Add Structures for lock-free initialization (#51)

Author: josephlr

Cargo.toml

@@ -27,10 +27,6 @@ libc = "0.2.54"
 [target.'cfg(any(unix, target_os = "redox"))'.dependencies]
 lazy_static = "1.3.0"
 
-# For caching result of CPUID check for RDRAND
-[target.'cfg(target_os = "uefi")'.dependencies]
-lazy_static = { version = "1.3.0", features = ["spin_no_std"] }
-
 [target.wasm32-unknown-unknown.dependencies]
 wasm-bindgen = { version = "0.2.29", optional = true }
 stdweb = { version = "0.4.9", optional = true }

src/lib.rs

@@ -142,6 +142,12 @@ extern crate std;
 
 mod error;
 pub use crate::error::Error;
+#[allow(dead_code)]
+mod util;
+
+// These targets need weak linkage to libc randomness functions.
+#[cfg(any(target_os = "macos", target_os = "solaris", target_os = "illumos"))]
+mod util_libc;
 
 // System-specific implementations.
 //

src/linux_android.rs

@@ -9,9 +9,9 @@
 //! Implementation for Linux / Android
 extern crate std;
 
+use crate::util::LazyBool;
 use crate::{use_file, Error};
 use core::num::NonZeroU32;
-use lazy_static::lazy_static;
 use std::io;
 
 fn syscall_getrandom(dest: &mut [u8], block: bool) -> Result<usize, io::Error> {
@@ -29,18 +29,15 @@ fn syscall_getrandom(dest: &mut [u8], block: bool) -> Result<usize, io::Error> {
 }
 
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
-    lazy_static! {
-        static ref HAS_GETRANDOM: bool = is_getrandom_available();
-    }
-    match *HAS_GETRANDOM {
-        true => {
-            let mut start = 0;
-            while start < dest.len() {
-                start += syscall_getrandom(&mut dest[start..], true)?;
-            }
-            Ok(())
+    static HAS_GETRANDOM: LazyBool = LazyBool::new();
+    if HAS_GETRANDOM.unsync_init(is_getrandom_available) {
+        let mut start = 0;
+        while start < dest.len() {
+            start += syscall_getrandom(&mut dest[start..], true)?;
         }
-        false => use_file::getrandom_inner(dest),
+        Ok(())
+    } else {
+        use_file::getrandom_inner(dest)
     }
 }
 

src/macos.rs

@@ -9,27 +9,20 @@
 //! Implementation for macOS
 extern crate std;
 
+use crate::util_libc::Weak;
 use crate::{use_file, Error};
 use core::mem;
 use core::num::NonZeroU32;
-use lazy_static::lazy_static;
 use std::io;
 
 type GetEntropyFn = unsafe extern "C" fn(*mut u8, libc::size_t) -> libc::c_int;
 
-fn fetch_getentropy() -> Option<GetEntropyFn> {
-    let name = "getentropy\0";
-    let addr = unsafe { libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr() as *const _) };
-    unsafe { mem::transmute(addr) }
-}
-
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
-    lazy_static! {
-        static ref GETENTROPY_FUNC: Option<GetEntropyFn> = fetch_getentropy();
-    }
-    if let Some(fptr) = *GETENTROPY_FUNC {
+    static GETENTROPY: Weak = unsafe { Weak::new("getentropy\0") };
+    if let Some(fptr) = GETENTROPY.ptr() {
+        let func: GetEntropyFn = unsafe { mem::transmute(fptr) };
         for chunk in dest.chunks_mut(256) {
-            let ret = unsafe { fptr(chunk.as_mut_ptr(), chunk.len()) };
+            let ret = unsafe { func(chunk.as_mut_ptr(), chunk.len()) };
             if ret != 0 {
                 error!("getentropy syscall failed with ret={}", ret);
                 return Err(io::Error::last_os_error().into());

src/rdrand.rs

@@ -7,6 +7,7 @@
 // except according to those terms.
 
 //! Implementation for SGX using RDRAND instruction
+use crate::util::LazyBool;
 use crate::Error;
 use core::arch::x86_64::_rdrand64_step;
 use core::mem;
@@ -55,13 +56,10 @@ fn is_rdrand_supported() -> bool {
 #[cfg(not(target_feature = "rdrand"))]
 fn is_rdrand_supported() -> bool {
     use core::arch::x86_64::__cpuid;
-    use lazy_static::lazy_static;
     // SAFETY: All x86_64 CPUs support CPUID leaf 1
     const FLAG: u32 = 1 << 30;
-    lazy_static! {
-        static ref HAS_RDRAND: bool = unsafe { __cpuid(1).ecx & FLAG != 0 };
-    }
-    *HAS_RDRAND
+    static HAS_RDRAND: LazyBool = LazyBool::new();
+    HAS_RDRAND.unsync_init(|| unsafe { (__cpuid(1).ecx & FLAG) != 0 })
 }
 
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {

src/solaris_illumos.rs

@@ -19,48 +19,34 @@
 //! libc::dlsym.
 extern crate std;
 
+use crate::util_libc::Weak;
 use crate::{use_file, Error};
 use core::mem;
 use core::num::NonZeroU32;
-use lazy_static::lazy_static;
 use std::io;
 
 #[cfg(target_os = "illumos")]
 type GetRandomFn = unsafe extern "C" fn(*mut u8, libc::size_t, libc::c_uint) -> libc::ssize_t;
 #[cfg(target_os = "solaris")]
 type GetRandomFn = unsafe extern "C" fn(*mut u8, libc::size_t, libc::c_uint) -> libc::c_int;
 
-fn libc_getrandom(rand: GetRandomFn, dest: &mut [u8]) -> Result<(), Error> {
-    let ret = unsafe { rand(dest.as_mut_ptr(), dest.len(), 0) as libc::ssize_t };
-
-    if ret == -1 || ret != dest.len() as libc::ssize_t {
-        error!("getrandom syscall failed with ret={}", ret);
-        Err(io::Error::last_os_error().into())
-    } else {
-        Ok(())
-    }
-}
-
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
-    lazy_static! {
-        static ref GETRANDOM_FUNC: Option<GetRandomFn> = fetch_getrandom();
-    }
-
-    // 256 bytes is the lowest common denominator across all the Solaris
-    // derived platforms for atomically obtaining random data.
-    for chunk in dest.chunks_mut(256) {
-        match *GETRANDOM_FUNC {
-            Some(fptr) => libc_getrandom(fptr, chunk)?,
-            None => use_file::getrandom_inner(chunk)?,
-        };
+    static GETRANDOM: Weak = unsafe { Weak::new("getrandom\0") };
+    if let Some(fptr) = GETRANDOM.ptr() {
+        let func: GetRandomFn = unsafe { mem::transmute(fptr) };
+        // 256 bytes is the lowest common denominator across all the Solaris
+        // derived platforms for atomically obtaining random data.
+        for chunk in dest.chunks_mut(256) {
+            let ret = unsafe { func(chunk.as_mut_ptr(), chunk.len(), 0) };
+            if ret != chunk.len() as _ {
+                error!("getrandom syscall failed with ret={}", ret);
+                return Err(io::Error::last_os_error().into());
+            }
+        }
+        Ok(())
+    } else {
+        use_file::getrandom_inner(dest)
     }
-    Ok(())
-}
-
-fn fetch_getrandom() -> Option<GetRandomFn> {
-    let name = "getrandom\0";
-    let addr = unsafe { libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr() as *const _) };
-    unsafe { mem::transmute(addr) }
 }
 
 #[inline(always)]

src/util.rs

@@ -0,0 +1,49 @@
+// Copyright 2019 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.
+
+use core::sync::atomic::{AtomicUsize, Ordering};
+
+// This structure represents a laziliy initialized static usize value. Useful
+// when it is perferable to just rerun initialization instead of locking.
+pub struct LazyUsize(AtomicUsize);
+
+impl LazyUsize {
+    pub const fn new() -> Self {
+        Self(AtomicUsize::new(Self::UNINIT))
+    }
+
+    // The initialization is not completed.
+    pub const UNINIT: usize = usize::max_value();
+
+    // Runs the init() function at least once, returning the value of some run
+    // of init(). Unlike std::sync::Once, the init() function may be run
+    // multiple times. If init() returns UNINIT, future calls to unsync_init()
+    // will always retry. This makes UNINIT ideal for representing failure.
+    pub fn unsync_init(&self, init: impl FnOnce() -> usize) -> usize {
+        // Relaxed ordering is fine, as we only have a single atomic variable.
+        let mut val = self.0.load(Ordering::Relaxed);
+        if val == Self::UNINIT {
+            val = init();
+            self.0.store(val, Ordering::Relaxed);
+        }
+        val
+    }
+}
+
+// Identical to LazyUsize except with bool instead of usize.
+pub struct LazyBool(LazyUsize);
+
+impl LazyBool {
+    pub const fn new() -> Self {
+        Self(LazyUsize::new())
+    }
+
+    pub fn unsync_init(&self, init: impl FnOnce() -> bool) -> bool {
+        self.0.unsync_init(|| init() as usize) != 0
+    }
+}

src/util_libc.rs

@@ -0,0 +1,38 @@
+// Copyright 2019 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.
+
+use crate::util::LazyUsize;
+use core::ptr::NonNull;
+
+// A "weak" binding to a C function that may or may not be present at runtime.
+// Used for supporting newer OS features while still building on older systems.
+// F must be a function pointer of type `unsafe extern "C" fn`. Based off of the
+// weak! macro in libstd.
+pub struct Weak {
+    name: &'static str,
+    addr: LazyUsize,
+}
+
+impl Weak {
+    // Construct a binding to a C function with a given name. This function is
+    // unsafe because `name` _must_ be null terminated.
+    pub const unsafe fn new(name: &'static str) -> Self {
+        Self {
+            name,
+            addr: LazyUsize::new(),
+        }
+    }
+
+    // Return a function pointer if present at runtime. Otherwise, return null.
+    pub fn ptr(&self) -> Option<NonNull<libc::c_void>> {
+        let addr = self.addr.unsync_init(|| unsafe {
+            libc::dlsym(libc::RTLD_DEFAULT, self.name.as_ptr() as *const _) as usize
+        });
+        NonNull::new(addr as *mut _)
+    }
+}