// SPDX-License-Identifier: GPL-2.0 // Copyright (C) 2024 Google LLC. //! Files and file descriptors.//!//! C headers: [`include/linux/fs.h`](srctree/include/linux/fs.h) and//! [`include/linux/file.h`](srctree/include/linux/file.h) use crate::{    bindings,    cred::Credential,    error::{code::*, Error, Result},    types::{ARef, AlwaysRefCounted, NotThreadSafe, Opaque},};use core::ptr; /// Flags associated with a [`File`].pub mod flags {    /// File is opened in append mode.    pub const O_APPEND: u32 = bindings::O_APPEND;     /// Signal-driven I/O is enabled.    pub const O_ASYNC: u32 = bindings::FASYNC;     /// Close-on-exec flag is set.    pub const O_CLOEXEC: u32 = bindings::O_CLOEXEC;     /// File was created if it didn't already exist.    pub const O_CREAT: u32 = bindings::O_CREAT;     /// Direct I/O is enabled for this file.    pub const O_DIRECT: u32 = bindings::O_DIRECT;     /// File must be a directory.    pub const O_DIRECTORY: u32 = bindings::O_DIRECTORY;     /// Like [`O_SYNC`] except metadata is not synced.    pub const O_DSYNC: u32 = bindings::O_DSYNC;     /// Ensure that this file is created with the `open(2)` call.    pub const O_EXCL: u32 = bindings::O_EXCL;     /// Large file size enabled (`off64_t` over `off_t`).    pub const O_LARGEFILE: u32 = bindings::O_LARGEFILE;     /// Do not update the file last access time.    pub const O_NOATIME: u32 = bindings::O_NOATIME;     /// File should not be used as process's controlling terminal.    pub const O_NOCTTY: u32 = bindings::O_NOCTTY;     /// If basename of path is a symbolic link, fail open.    pub const O_NOFOLLOW: u32 = bindings::O_NOFOLLOW;     /// File is using nonblocking I/O.    pub const O_NONBLOCK: u32 = bindings::O_NONBLOCK;     /// File is using nonblocking I/O.    ///    /// This is effectively the same flag as [`O_NONBLOCK`] on all architectures    /// except SPARC64.    pub const O_NDELAY: u32 = bindings::O_NDELAY;     /// Used to obtain a path file descriptor.    pub const O_PATH: u32 = bindings::O_PATH;     /// Write operations on this file will flush data and metadata.    pub const O_SYNC: u32 = bindings::O_SYNC;     /// This file is an unnamed temporary regular file.    pub const O_TMPFILE: u32 = bindings::O_TMPFILE;     /// File should be truncated to length 0.    pub const O_TRUNC: u32 = bindings::O_TRUNC;     /// Bitmask for access mode flags.    ///    /// # Examples    ///    /// ```    /// use kernel::fs::file;    /// # fn do_something() {}    /// # let flags = 0;    /// if (flags & file::flags::O_ACCMODE) == file::flags::O_RDONLY {    ///     do_something();    /// }    /// ```    pub const O_ACCMODE: u32 = bindings::O_ACCMODE;     /// File is read only.    pub const O_RDONLY: u32 = bindings::O_RDONLY;     /// File is write only.    pub const O_WRONLY: u32 = bindings::O_WRONLY;     /// File can be both read and written.    pub const O_RDWR: u32 = bindings::O_RDWR;} /// Wraps the kernel's `struct file`. Thread safe.////// This represents an open file rather than a file on a filesystem. Processes generally reference/// open files using file descriptors. However, file descriptors are not the same as files. A file/// descriptor is just an integer that corresponds to a file, and a single file may be referenced/// by multiple file descriptors.////// # Refcounting////// Instances of this type are reference-counted. The reference count is incremented by the/// `fget`/`get_file` functions and decremented by `fput`. The Rust type `ARef<File>` represents a/// pointer that owns a reference count on the file.////// Whenever a process opens a file descriptor (fd), it stores a pointer to the file in its fd/// table (`struct files_struct`). This pointer owns a reference count to the file, ensuring the/// file isn't prematurely deleted while the file descriptor is open. In Rust terminology, the/// pointers in `struct files_struct` are `ARef<File>` pointers.////// ## Light refcounts////// Whenever a process has an fd to a file, it may use something called a "light refcount" as a/// performance optimization. Light refcounts are acquired by calling `fdget` and released with/// `fdput`. The idea behind light refcounts is that if the fd is not closed between the calls to/// `fdget` and `fdput`, then the refcount cannot hit zero during that time, as the `struct/// files_struct` holds a reference until the fd is closed. This means that it's safe to access the/// file even if `fdget` does not increment the refcount.////// The requirement that the fd is not closed during a light refcount applies globally across all/// threads - not just on the thread using the light refcount. For this reason, light refcounts are/// only used when the `struct files_struct` is not shared with other threads, since this ensures/// that other unrelated threads cannot suddenly start using the fd and close it. Therefore,/// calling `fdget` on a shared `struct files_struct` creates a normal refcount instead of a light/// refcount.////// Light reference counts must be released with `fdput` before the system call returns to/// userspace. This means that if you wait until the current system call returns to userspace, then/// all light refcounts that existed at the time have gone away.////// ### The file position////// Each `struct file` has a position integer, which is protected by the `f_pos_lock` mutex./// However, if the `struct file` is not shared, then the kernel may avoid taking the lock as a/// performance optimization.////// The condition for avoiding the `f_pos_lock` mutex is different from the condition for using/// `fdget`. With `fdget`, you may avoid incrementing the refcount as long as the current fd table/// is not shared; it is okay if there are other fd tables that also reference the same `struct/// file`. However, `fdget_pos` can only avoid taking the `f_pos_lock` if the entire `struct file`/// is not shared, as different processes with an fd to the same `struct file` share the same/// position.////// To represent files that are not thread safe due to this optimization, the [`LocalFile`] type is/// used.////// ## Rust references////// The reference type `&File` is similar to light refcounts:////// * `&File` references don't own a reference count. They can only exist as long as the reference///   count stays positive, and can only be created when there is some mechanism in place to ensure///   this.////// * The Rust borrow-checker normally ensures this by enforcing that the `ARef<File>` from which///   a `&File` is created outlives the `&File`.////// * Using the unsafe [`File::from_raw_file`] means that it is up to the caller to ensure that the///   `&File` only exists while the reference count is positive.////// * You can think of `fdget` as using an fd to look up an `ARef<File>` in the `struct///   files_struct` and create an `&File` from it. The "fd cannot be closed" rule is like the Rust///   rule "the `ARef<File>` must outlive the `&File`".////// # Invariants////// * All instances of this type are refcounted using the `f_count` field./// * There must not be any active calls to `fdget_pos` on this file that did not take the///   `f_pos_lock` mutex.#[repr(transparent)]pub struct File {    inner: Opaque<bindings::file>,} // SAFETY: This file is known to not have any active `fdget_pos` calls that did not take the// `f_pos_lock` mutex, so it is safe to transfer it between threads.unsafe impl Send for File {} // SAFETY: This file is known to not have any active `fdget_pos` calls that did not take the// `f_pos_lock` mutex, so it is safe to access its methods from several threads in parallel.unsafe impl Sync for File {} // SAFETY: The type invariants guarantee that `File` is always ref-counted. This implementation// makes `ARef<File>` own a normal refcount.unsafe impl AlwaysRefCounted for File {    #[inline]    fn inc_ref(&self) {        // SAFETY: The existence of a shared reference means that the refcount is nonzero.        unsafe { bindings::get_file(self.as_ptr()) };    }     #[inline]    unsafe fn dec_ref(obj: ptr::NonNull<File>) {        // SAFETY: To call this method, the caller passes us ownership of a normal refcount, so we        // may drop it. The cast is okay since `File` has the same representation as `struct file`.        unsafe { bindings::fput(obj.cast().as_ptr()) }    }} /// Wraps the kernel's `struct file`. Not thread safe.////// This type represents a file that is not known to be safe to transfer across thread boundaries./// To obtain a thread-safe [`File`], use the [`assume_no_fdget_pos`] conversion.////// See the documentation for [`File`] for more information.////// # Invariants////// * All instances of this type are refcounted using the `f_count` field./// * If there is an active call to `fdget_pos` that did not take the `f_pos_lock` mutex, then it///   must be on the same thread as this file.////// [`assume_no_fdget_pos`]: LocalFile::assume_no_fdget_pospub struct LocalFile {    inner: Opaque<bindings::file>,} // SAFETY: The type invariants guarantee that `LocalFile` is always ref-counted. This implementation// makes `ARef<File>` own a normal refcount.unsafe impl AlwaysRefCounted for LocalFile {    #[inline]    fn inc_ref(&self) {        // SAFETY: The existence of a shared reference means that the refcount is nonzero.        unsafe { bindings::get_file(self.as_ptr()) };    }     #[inline]    unsafe fn dec_ref(obj: ptr::NonNull<LocalFile>) {        // SAFETY: To call this method, the caller passes us ownership of a normal refcount, so we        // may drop it. The cast is okay since `File` has the same representation as `struct file`.        unsafe { bindings::fput(obj.cast().as_ptr()) }    }} impl LocalFile {    /// Constructs a new `struct file` wrapper from a file descriptor.    ///    /// The file descriptor belongs to the current process, and there might be active local calls    /// to `fdget_pos` on the same file.    ///    /// To obtain an `ARef<File>`, use the [`assume_no_fdget_pos`] function to convert.    ///    /// [`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 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`.        //        // INVARIANT: This file is in the fd table on this thread, so either all `fdget_pos` calls        // are on this thread, or the file is shared, in which case `fdget_pos` calls took the        // `f_pos_lock` mutex.        Ok(unsafe { ARef::from_raw(ptr.cast()) })    }     /// Creates a reference to a [`LocalFile`] from a valid pointer.    ///    /// # Safety    ///    /// * The caller must ensure that `ptr` points at a valid file and that the file's refcount is    ///   positive for the duration of 'a.    /// * The caller must ensure that if there is an active call to `fdget_pos` that did not take    ///   the `f_pos_lock` mutex, then that call is on the current thread.    #[inline]    pub unsafe fn from_raw_file<'a>(ptr: *const bindings::file) -> &'a LocalFile {        // SAFETY: The caller guarantees that the pointer is not dangling and stays valid for the        // duration of 'a. The cast is okay because `File` is `repr(transparent)`.        //        // INVARIANT: The caller guarantees that there are no problematic `fdget_pos` calls.        unsafe { &*ptr.cast() }    }     /// Assume that there are no active `fdget_pos` calls that prevent us from sharing this file.    ///    /// This makes it safe to transfer this file to other threads. No checks are performed, and    /// using it incorrectly may lead to a data race on the file position if the file is shared    /// with another thread.    ///    /// This method is intended to be used together with [`LocalFile::fget`] when the caller knows    /// statically that there are no `fdget_pos` calls on the current thread. For example, you    /// might use it when calling `fget` from an ioctl, since ioctls usually do not touch the file    /// position.    ///    /// # Safety    ///    /// There must not be any active `fdget_pos` calls on the current thread.    #[inline]    pub unsafe fn assume_no_fdget_pos(me: ARef<LocalFile>) -> ARef<File> {        // INVARIANT: There are no `fdget_pos` calls on the current thread, and by the type        // invariants, if there is a `fdget_pos` call on another thread, then it took the        // `f_pos_lock` mutex.        //        // SAFETY: `LocalFile` and `File` have the same layout.        unsafe { ARef::from_raw(ARef::into_raw(me).cast()) }    }     /// Returns a raw pointer to the inner C struct.    #[inline]    pub fn as_ptr(&self) -> *mut bindings::file {        self.inner.get()    }     /// Returns the credentials of the task that originally opened the file.    pub fn cred(&self) -> &Credential {        // SAFETY: It's okay to read the `f_cred` field without synchronization because `f_cred` is        // never changed after initialization of the file.        let ptr = unsafe { (*self.as_ptr()).f_cred };         // SAFETY: The signature of this function ensures that the caller will only access the        // returned credential while the file is still valid, and the C side ensures that the        // credential stays valid at least as long as the file.        unsafe { Credential::from_ptr(ptr) }    }     /// Returns the flags associated with the file.    ///    /// The flags are a combination of the constants in [`flags`].    #[inline]    pub fn flags(&self) -> u32 {        // This `read_volatile` is intended to correspond to a READ_ONCE call.        //        // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.        //        // FIXME(read_once): Replace with `read_once` when available on the Rust side.        unsafe { core::ptr::addr_of!((*self.as_ptr()).f_flags).read_volatile() }    }} impl File {    /// Creates a reference to a [`File`] from a valid pointer.    ///    /// # Safety    ///    /// * The caller must ensure that `ptr` points at a valid file and that the file's refcount is    ///   positive for the duration of 'a.    /// * The caller must ensure that if there are active `fdget_pos` calls on this file, then they    ///   took the `f_pos_lock` mutex.    #[inline]    pub unsafe fn from_raw_file<'a>(ptr: *const bindings::file) -> &'a File {        // SAFETY: The caller guarantees that the pointer is not dangling and stays valid for the        // duration of 'a. The cast is okay because `File` is `repr(transparent)`.        //        // INVARIANT: The caller guarantees that there are no problematic `fdget_pos` calls.        unsafe { &*ptr.cast() }    }} // Make LocalFile methods available on File.impl core::ops::Deref for File {    type Target = LocalFile;    #[inline]    fn deref(&self) -> &LocalFile {        // SAFETY: The caller provides a `&File`, and since it is a reference, it must point at a        // valid file for the desired duration.        //        // By the type invariants, there are no `fdget_pos` calls that did not take the        // `f_pos_lock` mutex.        unsafe { LocalFile::from_raw_file(self as *const File as *const bindings::file) }    }} /// 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<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 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<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.        //        // 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`.#[derive(Copy, Clone, Eq, PartialEq)]pub struct BadFdError; impl From<BadFdError> for Error {    #[inline]    fn from(_: BadFdError) -> Error {        EBADF    }} impl core::fmt::Debug for BadFdError {    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {        f.pad("EBADF")    }}