rust: file: Add FileFlags type for File flags

This commit addes a wrapper struct, `FileFlags`, that defines constants
for `struct file`'s `f_flags`.

This struct is desirable b/c callers will no longer need to access
soon-to-be-private `bindings` module. It also documents the previously
ambiguous u32 return value on `flags()`.

This closes torvalds#606.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>

rust/kernel/file.rs

@@ -8,6 +8,105 @@
 use crate::{bindings, cred::CredentialRef, error::Error, Result};
 use core::{mem::ManuallyDrop, ops::Deref};
 
+/// Access mode associated with a file
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub enum AccessMode {
+    /// File is read only.
+    ReadOnly,
+    /// File is write only.
+    WriteOnly,
+    /// File is readable and writeable.
+    ReadWrite,
+}
+
+/// Flags associated with a [`File`].
+///
+/// It is tagged with `non_exhaustive` to prevent users from instantiating it.
+#[non_exhaustive]
+pub struct FileFlags;
+
+impl FileFlags {
+    /// 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 `Self::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;
+
+    /// Also known as `O_NDELAY`.
+    ///
+    /// This is effectively the same flag as [`Self::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::file::FileFlags;
+    /// # fn do_something() {}
+    /// # let flags = 0;
+    /// if (flags & FileFlags::O_ACCMODE) == FileFlags::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`.
 ///
 /// # Invariants
@@ -56,6 +155,8 @@ impl File {
     }
 
     /// Returns the flags associated with the file.
+    ///
+    /// The flags are a combination of the constants in [`FileFlags`].
     pub fn flags(&self) -> u32 {
         // SAFETY: `File::ptr` is guaranteed to be valid by the type invariants.
         unsafe { (*self.ptr).f_flags }