getrandom/backends/
linux_android_with_fallback.rs

1//! Implementation for Linux / Android with `/dev/urandom` fallback
2use super::use_file;
3use crate::Error;
4use core::{
5    ffi::c_void,
6    mem::{MaybeUninit, transmute},
7    ptr::{self, NonNull},
8};
9use use_file::utils;
10
11pub use crate::util::{inner_u32, inner_u64};
12
13type GetRandomFn = unsafe extern "C" fn(*mut c_void, libc::size_t, libc::c_uint) -> libc::ssize_t;
14
15/// Sentinel value which indicates that `libc::getrandom` either not available,
16/// or not supported by kernel.
17const NOT_AVAILABLE: NonNull<c_void> = unsafe { NonNull::new_unchecked(usize::MAX as *mut c_void) };
18
19#[cold]
20#[inline(never)]
21fn init() -> NonNull<c_void> {
22    // Use static linking to `libc::getrandom` on MUSL targets and `dlsym` everywhere else
23    #[cfg(not(target_env = "musl"))]
24    let raw_ptr = unsafe { libc::dlsym(libc::RTLD_DEFAULT, c"getrandom".as_ptr()) };
25    #[cfg(target_env = "musl")]
26    let raw_ptr = {
27        let fptr: GetRandomFn = libc::getrandom;
28        unsafe { transmute::<GetRandomFn, *mut c_void>(fptr) }
29    };
30
31    let res_ptr = match NonNull::new(raw_ptr) {
32        Some(fptr) => {
33            let getrandom_fn = unsafe { transmute::<*mut c_void, GetRandomFn>(fptr.as_ptr()) };
34            // Check that `getrandom` syscall is supported by kernel
35            let res = unsafe { getrandom_fn(ptr::dangling_mut(), 0, 0) };
36            if cfg!(getrandom_test_linux_fallback) {
37                NOT_AVAILABLE
38            } else if res.is_negative() {
39                match utils::get_errno() {
40                    libc::ENOSYS => NOT_AVAILABLE, // No kernel support
41                    // The fallback on EPERM is intentionally not done on Android since this workaround
42                    // seems to be needed only for specific Linux-based products that aren't based
43                    // on Android. See https://github.com/rust-random/getrandom/issues/229.
44                    #[cfg(target_os = "linux")]
45                    libc::EPERM => NOT_AVAILABLE, // Blocked by seccomp
46                    _ => fptr,
47                }
48            } else {
49                fptr
50            }
51        }
52        None => NOT_AVAILABLE,
53    };
54
55    #[cfg(getrandom_test_linux_without_fallback)]
56    if res_ptr == NOT_AVAILABLE {
57        panic!("Fallback is triggered with enabled `getrandom_test_linux_without_fallback`")
58    }
59
60    res_ptr
61}
62
63// Prevent inlining of the fallback implementation
64#[inline(never)]
65fn use_file_fallback(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
66    use_file::fill_inner(dest)
67}
68
69#[inline]
70pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
71    #[path = "../utils/lazy_ptr.rs"]
72    mod lazy;
73
74    static GETRANDOM_FN: lazy::LazyPtr<c_void> = lazy::LazyPtr::new();
75    let fptr = GETRANDOM_FN.unsync_init(init);
76
77    if fptr == NOT_AVAILABLE {
78        use_file_fallback(dest)
79    } else {
80        // note: `transmute` is currently the only way to convert a pointer into a function reference
81        let getrandom_fn = unsafe { transmute::<*mut c_void, GetRandomFn>(fptr.as_ptr()) };
82        utils::sys_fill_exact(dest, |buf| unsafe {
83            getrandom_fn(buf.as_mut_ptr().cast(), buf.len(), 0)
84        })
85    }
86}
getrandom/backends/linux_android_with_fallback.rs

getrandom/backends/
linux_android_with_fallback.rs
