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a=openpgp-sha256; l=5412; i=aliceryhl@google.com; h=from:subject:message-id; bh=UusuKczZgpbZafCOu2SHAqb0ksf74QfeTn7LXrO42A0=; b=owEBbQKS/ZANAwAKAQRYvu5YxjlGAcsmYgBm5u/DJeMSoYje+0B76NnFNXI/hIns7KoesrsKS 9PwYauTUkWJAjMEAAEKAB0WIQSDkqKUTWQHCvFIvbIEWL7uWMY5RgUCZubvwwAKCRAEWL7uWMY5 RquqD/9TQ3EwSzosmOZT9P7R4KT0im5i2FrNhk8HJ76gol9DIi9UU4ScEWMCM+NFetVWf4RehPK LcXPxfYku7nxRS1Q0CpAKD1WXgbsImT3eD7/3DJ4wwgkMl6PXD6FSuNRb9dRmjAmjxK1Js5gG6b eJJ/LvGdExSCRFQ7h520CpWsagx324C7ZyuSn/xdJfaFMxCjYkxiVneQerAgqjuY5UKrqPQ+aOd B5EG3Hl7plv925Q7Ws7zDyAnT19mFD10H7Mj36M0C/AXt5VQbC4WmrxFt5iz+UPZVn0mT4uHRWq 2v8/pgihao9PgKIKBSJ9VvV8wOuwIpmZcY71mfndZSL3/0AY0UEQXXCg5ql7czStjxlbmTheNL8 8EUZ6qE3ycprb2DIg7GoLhPHblvUkFtPlp4874CNBcGyLHdfsvfvgcVDfnwt1q19ELxRZmC2zDg XZl7eKvZyXfQIH12aPJFCPzwGKOGxgNAEiBHCLXhIRAWiW336iLK9FH2d2wlkWTMA19/9HVN5Vk yvRkcHXR7Ksgd9z6DSqR0E7qBPfqtVa0X8EAOj6wsIGVRvRQ53oc4ghtapB6icS2v8MegGPHHKh y0QxlWRmLhKktA4SiVunzSA4WtQLGeJ0EqcADNPHXTxjg97pRfKBJCk34qJTlh0wYtEp+yaLIj1 VFIJRgUPq/BkiiQ== X-Mailer: b4 0.13.0 Message-ID: <20240915-alice-file-v10-6-88484f7a3dcf@google.com> Subject: [PATCH v10 6/8] rust: file: add `FileDescriptorReservation` From: Alice Ryhl To: Paul Moore , James Morris , "Serge E. Hallyn" , Miguel Ojeda , Christian Brauner Cc: Alex Gaynor , Wedson Almeida Filho , Boqun Feng , Gary Guo , " =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= " , Benno Lossin , Andreas Hindborg , Peter Zijlstra , Alexander Viro , Greg Kroah-Hartman , " =?utf-8?q?Arve_Hj=C3=B8?= =?utf-8?q?nnev=C3=A5g?= " , Todd Kjos , Martijn Coenen , Joel Fernandes , Carlos Llamas , Suren Baghdasaryan , Dan Williams , Matthew Wilcox , Thomas Gleixner , Daniel Xu , Martin Rodriguez Reboredo , Trevor Gross , linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, rust-for-linux@vger.kernel.org, linux-fsdevel@vger.kernel.org, Alice Ryhl , Kees Cook From: Wedson Almeida Filho 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 Co-developed-by: Alice Ryhl Reviewed-by: Benno Lossin Reviewed-by: Martin Rodriguez Reboredo Reviewed-by: Trevor Gross Reviewed-by: Gary Guo Signed-off-by: Alice Ryhl --- rust/kernel/fs/file.rs | 75 +++++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 74 insertions(+), 1 deletion(-) diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs index 3c1f51719804..e03dbe14d62a 100644 --- a/rust/kernel/fs/file.rs +++ b/rust/kernel/fs/file.rs @@ -11,7 +11,7 @@ bindings, cred::Credential, error::{code::*, Error, Result}, - types::{ARef, AlwaysRefCounted, Opaque}, + types::{ARef, AlwaysRefCounted, NotThreadSafe, Opaque}, }; use core::ptr; @@ -368,6 +368,79 @@ fn deref(&self) -> &LocalFile { } } +/// 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. + /// + /// The `fd_install` and `put_unused_fd` functions assume that the value of `current` is + /// unchanged since the call to `get_unused_fd_flags`. By adding this marker to this type, we + /// prevent it from being moved across task boundaries, which ensures that `current` does not + /// change while this value exists. + _not_send: NotThreadSafe, +} + +impl FileDescriptorReservation { + /// Creates a new file descriptor reservation. + pub fn get_unused_fd_flags(flags: u32) -> Result { + // 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 u32, + _not_send: NotThreadSafe, + }) + } + + /// 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 fd_install(self, file: ARef) { + // 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. + // + // 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. + // Additionally, the file is known to not have any non-shared `fdget_pos` calls, so even if + // this process starts using the file position, this will not result in a data race on the + // file position. + unsafe { bindings::fd_install(self.fd, file.as_ptr()) }; + + // `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: 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. + unsafe { bindings::put_unused_fd(self.fd) }; + } +} + /// Represents the `EBADF` error code. /// /// Used for methods that can only fail with `EBADF`.