On 18.01.24 15:36, Alice Ryhl wrote:
> + /// Schedule a task work that closes the file descriptor when this task returns to userspace.
> + ///
> + /// Fails if this is called from a context where we cannot run work when returning to
> + /// userspace. (E.g., from a kthread.)
> + pub fn close_fd(self, fd: u32) -> Result<(), DeferredFdCloseError> {
> + use bindings::task_work_notify_mode_TWA_RESUME as TWA_RESUME;
> +
> + // In this method, we schedule the task work before closing the file. This is because
> + // scheduling a task work is fallible, and we need to know whether it will fail before we
> + // attempt to close the file.
> +
> + // Task works are not available on kthreads.
> + let current = crate::current!();
> + if current.is_kthread() {
> + return Err(DeferredFdCloseError::TaskWorkUnavailable);
> + }
> +
> + // Transfer ownership of the box's allocation to a raw pointer. This disables the
> + // destructor, so we must manually convert it back to a Box to drop it.
> + //
> + // Until we convert it back to a `Box`, there are no aliasing requirements on this
> + // pointer.
> + let inner = Box::into_raw(self.inner);
> +
> + // The `callback_head` field is first in the struct, so this cast correctly gives us a
> + // pointer to the field.
> + let callback_head = inner.cast::<bindings::callback_head>();
> + // SAFETY: This pointer offset operation does not go out-of-bounds.
> + let file_field = unsafe { core::ptr::addr_of_mut!((*inner).file) };
> +
> + let current = current.as_raw();
> +
> + // SAFETY: This function currently has exclusive access to the `DeferredFdCloserInner`, so
> + // it is okay for us to perform unsynchronized writes to its `callback_head` field.
> + unsafe { bindings::init_task_work(callback_head, Some(Self::do_close_fd)) };
> +
> + // SAFETY: This inserts the `DeferredFdCloserInner` into the task workqueue for the current
> + // task. If this operation is successful, then this transfers exclusive ownership of the
> + // `callback_head` field to the C side until it calls `do_close_fd`, and we don't touch or
> + // invalidate the field during that time.
> + //
> + // When the C side calls `do_close_fd`, the safety requirements of that method are
> + // satisfied because when a task work is executed, the callback is given ownership of the
> + // pointer.
> + //
> + // The file pointer is currently null. If it is changed to be non-null before `do_close_fd`
> + // is called, then that change happens due to the write at the end of this function, and
> + // that write has a safety comment that explains why the refcount can be dropped when
> + // `do_close_fd` runs.
> + let res = unsafe { bindings::task_work_add(current, callback_head, TWA_RESUME) };
> +
> + if res != 0 {
> + // SAFETY: Scheduling the task work failed, so we still have ownership of the box, so
> + // we may destroy it.
> + unsafe { drop(Box::from_raw(inner)) };
> +
> + return Err(DeferredFdCloseError::TaskWorkUnavailable);
> + }
> +
> + // SAFETY: This is safe no matter what `fd` is. If the `fd` is valid (that is, if the
> + // pointer is non-null), then we call `filp_close` on the returned pointer as required by
> + // `close_fd_get_file`.
> + let file = unsafe { bindings::close_fd_get_file(fd) };
> + if file.is_null() {
> + // We don't clean up the task work since that might be expensive if the task work queue
> + // is long. Just let it execute and let it clean up for itself.
> + return Err(DeferredFdCloseError::BadFd);
> + }
> +
> + // Acquire a refcount to the file.
> + //
> + // SAFETY: The `file` pointer points at a file with a non-zero refcount.
> + unsafe { bindings::get_file(file) };
> +
> + // SAFETY: The `file` pointer is valid. Passing `current->files` as the file table to close
> + // it in is correct, since we just got the `fd` from `close_fd_get_file` which also uses
> + // `current->files`.
> + //
> + // This method closes the fd. There could be active light refcounts created from that fd,
> + // so we must ensure that the file has a positive refcount for the duration of those active
> + // light refcounts.
This doesn't seem to be part of the SAFETY comment, so maybe move
this comment above it?
> + //
> + // Note: fl_owner_t is currently a void pointer.
> + unsafe { bindings::filp_close(file, (*current).files as bindings::fl_owner_t) };
> +
> + // We update the file pointer that the task work is supposed to fput. This transfers
> + // ownership of our last refcount.
I think it is very good that you mention how many refcounts you have
here, but I am missing that in the code above. IIRC `closed_fd_get_file`
acquires a refcount and `filp_close` consumes one.
> + //
> + // INVARIANT: This changes the `file` field of a `DeferredFdCloserInner` from null to
> + // non-null. This doesn't break the type invariant for `DeferredFdCloserInner` because we
> + // still own a refcount to the file, so we can pass ownership of that refcount to the
> + // `DeferredFdCloserInner`.
> + //
> + // SAFETY: Task works are executed on the current thread right before we return to
> + // userspace, so this write is guaranteed to happen before `do_close_fd` is called, which
> + // means that a race is not possible here.
> + //
> + // When `do_close_fd` runs, it must be safe for it to `fput` the refcount. However, this is
> + // the case because all light refcounts that are associated with the fd we closed
> + // previously must be dropped when `do_close_fd`, since light refcounts must be dropped
> + // before returning to userspace.
This also doesn't seem to be part of the SAFETY comment.
--
Cheers,
Benno
> + unsafe { *file_field = file };
> +
> + Ok(())
> + }
> +
> + /// # Safety
> + ///
> + /// The provided pointer must point at the `twork` field of a `DeferredFdCloserInner` stored in
> + /// a `Box`, and the caller must pass exclusive ownership of that `Box`. Furthermore, if the
> + /// file pointer is non-null, then it must be okay to release the refcount by calling `fput`.
> + unsafe extern "C" fn do_close_fd(inner: *mut bindings::callback_head) {
> + // SAFETY: The caller just passed us ownership of this box.
> + let inner = unsafe { Box::from_raw(inner.cast::<DeferredFdCloserInner>()) };
> + if !inner.file.is_null() {
> + // SAFETY: By the type invariants, we own a refcount to this file, and the caller
> + // guarantees that dropping the refcount now is okay.
> + unsafe { bindings::fput(inner.file) };
> + }
> + // The allocation is freed when `inner` goes out of scope.
> + }
> +}
