diff options
Diffstat (limited to 'src/util_libc.rs')
-rw-r--r-- | src/util_libc.rs | 77 |
1 files changed, 61 insertions, 16 deletions
diff --git a/src/util_libc.rs b/src/util_libc.rs index 6823609..d057071 100644 --- a/src/util_libc.rs +++ b/src/util_libc.rs @@ -6,8 +6,13 @@ // option. This file may not be copied, modified, or distributed // except according to those terms. #![allow(dead_code)] -use crate::{util::LazyUsize, Error}; -use core::{num::NonZeroU32, ptr::NonNull}; +use crate::Error; +use core::{ + num::NonZeroU32, + ptr::NonNull, + sync::atomic::{fence, AtomicPtr, Ordering}, +}; +use libc::c_void; cfg_if! { if #[cfg(any(target_os = "netbsd", target_os = "openbsd", target_os = "android"))] { @@ -20,6 +25,12 @@ cfg_if! { use libc::__error as errno_location; } else if #[cfg(target_os = "haiku")] { use libc::_errnop as errno_location; + } else if #[cfg(all(target_os = "horizon", target_arch = "arm"))] { + extern "C" { + // Not provided by libc: https://github.com/rust-lang/libc/issues/1995 + fn __errno() -> *mut libc::c_int; + } + use __errno as errno_location; } } @@ -70,29 +81,57 @@ pub fn sys_fill_exact( // A "weak" binding to a C function that may or may not be present at runtime. // Used for supporting newer OS features while still building on older systems. -// F must be a function pointer of type `unsafe extern "C" fn`. Based off of the -// weak! macro in libstd. +// Based off of the DlsymWeak struct in libstd: +// https://github.com/rust-lang/rust/blob/1.61.0/library/std/src/sys/unix/weak.rs#L84 +// except that the caller must manually cast self.ptr() to a function pointer. pub struct Weak { name: &'static str, - addr: LazyUsize, + addr: AtomicPtr<c_void>, } impl Weak { + // A non-null pointer value which indicates we are uninitialized. This + // constant should ideally not be a valid address of a function pointer. + // However, if by chance libc::dlsym does return UNINIT, there will not + // be undefined behavior. libc::dlsym will just be called each time ptr() + // is called. This would be inefficient, but correct. + // TODO: Replace with core::ptr::invalid_mut(1) when that is stable. + const UNINIT: *mut c_void = 1 as *mut c_void; + // Construct a binding to a C function with a given name. This function is // unsafe because `name` _must_ be null terminated. pub const unsafe fn new(name: &'static str) -> Self { Self { name, - addr: LazyUsize::new(), + addr: AtomicPtr::new(Self::UNINIT), } } - // Return a function pointer if present at runtime. Otherwise, return null. - pub fn ptr(&self) -> Option<NonNull<libc::c_void>> { - let addr = self.addr.unsync_init(|| unsafe { - libc::dlsym(libc::RTLD_DEFAULT, self.name.as_ptr() as *const _) as usize - }); - NonNull::new(addr as *mut _) + // Return the address of a function if present at runtime. Otherwise, + // return None. Multiple callers can call ptr() concurrently. It will + // always return _some_ value returned by libc::dlsym. However, the + // dlsym function may be called multiple times. + pub fn ptr(&self) -> Option<NonNull<c_void>> { + // Despite having only a single atomic variable (self.addr), we still + // cannot always use Ordering::Relaxed, as we need to make sure a + // successful call to dlsym() 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. + match self.addr.load(Ordering::Relaxed) { + Self::UNINIT => { + let symbol = self.name.as_ptr() as *const _; + let addr = unsafe { libc::dlsym(libc::RTLD_DEFAULT, symbol) }; + // Synchronizes with the Acquire fence below + self.addr.store(addr, Ordering::Release); + NonNull::new(addr) + } + addr => { + let func = NonNull::new(addr)?; + fence(Ordering::Acquire); + Some(func) + } + } } } @@ -107,9 +146,15 @@ cfg_if! { // SAFETY: path must be null terminated, FD must be manually closed. pub unsafe fn open_readonly(path: &str) -> Result<libc::c_int, Error> { debug_assert_eq!(path.as_bytes().last(), Some(&0)); - let fd = open(path.as_ptr() as *const _, libc::O_RDONLY | libc::O_CLOEXEC); - if fd < 0 { - return Err(last_os_error()); + loop { + let fd = open(path.as_ptr() as *const _, libc::O_RDONLY | libc::O_CLOEXEC); + if fd >= 0 { + return Ok(fd); + } + let err = last_os_error(); + // We should try again if open() was interrupted. + if err.raw_os_error() != Some(libc::EINTR) { + return Err(err); + } } - Ok(fd) } |