getrandom/
util_libc.rs

1#![allow(dead_code)]
2use crate::Error;
3use core::{
4    mem::MaybeUninit,
5    num::NonZeroU32,
6    ptr::NonNull,
7    sync::atomic::{fence, AtomicPtr, Ordering},
8};
9use libc::c_void;
10
11cfg_if! {
12    if #[cfg(any(target_os = "netbsd", target_os = "openbsd", target_os = "android"))] {
13        use libc::__errno as errno_location;
14    } else if #[cfg(any(target_os = "linux", target_os = "emscripten", target_os = "hurd", target_os = "redox", target_os = "dragonfly"))] {
15        use libc::__errno_location as errno_location;
16    } else if #[cfg(any(target_os = "solaris", target_os = "illumos"))] {
17        use libc::___errno as errno_location;
18    } else if #[cfg(any(target_os = "macos", target_os = "freebsd"))] {
19        use libc::__error as errno_location;
20    } else if #[cfg(target_os = "haiku")] {
21        use libc::_errnop as errno_location;
22    } else if #[cfg(target_os = "nto")] {
23        use libc::__get_errno_ptr as errno_location;
24    } else if #[cfg(any(all(target_os = "horizon", target_arch = "arm"), target_os = "vita"))] {
25        extern "C" {
26            // Not provided by libc: https://github.com/rust-lang/libc/issues/1995
27            fn __errno() -> *mut libc::c_int;
28        }
29        use __errno as errno_location;
30    } else if #[cfg(target_os = "aix")] {
31        use libc::_Errno as errno_location;
32    }
33}
34
35cfg_if! {
36    if #[cfg(target_os = "vxworks")] {
37        use libc::errnoGet as get_errno;
38    } else {
39        unsafe fn get_errno() -> libc::c_int { *errno_location() }
40    }
41}
42
43pub fn last_os_error() -> Error {
44    let errno = unsafe { get_errno() };
45    if errno > 0 {
46        Error::from(NonZeroU32::new(errno as u32).unwrap())
47    } else {
48        Error::ERRNO_NOT_POSITIVE
49    }
50}
51
52// Fill a buffer by repeatedly invoking a system call. The `sys_fill` function:
53//   - should return -1 and set errno on failure
54//   - should return the number of bytes written on success
55pub fn sys_fill_exact(
56    mut buf: &mut [MaybeUninit<u8>],
57    sys_fill: impl Fn(&mut [MaybeUninit<u8>]) -> libc::ssize_t,
58) -> Result<(), Error> {
59    while !buf.is_empty() {
60        let res = sys_fill(buf);
61        match res {
62            res if res > 0 => buf = buf.get_mut(res as usize..).ok_or(Error::UNEXPECTED)?,
63            -1 => {
64                let err = last_os_error();
65                // We should try again if the call was interrupted.
66                if err.raw_os_error() != Some(libc::EINTR) {
67                    return Err(err);
68                }
69            }
70            // Negative return codes not equal to -1 should be impossible.
71            // EOF (ret = 0) should be impossible, as the data we are reading
72            // should be an infinite stream of random bytes.
73            _ => return Err(Error::UNEXPECTED),
74        }
75    }
76    Ok(())
77}
78
79// A "weak" binding to a C function that may or may not be present at runtime.
80// Used for supporting newer OS features while still building on older systems.
81// Based off of the DlsymWeak struct in libstd:
82// https://github.com/rust-lang/rust/blob/1.61.0/library/std/src/sys/unix/weak.rs#L84
83// except that the caller must manually cast self.ptr() to a function pointer.
84pub struct Weak {
85    name: &'static str,
86    addr: AtomicPtr<c_void>,
87}
88
89impl Weak {
90    // A non-null pointer value which indicates we are uninitialized. This
91    // constant should ideally not be a valid address of a function pointer.
92    // However, if by chance libc::dlsym does return UNINIT, there will not
93    // be undefined behavior. libc::dlsym will just be called each time ptr()
94    // is called. This would be inefficient, but correct.
95    // TODO: Replace with core::ptr::invalid_mut(1) when that is stable.
96    const UNINIT: *mut c_void = 1 as *mut c_void;
97
98    // Construct a binding to a C function with a given name. This function is
99    // unsafe because `name` _must_ be null terminated.
100    pub const unsafe fn new(name: &'static str) -> Self {
101        Self {
102            name,
103            addr: AtomicPtr::new(Self::UNINIT),
104        }
105    }
106
107    // Return the address of a function if present at runtime. Otherwise,
108    // return None. Multiple callers can call ptr() concurrently. It will
109    // always return _some_ value returned by libc::dlsym. However, the
110    // dlsym function may be called multiple times.
111    pub fn ptr(&self) -> Option<NonNull<c_void>> {
112        // Despite having only a single atomic variable (self.addr), we still
113        // cannot always use Ordering::Relaxed, as we need to make sure a
114        // successful call to dlsym() is "ordered before" any data read through
115        // the returned pointer (which occurs when the function is called).
116        // Our implementation mirrors that of the one in libstd, meaning that
117        // the use of non-Relaxed operations is probably unnecessary.
118        match self.addr.load(Ordering::Relaxed) {
119            Self::UNINIT => {
120                let symbol = self.name.as_ptr() as *const _;
121                let addr = unsafe { libc::dlsym(libc::RTLD_DEFAULT, symbol) };
122                // Synchronizes with the Acquire fence below
123                self.addr.store(addr, Ordering::Release);
124                NonNull::new(addr)
125            }
126            addr => {
127                let func = NonNull::new(addr)?;
128                fence(Ordering::Acquire);
129                Some(func)
130            }
131        }
132    }
133}
134
135// SAFETY: path must be null terminated, FD must be manually closed.
136pub unsafe fn open_readonly(path: &str) -> Result<libc::c_int, Error> {
137    debug_assert_eq!(path.as_bytes().last(), Some(&0));
138    loop {
139        let fd = libc::open(path.as_ptr() as *const _, libc::O_RDONLY | libc::O_CLOEXEC);
140        if fd >= 0 {
141            return Ok(fd);
142        }
143        let err = last_os_error();
144        // We should try again if open() was interrupted.
145        if err.raw_os_error() != Some(libc::EINTR) {
146            return Err(err);
147        }
148    }
149}
150
151/// Thin wrapper around the `getrandom()` Linux system call
152#[cfg(any(target_os = "android", target_os = "linux"))]
153pub fn getrandom_syscall(buf: &mut [MaybeUninit<u8>]) -> libc::ssize_t {
154    unsafe {
155        libc::syscall(
156            libc::SYS_getrandom,
157            buf.as_mut_ptr() as *mut libc::c_void,
158            buf.len(),
159            0,
160        ) as libc::ssize_t
161    }
162}
getrandom/util_libc.rs

getrandom/
util_libc.rs
