Fix procfs's open_and_check_file to work in the presence of threads. (#1059)

* Fix `procfs`'s `open_and_check_file` to work in the presence of threads.

Have `open_and_check_file` create a new file descriptor before scanning
for directory entries, so that it doesn't share a directory position
with other threads.

Fixes #1050.

* Fix build errors on powerpc64-ibm-aix.

Author: sunfishcode

src/backend/libc/conv.rs

@@ -13,7 +13,7 @@ use crate::io;
 #[cfg(not(windows))]
 #[inline]
 pub(super) fn c_str(c: &CStr) -> *const c::c_char {
-    c.as_ptr()
+    c.as_ptr().cast()
 }
 
 #[cfg(not(any(windows, target_os = "espidf", target_os = "vita", target_os = "wasi")))]

src/backend/libc/net/read_sockaddr.rs

@@ -185,7 +185,9 @@ pub(crate) unsafe fn read_sockaddr(
                         return Err(io::Errno::INVAL);
                     }
                     debug_assert_eq!(
-                        CStr::from_ptr(decode.sun_path.as_ptr()).to_bytes().len(),
+                        CStr::from_ptr(decode.sun_path.as_ptr().cast())
+                            .to_bytes()
+                            .len(),
                         provided_len
                     );
                     &decode.sun_path[..provided_len]

src/procfs.rs

@@ -228,7 +228,8 @@ fn is_mountpoint(file: BorrowedFd<'_>) -> bool {
 fn proc_opendirat<P: crate::path::Arg, Fd: AsFd>(dirfd: Fd, path: P) -> io::Result<OwnedFd> {
     // We don't add `PATH` here because that disables `DIRECTORY`. And we don't
     // add `NOATIME` for the same reason as the comment in `open_and_check_file`.
-    let oflags = OFlags::NOFOLLOW | OFlags::DIRECTORY | OFlags::CLOEXEC | OFlags::NOCTTY;
+    let oflags =
+        OFlags::RDONLY | OFlags::NOFOLLOW | OFlags::DIRECTORY | OFlags::CLOEXEC | OFlags::NOCTTY;
     openat(dirfd, path, oflags, Mode::empty()).map_err(|_err| io::Errno::NOTSUP)
 }
 
@@ -488,8 +489,18 @@ fn open_and_check_file(
     let mut found_file = false;
     let mut found_dot = false;
 
+    // Open a new fd, so that if we're called on multiple threads, they don't
+    // share a seek position.
+    let oflags =
+        OFlags::RDONLY | OFlags::CLOEXEC | OFlags::NOFOLLOW | OFlags::NOCTTY | OFlags::DIRECTORY;
+    let dir = openat(dir, cstr!("."), oflags, Mode::empty()).map_err(|_err| io::Errno::NOTSUP)?;
+    let check_dir_stat = fstat(&dir)?;
+    if check_dir_stat.st_dev != dir_stat.st_dev || check_dir_stat.st_ino != dir_stat.st_ino {
+        return Err(io::Errno::NOTSUP);
+    }
+
     // Position the directory iteration at the start.
-    seek(dir, SeekFrom::Start(0))?;
+    seek(&dir, SeekFrom::Start(0))?;
 
     let mut buf = [MaybeUninit::uninit(); 2048];
     let mut iter = RawDir::new(dir, &mut buf);

tests/procfs/basic.rs

@@ -14,3 +14,20 @@ fn test_status_twice() {
     let fd = rustix::procfs::proc_self_status().unwrap();
     drop(fd);
 }
+
+#[test]
+fn parallel_self_proc_status() {
+    const THREADS: usize = 3;
+
+    fn self_proc_status() {
+        rustix::procfs::proc_self_status().expect("error getting proc/self/status pid");
+    }
+
+    let mut handles = Vec::with_capacity(THREADS);
+    for _ in 0..THREADS {
+        handles.push(std::thread::spawn(self_proc_status));
+    }
+    for handle in handles.drain(..) {
+        handle.join().expect("thread crashed");
+    }
+}