kernel/fs/file.rs
1// SPDX-License-Identifier: GPL-2.0
2
3// Copyright (C) 2024 Google LLC.
4
5//! Files and file descriptors.
6//!
7//! C headers: [`include/linux/fs.h`](srctree/include/linux/fs.h) and
8//! [`include/linux/file.h`](srctree/include/linux/file.h)
9
10use crate::{
11 bindings,
12 cred::Credential,
13 error::{code::*, to_result, Error, Result},
14 fmt,
15 sync::aref::{ARef, AlwaysRefCounted},
16 types::{NotThreadSafe, Opaque},
17};
18use core::ptr;
19
20/// Flags associated with a [`File`].
21pub mod flags {
22 /// File is opened in append mode.
23 pub const O_APPEND: u32 = bindings::O_APPEND;
24
25 /// Signal-driven I/O is enabled.
26 pub const O_ASYNC: u32 = bindings::FASYNC;
27
28 /// Close-on-exec flag is set.
29 pub const O_CLOEXEC: u32 = bindings::O_CLOEXEC;
30
31 /// File was created if it didn't already exist.
32 pub const O_CREAT: u32 = bindings::O_CREAT;
33
34 /// Direct I/O is enabled for this file.
35 pub const O_DIRECT: u32 = bindings::O_DIRECT;
36
37 /// File must be a directory.
38 pub const O_DIRECTORY: u32 = bindings::O_DIRECTORY;
39
40 /// Like [`O_SYNC`] except metadata is not synced.
41 pub const O_DSYNC: u32 = bindings::O_DSYNC;
42
43 /// Ensure that this file is created with the `open(2)` call.
44 pub const O_EXCL: u32 = bindings::O_EXCL;
45
46 /// Large file size enabled (`off64_t` over `off_t`).
47 pub const O_LARGEFILE: u32 = bindings::O_LARGEFILE;
48
49 /// Do not update the file last access time.
50 pub const O_NOATIME: u32 = bindings::O_NOATIME;
51
52 /// File should not be used as process's controlling terminal.
53 pub const O_NOCTTY: u32 = bindings::O_NOCTTY;
54
55 /// If basename of path is a symbolic link, fail open.
56 pub const O_NOFOLLOW: u32 = bindings::O_NOFOLLOW;
57
58 /// File is using nonblocking I/O.
59 pub const O_NONBLOCK: u32 = bindings::O_NONBLOCK;
60
61 /// File is using nonblocking I/O.
62 ///
63 /// This is effectively the same flag as [`O_NONBLOCK`] on all architectures
64 /// except SPARC64.
65 pub const O_NDELAY: u32 = bindings::O_NDELAY;
66
67 /// Used to obtain a path file descriptor.
68 pub const O_PATH: u32 = bindings::O_PATH;
69
70 /// Write operations on this file will flush data and metadata.
71 pub const O_SYNC: u32 = bindings::O_SYNC;
72
73 /// This file is an unnamed temporary regular file.
74 pub const O_TMPFILE: u32 = bindings::O_TMPFILE;
75
76 /// File should be truncated to length 0.
77 pub const O_TRUNC: u32 = bindings::O_TRUNC;
78
79 /// Bitmask for access mode flags.
80 ///
81 /// # Examples
82 ///
83 /// ```
84 /// use kernel::fs::file;
85 /// # fn do_something() {}
86 /// # let flags = 0;
87 /// if (flags & file::flags::O_ACCMODE) == file::flags::O_RDONLY {
88 /// do_something();
89 /// }
90 /// ```
91 pub const O_ACCMODE: u32 = bindings::O_ACCMODE;
92
93 /// File is read only.
94 pub const O_RDONLY: u32 = bindings::O_RDONLY;
95
96 /// File is write only.
97 pub const O_WRONLY: u32 = bindings::O_WRONLY;
98
99 /// File can be both read and written.
100 pub const O_RDWR: u32 = bindings::O_RDWR;
101}
102
103/// Wraps the kernel's `struct file`. Thread safe.
104///
105/// This represents an open file rather than a file on a filesystem. Processes generally reference
106/// open files using file descriptors. However, file descriptors are not the same as files. A file
107/// descriptor is just an integer that corresponds to a file, and a single file may be referenced
108/// by multiple file descriptors.
109///
110/// # Refcounting
111///
112/// Instances of this type are reference-counted. The reference count is incremented by the
113/// `fget`/`get_file` functions and decremented by `fput`. The Rust type `ARef<File>` represents a
114/// pointer that owns a reference count on the file.
115///
116/// Whenever a process opens a file descriptor (fd), it stores a pointer to the file in its fd
117/// table (`struct files_struct`). This pointer owns a reference count to the file, ensuring the
118/// file isn't prematurely deleted while the file descriptor is open. In Rust terminology, the
119/// pointers in `struct files_struct` are `ARef<File>` pointers.
120///
121/// ## Light refcounts
122///
123/// Whenever a process has an fd to a file, it may use something called a "light refcount" as a
124/// performance optimization. Light refcounts are acquired by calling `fdget` and released with
125/// `fdput`. The idea behind light refcounts is that if the fd is not closed between the calls to
126/// `fdget` and `fdput`, then the refcount cannot hit zero during that time, as the `struct
127/// files_struct` holds a reference until the fd is closed. This means that it's safe to access the
128/// file even if `fdget` does not increment the refcount.
129///
130/// The requirement that the fd is not closed during a light refcount applies globally across all
131/// threads - not just on the thread using the light refcount. For this reason, light refcounts are
132/// only used when the `struct files_struct` is not shared with other threads, since this ensures
133/// that other unrelated threads cannot suddenly start using the fd and close it. Therefore,
134/// calling `fdget` on a shared `struct files_struct` creates a normal refcount instead of a light
135/// refcount.
136///
137/// Light reference counts must be released with `fdput` before the system call returns to
138/// userspace. This means that if you wait until the current system call returns to userspace, then
139/// all light refcounts that existed at the time have gone away.
140///
141/// ### The file position
142///
143/// Each `struct file` has a position integer, which is protected by the `f_pos_lock` mutex.
144/// However, if the `struct file` is not shared, then the kernel may avoid taking the lock as a
145/// performance optimization.
146///
147/// The condition for avoiding the `f_pos_lock` mutex is different from the condition for using
148/// `fdget`. With `fdget`, you may avoid incrementing the refcount as long as the current fd table
149/// is not shared; it is okay if there are other fd tables that also reference the same `struct
150/// file`. However, `fdget_pos` can only avoid taking the `f_pos_lock` if the entire `struct file`
151/// is not shared, as different processes with an fd to the same `struct file` share the same
152/// position.
153///
154/// To represent files that are not thread safe due to this optimization, the [`LocalFile`] type is
155/// used.
156///
157/// ## Rust references
158///
159/// The reference type `&File` is similar to light refcounts:
160///
161/// * `&File` references don't own a reference count. They can only exist as long as the reference
162/// count stays positive, and can only be created when there is some mechanism in place to ensure
163/// this.
164///
165/// * The Rust borrow-checker normally ensures this by enforcing that the `ARef<File>` from which
166/// a `&File` is created outlives the `&File`.
167///
168/// * Using the unsafe [`File::from_raw_file`] means that it is up to the caller to ensure that the
169/// `&File` only exists while the reference count is positive.
170///
171/// * You can think of `fdget` as using an fd to look up an `ARef<File>` in the `struct
172/// files_struct` and create an `&File` from it. The "fd cannot be closed" rule is like the Rust
173/// rule "the `ARef<File>` must outlive the `&File`".
174///
175/// # Invariants
176///
177/// * All instances of this type are refcounted using the `f_count` field.
178/// * There must not be any active calls to `fdget_pos` on this file that did not take the
179/// `f_pos_lock` mutex.
180#[repr(transparent)]
181pub struct File {
182 inner: Opaque<bindings::file>,
183}
184
185// SAFETY: This file is known to not have any active `fdget_pos` calls that did not take the
186// `f_pos_lock` mutex, so it is safe to transfer it between threads.
187unsafe impl Send for File {}
188
189// SAFETY: This file is known to not have any active `fdget_pos` calls that did not take the
190// `f_pos_lock` mutex, so it is safe to access its methods from several threads in parallel.
191unsafe impl Sync for File {}
192
193// SAFETY: The type invariants guarantee that `File` is always ref-counted. This implementation
194// makes `ARef<File>` own a normal refcount.
195unsafe impl AlwaysRefCounted for File {
196 #[inline]
197 fn inc_ref(&self) {
198 // SAFETY: The existence of a shared reference means that the refcount is nonzero.
199 unsafe { bindings::get_file(self.as_ptr()) };
200 }
201
202 #[inline]
203 unsafe fn dec_ref(obj: ptr::NonNull<File>) {
204 // SAFETY: To call this method, the caller passes us ownership of a normal refcount, so we
205 // may drop it. The cast is okay since `File` has the same representation as `struct file`.
206 unsafe { bindings::fput(obj.cast().as_ptr()) }
207 }
208}
209
210/// Wraps the kernel's `struct file`. Not thread safe.
211///
212/// This type represents a file that is not known to be safe to transfer across thread boundaries.
213/// To obtain a thread-safe [`File`], use the [`assume_no_fdget_pos`] conversion.
214///
215/// See the documentation for [`File`] for more information.
216///
217/// # Invariants
218///
219/// * All instances of this type are refcounted using the `f_count` field.
220/// * If there is an active call to `fdget_pos` that did not take the `f_pos_lock` mutex, then it
221/// must be on the same thread as this file.
222///
223/// [`assume_no_fdget_pos`]: LocalFile::assume_no_fdget_pos
224#[repr(transparent)]
225pub struct LocalFile {
226 inner: Opaque<bindings::file>,
227}
228
229// SAFETY: The type invariants guarantee that `LocalFile` is always ref-counted. This implementation
230// makes `ARef<LocalFile>` own a normal refcount.
231unsafe impl AlwaysRefCounted for LocalFile {
232 #[inline]
233 fn inc_ref(&self) {
234 // SAFETY: The existence of a shared reference means that the refcount is nonzero.
235 unsafe { bindings::get_file(self.as_ptr()) };
236 }
237
238 #[inline]
239 unsafe fn dec_ref(obj: ptr::NonNull<LocalFile>) {
240 // SAFETY: To call this method, the caller passes us ownership of a normal refcount, so we
241 // may drop it. The cast is okay since `LocalFile` has the same representation as
242 // `struct file`.
243 unsafe { bindings::fput(obj.cast().as_ptr()) }
244 }
245}
246
247impl LocalFile {
248 /// Constructs a new `struct file` wrapper from a file descriptor.
249 ///
250 /// The file descriptor belongs to the current process, and there might be active local calls
251 /// to `fdget_pos` on the same file.
252 ///
253 /// To obtain an `ARef<File>`, use the [`assume_no_fdget_pos`] function to convert.
254 ///
255 /// [`assume_no_fdget_pos`]: LocalFile::assume_no_fdget_pos
256 #[inline]
257 pub fn fget(fd: u32) -> Result<ARef<LocalFile>, BadFdError> {
258 // SAFETY: FFI call, there are no requirements on `fd`.
259 let ptr = ptr::NonNull::new(unsafe { bindings::fget(fd) }).ok_or(BadFdError)?;
260
261 // SAFETY: `bindings::fget` created a refcount, and we pass ownership of it to the `ARef`.
262 //
263 // INVARIANT: This file is in the fd table on this thread, so either all `fdget_pos` calls
264 // are on this thread, or the file is shared, in which case `fdget_pos` calls took the
265 // `f_pos_lock` mutex.
266 Ok(unsafe { ARef::from_raw(ptr.cast()) })
267 }
268
269 /// Creates a reference to a [`LocalFile`] from a valid pointer.
270 ///
271 /// # Safety
272 ///
273 /// * The caller must ensure that `ptr` points at a valid file and that the file's refcount is
274 /// positive for the duration of `'a`.
275 /// * The caller must ensure that if there is an active call to `fdget_pos` that did not take
276 /// the `f_pos_lock` mutex, then that call is on the current thread.
277 #[inline]
278 pub unsafe fn from_raw_file<'a>(ptr: *const bindings::file) -> &'a LocalFile {
279 // SAFETY: The caller guarantees that the pointer is not dangling and stays valid for the
280 // duration of `'a`. The cast is okay because `LocalFile` is `repr(transparent)`.
281 //
282 // INVARIANT: The caller guarantees that there are no problematic `fdget_pos` calls.
283 unsafe { &*ptr.cast() }
284 }
285
286 /// Assume that there are no active `fdget_pos` calls that prevent us from sharing this file.
287 ///
288 /// This makes it safe to transfer this file to other threads. No checks are performed, and
289 /// using it incorrectly may lead to a data race on the file position if the file is shared
290 /// with another thread.
291 ///
292 /// This method is intended to be used together with [`LocalFile::fget`] when the caller knows
293 /// statically that there are no `fdget_pos` calls on the current thread. For example, you
294 /// might use it when calling `fget` from an ioctl, since ioctls usually do not touch the file
295 /// position.
296 ///
297 /// # Safety
298 ///
299 /// There must not be any active `fdget_pos` calls on the current thread.
300 #[inline]
301 pub unsafe fn assume_no_fdget_pos(me: ARef<LocalFile>) -> ARef<File> {
302 // INVARIANT: There are no `fdget_pos` calls on the current thread, and by the type
303 // invariants, if there is a `fdget_pos` call on another thread, then it took the
304 // `f_pos_lock` mutex.
305 //
306 // SAFETY: `LocalFile` and `File` have the same layout.
307 unsafe { ARef::from_raw(ARef::into_raw(me).cast()) }
308 }
309
310 /// Returns a raw pointer to the inner C struct.
311 #[inline]
312 pub fn as_ptr(&self) -> *mut bindings::file {
313 self.inner.get()
314 }
315
316 /// Returns the credentials of the task that originally opened the file.
317 pub fn cred(&self) -> &Credential {
318 // SAFETY: It's okay to read the `f_cred` field without synchronization because `f_cred` is
319 // never changed after initialization of the file.
320 let ptr = unsafe { (*self.as_ptr()).f_cred };
321
322 // SAFETY: The signature of this function ensures that the caller will only access the
323 // returned credential while the file is still valid, and the C side ensures that the
324 // credential stays valid at least as long as the file.
325 unsafe { Credential::from_ptr(ptr) }
326 }
327
328 /// Returns the flags associated with the file.
329 ///
330 /// The flags are a combination of the constants in [`flags`].
331 #[inline]
332 pub fn flags(&self) -> u32 {
333 // This `read_volatile` is intended to correspond to a READ_ONCE call.
334 //
335 // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
336 //
337 // FIXME(read_once): Replace with `read_once` when available on the Rust side.
338 unsafe { core::ptr::addr_of!((*self.as_ptr()).f_flags).read_volatile() }
339 }
340}
341
342impl File {
343 /// Creates a reference to a [`File`] from a valid pointer.
344 ///
345 /// # Safety
346 ///
347 /// * The caller must ensure that `ptr` points at a valid file and that the file's refcount is
348 /// positive for the duration of `'a`.
349 /// * The caller must ensure that if there are active `fdget_pos` calls on this file, then they
350 /// took the `f_pos_lock` mutex.
351 #[inline]
352 pub unsafe fn from_raw_file<'a>(ptr: *const bindings::file) -> &'a File {
353 // SAFETY: The caller guarantees that the pointer is not dangling and stays valid for the
354 // duration of `'a`. The cast is okay because `File` is `repr(transparent)`.
355 //
356 // INVARIANT: The caller guarantees that there are no problematic `fdget_pos` calls.
357 unsafe { &*ptr.cast() }
358 }
359}
360
361// Make LocalFile methods available on File.
362impl core::ops::Deref for File {
363 type Target = LocalFile;
364 #[inline]
365 fn deref(&self) -> &LocalFile {
366 // SAFETY: The caller provides a `&File`, and since it is a reference, it must point at a
367 // valid file for the desired duration.
368 //
369 // By the type invariants, there are no `fdget_pos` calls that did not take the
370 // `f_pos_lock` mutex.
371 unsafe { LocalFile::from_raw_file(core::ptr::from_ref(self).cast()) }
372 }
373}
374
375/// A file descriptor reservation.
376///
377/// This allows the creation of a file descriptor in two steps: first, we reserve a slot for it,
378/// then we commit or drop the reservation. The first step may fail (e.g., the current process ran
379/// out of available slots), but commit and drop never fail (and are mutually exclusive).
380///
381/// Dropping the reservation happens in the destructor of this type.
382///
383/// # Invariants
384///
385/// The fd stored in this struct must correspond to a reserved file descriptor of the current task.
386pub struct FileDescriptorReservation {
387 fd: u32,
388 /// Prevent values of this type from being moved to a different task.
389 ///
390 /// The `fd_install` and `put_unused_fd` functions assume that the value of `current` is
391 /// unchanged since the call to `get_unused_fd_flags`. By adding this marker to this type, we
392 /// prevent it from being moved across task boundaries, which ensures that `current` does not
393 /// change while this value exists.
394 _not_send: NotThreadSafe,
395}
396
397impl FileDescriptorReservation {
398 /// Creates a new file descriptor reservation.
399 #[inline]
400 pub fn get_unused_fd_flags(flags: u32) -> Result<Self> {
401 // SAFETY: FFI call, there are no safety requirements on `flags`.
402 let fd: i32 = unsafe { bindings::get_unused_fd_flags(flags) };
403 to_result(fd)?;
404
405 Ok(Self {
406 fd: fd as u32,
407 _not_send: NotThreadSafe,
408 })
409 }
410
411 /// Returns the file descriptor number that was reserved.
412 #[inline]
413 pub fn reserved_fd(&self) -> u32 {
414 self.fd
415 }
416
417 /// Commits the reservation.
418 ///
419 /// The previously reserved file descriptor is bound to `file`. This method consumes the
420 /// [`FileDescriptorReservation`], so it will not be usable after this call.
421 #[inline]
422 pub fn fd_install(self, file: ARef<File>) {
423 // SAFETY: `self.fd` was previously returned by `get_unused_fd_flags`. We have not yet used
424 // the fd, so it is still valid, and `current` still refers to the same task, as this type
425 // cannot be moved across task boundaries.
426 //
427 // Furthermore, the file pointer is guaranteed to own a refcount by its type invariants,
428 // and we take ownership of that refcount by not running the destructor below.
429 // Additionally, the file is known to not have any non-shared `fdget_pos` calls, so even if
430 // this process starts using the file position, this will not result in a data race on the
431 // file position.
432 unsafe { bindings::fd_install(self.fd, file.as_ptr()) };
433
434 // `fd_install` consumes both the file descriptor and the file reference, so we cannot run
435 // the destructors.
436 core::mem::forget(self);
437 core::mem::forget(file);
438 }
439}
440
441impl Drop for FileDescriptorReservation {
442 #[inline]
443 fn drop(&mut self) {
444 // SAFETY: By the type invariants of this type, `self.fd` was previously returned by
445 // `get_unused_fd_flags`. We have not yet used the fd, so it is still valid, and `current`
446 // still refers to the same task, as this type cannot be moved across task boundaries.
447 unsafe { bindings::put_unused_fd(self.fd) };
448 }
449}
450
451/// Represents the [`EBADF`] error code.
452///
453/// Used for methods that can only fail with [`EBADF`].
454#[derive(Copy, Clone, Eq, PartialEq)]
455pub struct BadFdError;
456
457impl From<BadFdError> for Error {
458 #[inline]
459 fn from(_: BadFdError) -> Error {
460 EBADF
461 }
462}
463
464impl fmt::Debug for BadFdError {
465 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
466 f.pad("EBADF")
467 }
468}