaxed) {
-            Self::UNINIT | Self::ACTIVE => {}
-            val => return val,
-        }
-        // Relaxed ordering is fine, as we only have a single atomic variable.
-        loop {
-            match self.0.compare_and_swap(Self::UNINIT, Self::ACTIVE, Relaxed) {
-                Self::UNINIT => {
-                    let val = init();
-                    self.0.store(
-                        match val {
-                            Self::UNINIT | Self::ACTIVE => Self::UNINIT,
-                            val => val,
-                        },
-                        Relaxed,
-                    );
-                    return val;
-                }
-                Self::ACTIVE => wait(),
-                val => return val,
-            }
-        }
-    }
 }
 
 // Identical to LazyUsize except with bool instead of usize.

src/util_libc.rs

@@ -89,37 +89,6 @@ impl Weak {
     }
 }
 
-pub struct LazyFd(LazyUsize);
-
-impl LazyFd {
-    pub const fn new() -> Self {
-        Self(LazyUsize::new())
-    }
-
-    // If init() returns Some(x), x should be nonnegative.
-    pub fn init(&self, init: impl FnOnce() -> Option<libc::c_int>) -> Option<libc::c_int> {
-        let fd = self.0.sync_init(
-            || match init() {
-                // OK as val >= 0 and val <= c_int::MAX < usize::MAX
-                Some(val) => val as usize,
-                None => LazyUsize::UNINIT,
-            },
-            || unsafe {
-                // We are usually waiting on an open(2) syscall to complete,
-                // which typically takes < 10us if the file is a device.
-                // However, we might end up waiting much longer if the entropy
-                // pool isn't initialized, but even in that case, this loop will
-                // consume a negligible amount of CPU on most platforms.
-                libc::usleep(10);
-            },
-        );
-        match fd {
-            LazyUsize::UNINIT => None,
-            val => Some(val as libc::c_int),
-        }
-    }
-}
-
 cfg_if! {
     if #[cfg(any(target_os = "linux", target_os = "emscripten"))] {
         use libc::open64 as open;
@@ -129,15 +98,15 @@ cfg_if! {
 }
 
 // SAFETY: path must be null terminated, FD must be manually closed.
-pub unsafe fn open_readonly(path: &str) -> Option<libc::c_int> {
+pub unsafe fn open_readonly(path: &str) -> Result<libc::c_int, Error> {
     debug_assert!(path.as_bytes().last() == Some(&0));
     let fd = open(path.as_ptr() as *mut _, libc::O_RDONLY | libc::O_CLOEXEC);
     if fd < 0 {
-        return None;
+        return Err(last_os_error());
     }
     // O_CLOEXEC works on all Unix targets except for older Linux kernels (pre
     // 2.6.23), so we also use an ioctl to make sure FD_CLOEXEC is set.
     #[cfg(target_os = "linux")]
     libc::ioctl(fd, libc::FIOCLEX);
-    Some(fd)
+    Ok(fd)
 }