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::{transmute, MaybeUninit},
7    ptr::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(usize::MAX as *mut c_void) };
19
20static GETRANDOM_FN: AtomicPtr<c_void> = AtomicPtr::new(core::ptr::null_mut());
21
22#[cold]
23#[inline(never)]
24fn init() -> NonNull<c_void> {
25    // Use static linking to `libc::getrandom` on MUSL targets and `dlsym` everywhere else
26    #[cfg(not(target_env = "musl"))]
27    let raw_ptr = {
28        static NAME: &[u8] = b"getrandom\0";
29        let name_ptr = NAME.as_ptr().cast::<libc::c_char>();
30        unsafe { libc::dlsym(libc::RTLD_DEFAULT, name_ptr) }
31    };
32    #[cfg(target_env = "musl")]
33    let raw_ptr = {
34        let fptr: GetRandomFn = libc::getrandom;
35        unsafe { transmute::<GetRandomFn, *mut c_void>(fptr) }
36    };
37
38    let res_ptr = match NonNull::new(raw_ptr) {
39        Some(fptr) => {
40            let getrandom_fn = unsafe { transmute::<NonNull<c_void>, GetRandomFn>(fptr) };
41            let dangling_ptr = NonNull::dangling().as_ptr();
42            // Check that `getrandom` syscall is supported by kernel
43            let res = unsafe { getrandom_fn(dangling_ptr, 0, 0) };
44            if cfg!(getrandom_test_linux_fallback) {
45                NOT_AVAILABLE
46            } else if res.is_negative() {
47                match util_libc::last_os_error().raw_os_error() {
48                    Some(libc::ENOSYS) => NOT_AVAILABLE, // No kernel support
49                    // The fallback on EPERM is intentionally not done on Android since this workaround
50                    // seems to be needed only for specific Linux-based products that aren't based
51                    // on Android. See https://github.com/rust-random/getrandom/issues/229.
52                    #[cfg(target_os = "linux")]
53                    Some(libc::EPERM) => NOT_AVAILABLE, // Blocked by seccomp
54                    _ => fptr,
55                }
56            } else {
57                fptr
58            }
59        }
60        None => NOT_AVAILABLE,
61    };
62
63    #[cfg(getrandom_test_linux_without_fallback)]
64    if res_ptr == NOT_AVAILABLE {
65        panic!("Fallback is triggered with enabled `getrandom_test_linux_without_fallback`")
66    }
67
68    GETRANDOM_FN.store(res_ptr.as_ptr(), Ordering::Release);
69    res_ptr
70}
71
72// Prevent inlining of the fallback implementation
73#[inline(never)]
74fn use_file_fallback(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
75    use_file::fill_inner(dest)
76}
77
78#[inline]
79pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
80    // Despite being only a single atomic variable, we still cannot always use
81    // Ordering::Relaxed, as we need to make sure a successful call to `init`
82    // is "ordered before" any data read through the returned pointer (which
83    // occurs when the function is called). Our implementation mirrors that of
84    // the one in libstd, meaning that the use of non-Relaxed operations is
85    // probably unnecessary.
86    let raw_ptr = GETRANDOM_FN.load(Ordering::Acquire);
87    let fptr = match NonNull::new(raw_ptr) {
88        Some(p) => p,
89        None => init(),
90    };
91
92    if fptr == NOT_AVAILABLE {
93        use_file_fallback(dest)
94    } else {
95        // note: `transmute` is currently the only way to convert a pointer into a function reference
96        let getrandom_fn = unsafe { transmute::<NonNull<c_void>, GetRandomFn>(fptr) };
97        util_libc::sys_fill_exact(dest, |buf| unsafe {
98            getrandom_fn(buf.as_mut_ptr().cast(), buf.len(), 0)
99        })
100    }
101}
getrandom/backends/linux_android_with_fallback.rs

getrandom/backends/
linux_android_with_fallback.rs
