Re: [PATCH] rust: file: handle fd table teardown in file descriptor APIs

["Gary Guo" <gary@garyguo.net>]

On Sun Sep 20, 2026 at 9:01 PM BST, Georgios Androutsopoulos wrote:
> Several Rust file descriptor APIs rely on `current->files` being
> available. However, `exit_files()` clears it while execution may still
> continue on the same task.
>
> This affects `LocalFile::fget()` and the
> `FileDescriptorReservation` operations that call
> `get_unused_fd_flags()`, `fd_install()`, and `put_unused_fd()`.
> `FileDescriptorReservation` cannot cross task boundaries, but remaining
> on the same task does not guarantee that `current->files` is still
> available when these operations are performed.
>
> Guard the affected operations against a missing `current->files`.
> `LocalFile::fget()` returns `EBADF` and
> `FileDescriptorReservation::get_unused_fd_flags()` returns `EMFILE`.
> For the infallible `fd_install()` and drop paths, warn once and avoid
> calling the corresponding C helper when the fd table is already gone.
>
> This prevents NULL dereferences through these safe Rust APIs after
> `exit_files()`.

I suppose we could add these checks to C side instead, although perhaps one may
say "its bad caller code and not worth checking"?

So having these checks on Rust side is okay to me.

>
> Fixes: 851849824bb5 ("rust: file: add Rust abstraction for `struct file`")
> Fixes: 5da9857b127e ("rust: file: add `FileDescriptorReservation`")
> Closes: https://github.com/Rust-for-Linux/linux/issues/1256
> Signed-off-by: Georgios Androutsopoulos <georgeandrout13@gmail.com>
> ---
>  rust/kernel/fs/file.rs | 69 ++++++++++++++++++++++++++++++++++++------
>  1 file changed, 60 insertions(+), 9 deletions(-)
>
> diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
> index 23ee689bd240..559f0985b12c 100644
> --- a/rust/kernel/fs/file.rs
> +++ b/rust/kernel/fs/file.rs
> @@ -260,7 +260,17 @@ impl LocalFile {
>      /// [`assume_no_fdget_pos`]: LocalFile::assume_no_fdget_pos
>      #[inline]
>      pub fn fget(fd: u32) -> Result<ARef<LocalFile>, BadFdError> {
> -        // SAFETY: FFI call, there are no requirements on `fd`.
> +        let current = crate::current!();
> +
> +        // SAFETY: `current` points to the currently executing task, so it is
> +        // valid to read its `files` pointer. The pointer may be null during
> +        // task teardown.
> +        if unsafe { (*current.as_ptr()).files.is_null() } {
> +            return Err(BadFdError);
> +        }

I think we want to add `unlikely()` on them (which is being added by
https://lore.kernel.org/rust-for-linux/20260406095820.465994-2-ojeda@kernel.org/).

> +
> +        // SAFETY: There are no requirements on `fd`. We checked above that the
> +        // current task still has a file descriptor table, which `fget` accesses.
>          let ptr = ptr::NonNull::new(unsafe { bindings::fget(fd) }).ok_or(BadFdError)?;
>  
>          // SAFETY: `bindings::fget` created a refcount, and we pass ownership of it to the `ARef`.
> @@ -403,7 +413,18 @@ impl FileDescriptorReservation {
>      /// Creates a new file descriptor reservation.
>      #[inline]
>      pub fn get_unused_fd_flags(flags: u32) -> Result<Self> {
> -        // SAFETY: FFI call, there are no safety requirements on `flags`.
> +        let current = crate::current!();
> +
> +        // SAFETY: `current` points to the currently executing task, so it is
> +        // valid to read its `files` pointer. The pointer may be null during
> +        // task teardown.
> +        if unsafe { (*current.as_ptr()).files.is_null() } {
> +            return Err(EMFILE);
> +        }
> +
> +        // SAFETY: There are no safety requirements on `flags`. We checked above
> +        // that the current task still has a file descriptor table, which
> +        // `get_unused_fd_flags` accesses.
>          let fd: i32 = unsafe { bindings::get_unused_fd_flags(flags) };
>          to_result(fd)?;
>  
> @@ -421,13 +442,30 @@ pub fn reserved_fd(&self) -> u32 {
>  
>      /// Commits the reservation.
>      ///
> -    /// The previously reserved file descriptor is bound to `file`. This method consumes the
> -    /// [`FileDescriptorReservation`], so it will not be usable after this call.
> +    /// The previously reserved file descriptor is bound to `file`. If the current task no longer
> +    /// has a file descriptor table, the reservation is abandoned instead. This method consumes the
> +    /// [`FileDescriptorReservation`] in either case.
>      #[inline]
>      pub fn fd_install(self, file: ARef<File>) {
> -        // SAFETY: `self.fd` was previously returned by `get_unused_fd_flags`. We have not yet used
> -        // the fd, so it is still valid, and `current` still refers to the same task, as this type
> -        // cannot be moved across task boundaries.
> +        let current = crate::current!();
> +
> +        // SAFETY: `current` points to the currently executing task, so it is
> +        // valid to read its `files` pointer. The pointer may be null during
> +        // task teardown.
> +        if unsafe { (*current.as_ptr()).files.is_null() } {
> +            crate::pr_warn_once!(
> +                "FileDescriptorReservation::fd_install called with current->files == NULL\n"
> +            );

I wonder if we should upgrade this to `WARN_ONCE`. As code being executed when
exiting are cleanup code, for this code path to be hit, it would mean that some
code is installing FD descriptor while being dropped -- which is likely a bug.

Putting a "BTW, some Rust code is installing a FD when process is exiting" in
dmesg is not going to be useful to understand what's going on. We'd want a full
backtrace.

On the other hand, dropping a `FileDescriptorReservation` is a more realistic,
so we perhaps might even want to declare it being okay (see below).

> +
> +            // `put_unused_fd` also requires `current->files` to be valid, so do not run
> +            // the reservation's destructor after the current task has lost its fd table.
> +            core::mem::forget(self);
> +            return;
> +        }
> +
> +        // SAFETY: `self.fd` was previously returned by `get_unused_fd_flags` and has not yet been
> +        // used. This type cannot be moved across task boundaries, so `current` still refers to the
> +        // same task, and we checked above that it still has an fd table.
>          //
>          // Furthermore, the file pointer is guaranteed to own a refcount by its type invariants,
>          // and we take ownership of that refcount by not running the destructor below.
> @@ -446,9 +484,22 @@ pub fn fd_install(self, file: ARef<File>) {
>  impl Drop for FileDescriptorReservation {
>      #[inline]
>      fn drop(&mut self) {
> +        let current = crate::current!();
> +
> +        // SAFETY: `current` points to the currently executing task, so it is
> +        // valid to read its `files` pointer. The pointer may be null during
> +        // task teardown.
> +        if unsafe { (*current.as_ptr()).files.is_null() } {
> +            crate::pr_warn_once!(
> +                "FileDescriptorReservation dropped with current->files == NULL\n"
> +            );

I think we can remove this warning. Skipping put_unused_fd isn't actually
leaking anything as the files_struct is cleaned up.

Best,
Gary


> +            return;
> +        }
> +
>          // SAFETY: By the type invariants of this type, `self.fd` was previously returned by
> -        // `get_unused_fd_flags`. We have not yet used the fd, so it is still valid, and `current`
> -        // still refers to the same task, as this type cannot be moved across task boundaries.
> +        // `get_unused_fd_flags` and has not yet been used. This type cannot be moved across task
> +        // boundaries, so `current` still refers to the same task, and we checked above that it
> +        // still has an fd table.
>          unsafe { bindings::put_unused_fd(self.fd) };
>      }
>  }