rust: file: Add FileFlags type for File flags

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

This wrapper is desirable b/c callers no longer have 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 +8,104 @@
 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`].
+///
+/// Wrapper around `struct file`'s `f_flags`.
+#[repr(transparent)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub struct FileFlags(u32);
+
+impl FileFlags {
+    /// Also known as `O_APPEND`.
+    pub const APPEND: u32 = bindings::O_APPEND;
+
+    /// Also known as `FASYNC`.
+    pub const ASYNC: u32 = bindings::FASYNC;
+
+    /// Also known as `O_CLOEXEC`.
+    pub const CLOSE_ON_EXEC: u32 = bindings::O_CLOEXEC;
+
+    /// Also known as `O_CREAT`.
+    pub const CREATE: u32 = bindings::O_CREAT;
+
+    /// Also known as `O_DIRECT`.
+    pub const DIRECT: u32 = bindings::O_DIRECT;
+
+    /// Also known as `O_DIRECTORY`.
+    pub const DIRECTORY: u32 = bindings::O_DIRECTORY;
+
+    /// Also known as `O_DSYNC`.
+    pub const DATA_SYNC: u32 = bindings::O_DSYNC;
+
+    /// Also known as `O_EXCL`.
+    pub const EXCLUSIVE: u32 = bindings::O_EXCL;
+
+    /// Also known as `O_LARGEFILE`.
+    pub const LARGE_FILE: u32 = bindings::O_LARGEFILE;
+
+    /// Also known as `O_NOATIME`.
+    pub const NO_ACCESS_TIME: u32 = bindings::O_NOATIME;
+
+    /// Also known as `O_NOCTTY`.
+    pub const NO_CONTROLLING_TTY: u32 = bindings::O_NOCTTY;
+
+    /// Also known as `O_NOFOLLOW`.
+    pub const NO_FOLLOW: u32 = bindings::O_NOFOLLOW;
+
+    /// Also known as `O_NONBLOCK`
+    pub const NON_BLOCK: u32 = bindings::O_NONBLOCK;
+
+    /// Also known as `O_NDELAY`.
+    ///
+    /// This is effectively the same flag as [`Self::NON_BLOCK`] on all architectures except
+    /// SPARC64.
+    pub const NO_DELAY: u32 = bindings::O_NDELAY;
+
+    /// Also known as `O_PATH`.
+    pub const PATH: u32 = bindings::O_PATH;
+
+    /// Also known as `O_SYNC`.
+    pub const SYNC: u32 = bindings::O_SYNC;
+
+    /// Also known as `O_TMPFILE`.
+    pub const TEMPORARY_FILE: u32 = bindings::O_TMPFILE;
+
+    /// Also known as `O_TRUNC`.
+    pub const TRUNCATE: u32 = bindings::O_TRUNC;
+
+    fn new(flags: u32) -> Self {
+        Self(flags)
+    }
+
+    /// Access bitmasked set of file flags.
+    ///
+    /// Use the above constants to interpret the bitmask.
+    pub fn flags(&self) -> u32 {
+        self.0
+    }
+
+    /// Access mode associated with the file.
+    pub fn access_mode(&self) -> AccessMode {
+        match self.0 & bindings::O_ACCMODE {
+            bindings::O_RDONLY => AccessMode::ReadOnly,
+            bindings::O_WRONLY => AccessMode::WriteOnly,
+            bindings::O_RDWR => AccessMode::ReadWrite,
+            _ => panic!("Unreachable. Kernel VFS layer guarantees one of the above flags set."),
+        }
+    }
+}
+
 /// Wraps the kernel's `struct file`.
 ///
 /// # Invariants
@@ -55,10 +153,10 @@ impl File {
         unsafe { CredentialRef::from_ptr(ptr) }
     }
 
-    /// Returns the flags associated with the file.
-    pub fn flags(&self) -> u32 {
+    /// Returns the [`FileFlags`] associated with the file.
+    pub fn flags(&self) -> FileFlags {
         // SAFETY: `File::ptr` is guaranteed to be valid by the type invariants.
-        unsafe { (*self.ptr).f_flags }
+        unsafe { FileFlags::new((*self.ptr).f_flags) }
     }
 }