[PATCH 4/7] rust: file: add `FileDescriptorReservation`

[Alice Ryhl <aliceryhl@google.com>]

From: Wedson Almeida Filho <wedsonaf@gmail.com>

Allow for the creation of a file descriptor in two steps: first, we
reserve a slot for it, then we commit or drop the reservation. The first
step may fail (e.g., the current process ran out of available slots),
but commit and drop never fail (and are mutually exclusive).

This is needed by Rust Binder when fds are sent from one process to
another. It has to be a two-step process to properly handle the case
where multiple fds are sent: The operation must fail or succeed
atomically, which we achieve by first reserving the fds we need, and
only installing the files once we have reserved enough fds to send the
files.

Fd reservations assume that the value of `current` does not change
between the call to get_unused_fd_flags and the call to fd_install (or
put_unused_fd). By not implementing the Send trait, this abstraction
ensures that the `FileDescriptorReservation` cannot be moved into a
different process.

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/file.rs | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 63 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/file.rs b/rust/kernel/file.rs
index f1f71c3d97e2..2186a6ea3f2f 100644
--- a/rust/kernel/file.rs
+++ b/rust/kernel/file.rs
@@ -11,7 +11,7 @@
     error::{code::*, Error, Result},
     types::{ARef, AlwaysRefCounted, Opaque},
 };
-use core::ptr;
+use core::{marker::PhantomData, ptr};
 
 /// Flags associated with a [`File`].
 pub mod flags {
@@ -180,6 +180,68 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
     }
 }
 
+/// A file descriptor reservation.
+///
+/// This allows the creation of a file descriptor in two steps: first, we reserve a slot for it,
+/// then we commit or drop the reservation. The first step may fail (e.g., the current process ran
+/// out of available slots), but commit and drop never fail (and are mutually exclusive).
+///
+/// Dropping the reservation happens in the destructor of this type.
+///
+/// # Invariants
+///
+/// The fd stored in this struct must correspond to a reserved file descriptor of the current task.
+pub struct FileDescriptorReservation {
+    fd: u32,
+    /// Prevent values of this type from being moved to a different task.
+    ///
+    /// This is necessary because the C FFI calls assume that `current` is set to the task that
+    /// owns the fd in question.
+    _not_send_sync: PhantomData<*mut ()>,
+}
+
+impl FileDescriptorReservation {
+    /// Creates a new file descriptor reservation.
+    pub fn new(flags: u32) -> Result<Self> {
+        // SAFETY: FFI call, there are no safety requirements on `flags`.
+        let fd: i32 = unsafe { bindings::get_unused_fd_flags(flags) };
+        if fd < 0 {
+            return Err(Error::from_errno(fd));
+        }
+        Ok(Self {
+            fd: fd as _,
+            _not_send_sync: PhantomData,
+        })
+    }
+
+    /// Returns the file descriptor number that was reserved.
+    pub fn reserved_fd(&self) -> u32 {
+        self.fd
+    }
+
+    /// 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.
+    pub fn commit(self, file: ARef<File>) {
+        // SAFETY: `self.fd` was previously returned by `get_unused_fd_flags`, and `file.ptr` is
+        // guaranteed to have an owned ref count by its type invariants.
+        unsafe { bindings::fd_install(self.fd, file.0.get()) };
+
+        // `fd_install` consumes both the file descriptor and the file reference, so we cannot run
+        // the destructors.
+        core::mem::forget(self);
+        core::mem::forget(file);
+    }
+}
+
+impl Drop for FileDescriptorReservation {
+    fn drop(&mut self) {
+        // SAFETY: `self.fd` was returned by a previous call to `get_unused_fd_flags`.
+        unsafe { bindings::put_unused_fd(self.fd) };
+    }
+}
+
 /// Represents the `EBADF` error code.
 ///
 /// Used for methods that can only fail with `EBADF`.

-- 
2.43.0.rc1.413.gea7ed67945-goog