Re: [PATCH v2 6/7] rust: file: add `DeferredFdCloser`

[Benno Lossin <benno.lossin@proton.me>]

On 12/6/23 12:59, 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.
> +
> +        // SAFETY: Getting a pointer to current is always safe.
> +        let current = unsafe { bindings::get_current() };
> +
> +        // SAFETY: Accessing the `flags` field of `current` is always safe.
> +        let is_kthread = (unsafe { (*current).flags } & bindings::PF_KTHREAD) != 0;

Since Boqun brought to my attention that we already have a wrapper for
`get_current()`, how about you use it here as well?

> +        if is_kthread {
> +            return Err(DeferredFdCloseError::TaskWorkUnavailable);
> +        }
> +
> +        // This disables the destructor of the box, so the allocation is not cleaned up
> +        // automatically below.
> +        let inner = Box::into_raw(self.inner);

Importantly this also lifts the uniqueness requirement (maybe add this
to the comment?).

> +
> +        // 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) };
> +
> +        // SAFETY: The `callback_head` pointer is compatible with the `do_close_fd` method.

Also, `callback_head` is valid, since it is derived from...

> +        unsafe { bindings::init_task_work(callback_head, Some(Self::do_close_fd)) };
> +        // SAFETY: The `callback_head` pointer points at a valid and fully initialized task work
> +        // that is ready to be scheduled.
> +        //
> +        // If the task work gets scheduled as-is, then it will be a no-op. However, we will update
> +        // the file pointer below to tell it which file to fput.
> +        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);

Just curious, what could make the `task_work_add` call fail? I imagine
an OOM situation, but is that all?

> +        }
> +
> +        // SAFETY: Just an FFI call. This is safe no matter what `fd` is.

I took a look at the C code and there I found this comment:

    /*
     * variant of close_fd that gets a ref on the file for later fput.
     * The caller must ensure that filp_close() called on the file.
     */

And while you do call `filp_close` later, this seems like a safety
requirement to me.
Also, you do not call it when `file` is null, which I imagine to be
fine, but I do not know that since the C comment does not cover that
case.

> +        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);
> +        }
> +
> +        // SAFETY: The `file` pointer points at a valid file.
> +        unsafe { bindings::get_file(file) };
> +
> +        // SAFETY: Due to the above `get_file`, even if the current task holds an `fdget` to
> +        // this file right now, the refcount will not drop to zero until after it is released
> +        // with `fdput`. This is because when using `fdget`, you must always use `fdput` before

Shouldn't this be "the refcount will not drop to zero until after it is
released with `fput`."?

Why is this the SAFETY comment for `filp_close`? I am not understanding
the requirement on that function that needs this. This seems more a
justification for accessing `file` inside `do_close_fd`. In which case I
think it would be better to make it a type invariant of
`DeferredFdCloserInner`.

> +        // returning to userspace, and our task work runs after any `fdget` users have returned
> +        // to userspace.
> +        //
> +        // 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.
> +        //
> +        // SAFETY: Task works are executed on the current thread once 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.
> +        //
> +        // It's okay to pass this pointer to the task work, since we just acquired a refcount with
> +        // the previous call to `get_file`. Furthermore, the refcount will not drop to zero during
> +        // an `fdget` call, since we defer the `fput` until after returning to userspace.
> +        unsafe { *file_field = file };

A synchronization question: who guarantees that this write is actually
available to the cpu that executes `do_close_fd`? Is there some
synchronization run when returning to userspace?

> +
> +        Ok(())
> +    }
> +
> +    // SAFETY: This function is an implementation detail of `close_fd`, so its safety comments
> +    // should be read in extension of that method.

Why not use this?:
- `inner` is a valid pointer to the `callback_head` field of a valid
  `DeferredFdCloserInner`.
- `inner` has exclusive access to the pointee and owns the allocation.
- `inner` originates from a call to `Box::into_raw`.

> +    unsafe extern "C" fn do_close_fd(inner: *mut bindings::callback_head) {
> +        // SAFETY: In `close_fd` we use this method together with a pointer that originates from a
> +        // `Box<DeferredFdCloserInner>`, and we have just been given ownership of that allocation.
> +        let inner = unsafe { Box::from_raw(inner as *mut DeferredFdCloserInner) };

Use `.cast()`.

> +        if !inner.file.is_null() {
> +            // SAFETY: This drops a refcount we acquired in `close_fd`. Since this callback runs in
> +            // a task work after we return to userspace, it is guaranteed that the current thread
> +            // doesn't hold this file with `fdget`, as `fdget` must be released before returning to
> +            // userspace.
> +            unsafe { bindings::fput(inner.file) };
> +        }
> +        // Free the allocation.
> +        drop(inner);
> +    }
> +}
> +
> +/// Represents a failure to close an fd in a deferred manner.
> +#[derive(Copy, Clone, Eq, PartialEq)]
> +pub enum DeferredFdCloseError {
> +    /// Closing the fd failed because we were unable to schedule a task work.
> +    TaskWorkUnavailable,
> +    /// Closing the fd failed because the fd does not exist.
> +    BadFd,
> +}
> +
> +impl From<DeferredFdCloseError> for Error {
> +    fn from(err: DeferredFdCloseError) -> Error {
> +        match err {
> +            DeferredFdCloseError::TaskWorkUnavailable => ESRCH,

This error reads "No such process", I am not sure if that is the best
way to express the problem in that situation. I took a quick look at the
other error codes, but could not find a better fit. Do you have any
better ideas? Or is this the error that C binder uses?

-- 
Cheers,
Benno

> +            DeferredFdCloseError::BadFd => EBADF,
> +        }
> +    }
> +}