[RFC PATCH v2 13/30] rust: fs: introduce `file::Operations::read`

[Wedson Almeida Filho <wedsonaf@gmail.com>]

From: Wedson Almeida Filho <walmeida@microsoft.com>

This allows file systems to customise their behaviour when callers want
to read from a file.

Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
---
 rust/kernel/fs/file.rs    | 35 ++++++++++++++++++++++++++++++++++-
 rust/kernel/user.rs       |  1 -
 samples/rust/rust_rofs.rs |  6 +++++-
 3 files changed, 39 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
index 77eb6d230568..2ba456a1eee1 100644
--- a/rust/kernel/fs/file.rs
+++ b/rust/kernel/fs/file.rs
@@ -12,6 +12,7 @@
     bindings,
     error::{code::*, from_result, Error, Result},
     types::{ARef, AlwaysRefCounted, Locked, Opaque},
+    user,
 };
 use core::{marker::PhantomData, mem::ManuallyDrop, ptr};
 use macros::vtable;
@@ -324,6 +325,15 @@ pub trait Operations {
     /// File system that these operations are compatible with.
     type FileSystem: FileSystem + ?Sized;
 
+    /// Reads data from this file into the caller's buffer.
+    fn read(
+        _file: &File<Self::FileSystem>,
+        _buffer: &mut user::Writer,
+        _offset: &mut Offset,
+    ) -> Result<usize> {
+        Err(EINVAL)
+    }
+
     /// Seeks the file to the given offset.
     fn seek(_file: &File<Self::FileSystem>, _offset: Offset, _whence: Whence) -> Result<Offset> {
         Err(EINVAL)
@@ -356,7 +366,11 @@ impl<T: Operations + ?Sized> Table<T> {
                 } else {
                     None
                 },
-                read: None,
+                read: if T::HAS_READ {
+                    Some(Self::read_callback)
+                } else {
+                    None
+                },
                 write: None,
                 read_iter: None,
                 write_iter: None,
@@ -407,6 +421,25 @@ impl<T: Operations + ?Sized> Table<T> {
                 })
             }
 
+            unsafe extern "C" fn read_callback(
+                file_ptr: *mut bindings::file,
+                ptr: *mut core::ffi::c_char,
+                len: usize,
+                offset: *mut bindings::loff_t,
+            ) -> isize {
+                from_result(|| {
+                    // SAFETY: The C API guarantees that `file` is valid for the duration of the
+                    // callback. Since this callback is specifically for filesystem T, we know `T`
+                    // is the right filesystem.
+                    let file = unsafe { File::from_raw(file_ptr) };
+                    let mut writer = user::Writer::new(ptr, len);
+
+                    // SAFETY: The C API guarantees that `offset` is valid for read and write.
+                    let read = T::read(file, &mut writer, unsafe { &mut *offset })?;
+                    Ok(isize::try_from(read)?)
+                })
+            }
+
             unsafe extern "C" fn read_dir_callback(
                 file_ptr: *mut bindings::file,
                 ctx_ptr: *mut bindings::dir_context,
diff --git a/rust/kernel/user.rs b/rust/kernel/user.rs
index 35a673ebcd58..20fb887f4640 100644
--- a/rust/kernel/user.rs
+++ b/rust/kernel/user.rs
@@ -11,7 +11,6 @@ pub struct Writer {
 }
 
 impl Writer {
-    #[allow(dead_code)]
     pub(crate) fn new(ptr: *mut i8, len: usize) -> Self {
         Self {
             ptr: ptr.cast::<u8>(),
diff --git a/samples/rust/rust_rofs.rs b/samples/rust/rust_rofs.rs
index abec084360da..f4be5908369c 100644
--- a/samples/rust/rust_rofs.rs
+++ b/samples/rust/rust_rofs.rs
@@ -4,7 +4,7 @@
 
 use kernel::fs::{dentry, file, file::File, inode, inode::INode, sb::SuperBlock, Offset};
 use kernel::prelude::*;
-use kernel::{c_str, fs, time::UNIX_EPOCH, types::Either, types::Locked};
+use kernel::{c_str, fs, time::UNIX_EPOCH, types::Either, types::Locked, user};
 
 kernel::module_fs! {
     type: RoFs,
@@ -80,6 +80,10 @@ fn seek(file: &File<Self>, offset: Offset, whence: file::Whence) -> Result<Offse
         file::generic_seek(file, offset, whence)
     }
 
+    fn read(_: &File<Self>, _: &mut user::Writer, _: &mut Offset) -> Result<usize> {
+        Err(EISDIR)
+    }
+
     fn read_dir(
         _file: &File<Self>,
         inode: &Locked<&INode<Self>, inode::ReadSem>,
-- 
2.34.1