1//! Implementation for Linux / Android with `/dev/urandom` fallback
2use super::use_file;
3use crate::Error;
4use core::{
5 ffi::c_void,
6 mem::{self, MaybeUninit},
7 ptr::{self, NonNull},
8 sync::atomic::{AtomicPtr, Ordering},
9};
10use use_file::util_libc;
11
12pub use crate::util::{inner_u32, inner_u64};
13
14type GetRandomFn = unsafe extern "C" fn(*mut c_void, libc::size_t, libc::c_uint) -> libc::ssize_t;
15
16/// Sentinel value which indicates that `libc::getrandom` either not available,
17/// or not supported by kernel.
18const NOT_AVAILABLE: NonNull<c_void> = unsafe { NonNull::new_unchecked(ptr:usize::MAX as *mut c_void) };
19
20static GETRANDOM_FN: AtomicPtr<c_void> = AtomicPtr::new(ptr::null_mut());
21
22#[cold]
23fn init() -> NonNull<c_void> {
24 static NAME: &[u8] = b"getrandom\0";
25 let name_ptr = NAME.as_ptr().cast::<libc::c_char>();
26 let raw_ptr = unsafe { libc::dlsym(libc::RTLD_DEFAULT, name_ptr) };
27 let res_ptr = match NonNull::new(raw_ptr) {
28 Some(fptr) => {
29 let getrandom_fn = unsafe { mem::transmute::<NonNull<c_void>, GetRandomFn>(fptr) };
30 let dangling_ptr = ptr::NonNull::dangling().as_ptr();
31 // Check that `getrandom` syscall is supported by kernel
32 let res = unsafe { getrandom_fn(dangling_ptr, 0, 0) };
33 if cfg!(getrandom_test_linux_fallback) {
34 NOT_AVAILABLE
35 } else if res.is_negative() {
36 match util_libc::last_os_error().raw_os_error() {
37 Some(libc::ENOSYS) => NOT_AVAILABLE, // No kernel support
38 // The fallback on EPERM is intentionally not done on Android since this workaround
39 // seems to be needed only for specific Linux-based products that aren't based
40 // on Android. See https://github.com/rust-random/getrandom/issues/229.
41 #[cfg(target_os = "linux")]
42 Some(libc::EPERM) => NOT_AVAILABLE, // Blocked by seccomp
43 _ => fptr,
44 }
45 } else {
46 fptr
47 }
48 }
49 None => NOT_AVAILABLE,
50 };
51
52 GETRANDOM_FN.store(res_ptr.as_ptr(), Ordering::Release);
53 res_ptr
54}
55
56// prevent inlining of the fallback implementation
57#[inline(never)]
58fn use_file_fallback(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
59 use_file::fill_inner(dest)
60}
61
62pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
63 // Despite being only a single atomic variable, we still cannot always use
64 // Ordering::Relaxed, as we need to make sure a successful call to `init`
65 // is "ordered before" any data read through the returned pointer (which
66 // occurs when the function is called). Our implementation mirrors that of
67 // the one in libstd, meaning that the use of non-Relaxed operations is
68 // probably unnecessary.
69 let raw_ptr: *mut c_void = GETRANDOM_FN.load(order:Ordering::Acquire);
70 let fptr: NonNull = match NonNull::new(raw_ptr) {
71 Some(p: NonNull) => p,
72 None => init(),
73 };
74
75 if fptr == NOT_AVAILABLE {
76 use_file_fallback(dest)
77 } else {
78 // note: `transume` is currently the only way to convert pointer into function reference
79 let getrandom_fn: unsafe fn(*mut c_void, usize, …) -> … = unsafe { mem::transmute::<NonNull<c_void>, GetRandomFn>(src:fptr) };
80 util_libc::sys_fill_exact(buf:dest, |buf: &mut [MaybeUninit]| unsafe {
81 getrandom_fn(buf.as_mut_ptr().cast(), buf.len(), 0)
82 })
83 }
84}
85