linux_android: use libc::getrandom (#508)

Use of `libc::getrandom` will automatically give us optimizations like
vDSO (#503) and can help with testing of fallback logic (#289).

It was also requested in #285. In that discussion we decided against
using this approach, but in the light of the vDSO optimization it may be
worth to reconsider it.

In `linux_android_with_fallback` use of `libc::syscall` is replaced by
`dlsym`-based code similar to what we use in the `netbsd` backend.

Author: newpavlov

src/lib.rs

@@ -384,10 +384,8 @@ cfg_if! {
             ),
         )
     ))] {
-        mod lazy;
         mod util_libc;
         mod use_file;
-        mod linux_android;
         #[path = "linux_android_with_fallback.rs"] mod imp;
     } else if #[cfg(any(target_os = "android", target_os = "linux"))] {
         mod util_libc;

src/linux_android.rs

@@ -6,21 +6,7 @@ use core::mem::MaybeUninit;
 compile_error!("`linux_getrandom` backend can be enabled only for Linux/Android targets!");
 
 pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
-    util_libc::sys_fill_exact(dest, getrandom_syscall)
-}
-
-pub fn getrandom_syscall(buf: &mut [MaybeUninit<u8>]) -> libc::ssize_t {
-    let res: libc::c_long = unsafe {
-        libc::syscall(
-            libc::SYS_getrandom,
-            buf.as_mut_ptr().cast::<core::ffi::c_void>(),
-            buf.len(),
-            0,
-        )
-    };
-
-    const _: () =
-        assert!(core::mem::size_of::<libc::c_long>() == core::mem::size_of::<libc::ssize_t>());
-    res.try_into()
-        .expect("c_long to ssize_t conversion is lossless")
+    util_libc::sys_fill_exact(dest, |buf| unsafe {
+        libc::getrandom(buf.as_mut_ptr().cast(), buf.len(), 0)
+    })
 }

src/linux_android_with_fallback.rs

@@ -1,37 +1,80 @@
 //! Implementation for Linux / Android with `/dev/urandom` fallback
-use crate::{lazy::LazyBool, linux_android, use_file, util_libc::last_os_error, Error};
-use core::mem::MaybeUninit;
+use crate::{use_file, util_libc, Error};
+use core::{
+    ffi::c_void,
+    mem::{self, MaybeUninit},
+    ptr::{self, NonNull},
+    sync::atomic::{AtomicPtr, Ordering},
+};
 
-pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
-    // getrandom(2) was introduced in Linux 3.17
-    static HAS_GETRANDOM: LazyBool = LazyBool::new();
-    if HAS_GETRANDOM.unsync_init(is_getrandom_available) {
-        linux_android::getrandom_inner(dest)
-    } else {
-        // prevent inlining of the fallback implementation
-        #[inline(never)]
-        fn inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
-            use_file::getrandom_inner(dest)
+type GetRandomFn = unsafe extern "C" fn(*mut c_void, libc::size_t, libc::c_uint) -> libc::ssize_t;
+
+/// Sentinel value which indicates that `libc::getrandom` either not available,
+/// or not supported by kernel.
+const NOT_AVAILABLE: NonNull<c_void> = unsafe { NonNull::new_unchecked(usize::MAX as *mut c_void) };
+
+static GETRANDOM_FN: AtomicPtr<c_void> = AtomicPtr::new(ptr::null_mut());
+
+#[cold]
+fn init() -> NonNull<c_void> {
+    static NAME: &[u8] = b"getrandom\0";
+    let name_ptr = NAME.as_ptr().cast::<libc::c_char>();
+    let raw_ptr = unsafe { libc::dlsym(libc::RTLD_DEFAULT, name_ptr) };
+    let res_ptr = match NonNull::new(raw_ptr) {
+        Some(fptr) => {
+            let getrandom_fn = unsafe { mem::transmute::<NonNull<c_void>, GetRandomFn>(fptr) };
+            let dangling_ptr = ptr::NonNull::dangling().as_ptr();
+            // Check that `getrandom` syscall is supported by kernel
+            let res = unsafe { getrandom_fn(dangling_ptr, 0, 0) };
+            if cfg!(getrandom_test_linux_fallback) {
+                NOT_AVAILABLE
+            } else if res.is_negative() {
+                match util_libc::last_os_error().raw_os_error() {
+                    Some(libc::ENOSYS) => NOT_AVAILABLE, // No kernel support
+                    // The fallback on EPERM is intentionally not done on Android since this workaround
+                    // seems to be needed only for specific Linux-based products that aren't based
+                    // on Android. See https://github.com/rust-random/getrandom/issues/229.
+                    #[cfg(target_os = "linux")]
+                    Some(libc::EPERM) => NOT_AVAILABLE, // Blocked by seccomp
+                    _ => fptr,
+                }
+            } else {
+                fptr
+            }
         }
+        None => NOT_AVAILABLE,
+    };
 
-        inner(dest)
-    }
+    GETRANDOM_FN.store(res_ptr.as_ptr(), Ordering::Release);
+    res_ptr
 }
 
-fn is_getrandom_available() -> bool {
-    if cfg!(getrandom_test_linux_fallback) {
-        false
-    } else if linux_android::getrandom_syscall(&mut []) < 0 {
-        match last_os_error().raw_os_error() {
-            Some(libc::ENOSYS) => false, // No kernel support
-            // The fallback on EPERM is intentionally not done on Android since this workaround
-            // seems to be needed only for specific Linux-based products that aren't based
-            // on Android. See https://github.com/rust-random/getrandom/issues/229.
-            #[cfg(target_os = "linux")]
-            Some(libc::EPERM) => false, // Blocked by seccomp
-            _ => true,
-        }
+// prevent inlining of the fallback implementation
+#[inline(never)]
+fn use_file_fallback(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
+    use_file::getrandom_inner(dest)
+}
+
+pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
+    // Despite being only a single atomic variable, we still cannot always use
+    // Ordering::Relaxed, as we need to make sure a successful call to `init`
+    // is "ordered before" any data read through the returned pointer (which
+    // occurs when the function is called). Our implementation mirrors that of
+    // the one in libstd, meaning that the use of non-Relaxed operations is
+    // probably unnecessary.
+    let raw_ptr = GETRANDOM_FN.load(Ordering::Acquire);
+    let fptr = match NonNull::new(raw_ptr) {
+        Some(p) => p,
+        None => init(),
+    };
+
+    if fptr == NOT_AVAILABLE {
+        use_file_fallback(dest)
     } else {
-        true
+        // note: `transume` is currently the only way to convert pointer into function reference
+        let getrandom_fn = unsafe { mem::transmute::<NonNull<c_void>, GetRandomFn>(fptr) };
+        util_libc::sys_fill_exact(dest, |buf| unsafe {
+            getrandom_fn(buf.as_mut_ptr().cast(), buf.len(), 0)
+        })
     }
 }