epoll: add data_race test

This test demonstrates the need to synchronize the clock
of the thread waking up from an epoll_wait from the thread
that issued the epoll awake event.

src/tools/miri/tests/fail-dep/libc/libc-epoll-blocking.rs

@@ -0,0 +1,79 @@
+//@only-target: linux
+// test_epoll_race depends on a deterministic schedule.
+//@compile-flags: -Zmiri-preemption-rate=0
+
+use std::convert::TryInto;
+use std::thread;
+
+fn main() {
+    test_epoll_race();
+}
+
+// Using `as` cast since `EPOLLET` wraps around
+const EPOLL_IN_OUT_ET: u32 = (libc::EPOLLIN | libc::EPOLLOUT | libc::EPOLLET) as _;
+
+#[track_caller]
+fn check_epoll_wait<const N: usize>(
+    epfd: i32,
+    expected_notifications: &[(u32, u64)],
+    timeout: i32,
+) {
+    let epoll_event = libc::epoll_event { events: 0, u64: 0 };
+    let mut array: [libc::epoll_event; N] = [epoll_event; N];
+    let maxsize = N;
+    let array_ptr = array.as_mut_ptr();
+    let res = unsafe { libc::epoll_wait(epfd, array_ptr, maxsize.try_into().unwrap(), timeout) };
+    if res < 0 {
+        panic!("epoll_wait failed: {}", std::io::Error::last_os_error());
+    }
+    assert_eq!(
+        res,
+        expected_notifications.len().try_into().unwrap(),
+        "got wrong number of notifications"
+    );
+    let slice = unsafe { std::slice::from_raw_parts(array_ptr, res.try_into().unwrap()) };
+    for (return_event, expected_event) in slice.iter().zip(expected_notifications.iter()) {
+        let event = return_event.events;
+        let data = return_event.u64;
+        assert_eq!(event, expected_event.0, "got wrong events");
+        assert_eq!(data, expected_event.1, "got wrong data");
+    }
+}
+
+// This test shows a data_race before epoll had vector clocks added.
+fn test_epoll_race() {
+    // Create an epoll instance.
+    let epfd = unsafe { libc::epoll_create1(0) };
+    assert_ne!(epfd, -1);
+
+    // Create an eventfd instance.
+    let flags = libc::EFD_NONBLOCK | libc::EFD_CLOEXEC;
+    let fd = unsafe { libc::eventfd(0, flags) };
+
+    // Register eventfd with the epoll instance.
+    let mut ev = libc::epoll_event { events: EPOLL_IN_OUT_ET, u64: fd as u64 };
+    let res = unsafe { libc::epoll_ctl(epfd, libc::EPOLL_CTL_ADD, fd, &mut ev) };
+    assert_eq!(res, 0);
+
+    static mut VAL: u8 = 0;
+    let thread1 = thread::spawn(move || {
+        // Write to the static mut variable.
+        unsafe { VAL = 1 };
+        // Write to the eventfd instance.
+        let sized_8_data: [u8; 8] = 1_u64.to_ne_bytes();
+        let res = unsafe { libc::write(fd, sized_8_data.as_ptr() as *const libc::c_void, 8) };
+        // read returns number of bytes that have been read, which is always 8.
+        assert_eq!(res, 8);
+    });
+    thread::yield_now();
+    // epoll_wait for the event to happen.
+    let expected_event = u32::try_from(libc::EPOLLIN | libc::EPOLLOUT).unwrap();
+    let expected_value = u64::try_from(fd).unwrap();
+    check_epoll_wait::<8>(epfd, &[(expected_event, expected_value)], -1);
+    // Read from the static mut variable.
+    #[allow(static_mut_refs)]
+    unsafe {
+        assert_eq!(VAL, 1) //~ ERROR: Data race detected
+    };
+    thread1.join().unwrap();
+}

src/tools/miri/tests/fail-dep/libc/libc-epoll-blocking.stderr

@@ -0,0 +1,29 @@
+error: Undefined Behavior: Data race detected between (1) non-atomic write on thread `unnamed-ID` and (2) retag read of type `u8` on thread `main` at ALLOC. (2) just happened here
+  --> tests/fail-dep/libc/libc-epoll-blocking.rs:LL:CC
+   |
+LL |         assert_eq!(VAL, 1)
+   |         ^^^^^^^^^^^^^^^^^^ Data race detected between (1) non-atomic write on thread `unnamed-ID` and (2) retag read of type `u8` on thread `main` at ALLOC. (2) just happened here
+   |
+help: and (1) occurred earlier here
+  --> tests/fail-dep/libc/libc-epoll-blocking.rs:LL:CC
+   |
+LL |         unsafe { VAL = 1 };
+   |                  ^^^^^^^
+   = help: retags occur on all (re)borrows and as well as when references are copied or moved
+   = help: retags permit optimizations that insert speculative reads or writes
+   = help: therefore from the perspective of data races, a retag has the same implications as a read or write
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE (of the first span):
+   = note: inside `test_epoll_race` at RUSTLIB/core/src/macros/mod.rs:LL:CC
+note: inside `main`
+  --> tests/fail-dep/libc/libc-epoll-blocking.rs:LL:CC
+   |
+LL |     test_epoll_race();
+   |     ^^^^^^^^^^^^^^^^^
+   = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+