This patch introduce erofs_readdir_rust as an alternative
for erofs_readdir written in Rust.

Signed-off-by: Yiyang Wu <toolmanp@tlmp.cc>
---
 fs/erofs/Makefile        |  2 +-
 fs/erofs/dir.c           |  2 ++
 fs/erofs/dir_rs.rs       | 57 ++++++++++++++++++++++++++++++++++++++++
 fs/erofs/internal.h      |  1 +
 fs/erofs/rust_bindings.h |  1 +
 5 files changed, 62 insertions(+), 1 deletion(-)
 create mode 100644 fs/erofs/dir_rs.rs

diff --git a/fs/erofs/Makefile b/fs/erofs/Makefile
index 0f748f3e0ff6..e086487971b6 100644
--- a/fs/erofs/Makefile
+++ b/fs/erofs/Makefile
@@ -9,4 +9,4 @@ erofs-$(CONFIG_EROFS_FS_ZIP_DEFLATE) += decompressor_deflate.o
 erofs-$(CONFIG_EROFS_FS_ZIP_ZSTD) += decompressor_zstd.o
 erofs-$(CONFIG_EROFS_FS_BACKED_BY_FILE) += fileio.o
 erofs-$(CONFIG_EROFS_FS_ONDEMAND) += fscache.o
-erofs-$(CONFIG_EROFS_FS_RUST) += super_rs.o inode_rs.o namei_rs.o rust_helpers.o
+erofs-$(CONFIG_EROFS_FS_RUST) += super_rs.o inode_rs.o namei_rs.o dir_rs.o rust_helpers.o
diff --git a/fs/erofs/dir.c b/fs/erofs/dir.c
index c3b90abdee37..0f5df8a4169b 100644
--- a/fs/erofs/dir.c
+++ b/fs/erofs/dir.c
@@ -6,6 +6,7 @@
  */
 #include "internal.h"
 
+#ifndef CONFIG_EROFS_FS_RUST
 static int erofs_fill_dentries(struct inode *dir, struct dir_context *ctx,
 			       void *dentry_blk, struct erofs_dirent *de,
 			       unsigned int nameoff0, unsigned int maxsize)
@@ -92,6 +93,7 @@ static int erofs_readdir(struct file *f, struct dir_context *ctx)
 	erofs_put_metabuf(&buf);
 	return err < 0 ? err : 0;
 }
+#endif
 
 const struct file_operations erofs_dir_fops = {
 	.llseek		= generic_file_llseek,
diff --git a/fs/erofs/dir_rs.rs b/fs/erofs/dir_rs.rs
new file mode 100644
index 000000000000..d965e6076242
--- /dev/null
+++ b/fs/erofs/dir_rs.rs
@@ -0,0 +1,57 @@
+// Copyright 2024 Yiyang Wu
+// SPDX-License-Identifier: MIT or GPL-2.0-or-later
+
+//! EROFS Rust Kernel Module Helpers Implementation
+//! This is only for experimental purpose. Feedback is always welcome.
+
+#[allow(dead_code)]
+#[allow(missing_docs)]
+pub(crate) mod rust;
+use core::ffi::*;
+use core::ptr::NonNull;
+
+use kernel::bindings::{dir_context, file};
+use kernel::container_of;
+
+use rust::{
+    erofs_sys::{inode::*, *},
+    kinode::*,
+    ksuperblock::*,
+};
+
+/// Exported as a replacement of erofs_readdir.
+#[no_mangle]
+pub unsafe extern "C" fn erofs_readdir_rust(
+    f: NonNull<file>,
+    mut ctx: NonNull<dir_context>,
+) -> c_int {
+    // SAFETY: inode is always initialized in file.
+    let k_inode = unsafe { f.as_ref().f_inode };
+    // SAFETY: We are sure that the inode is a Kernel Inode since alloc_inode is called
+    let erofs_inode = unsafe { &*container_of!(k_inode, KernelInode, k_inode) };
+    // SAFETY: The super_block is always initialized when calling iget5_locked.
+    let sb = unsafe { (*k_inode).i_sb };
+    let sbi = erofs_sbi(NonNull::new(sb).unwrap());
+    // SAFETY: ctx is nonnull.
+    let offset = unsafe { ctx.as_ref().pos };
+    match sbi
+        .filesystem
+        .fill_dentries(erofs_inode, offset as Off, &mut |dir, pos| unsafe {
+            // inline expansion from dir_emit
+            ctx.as_ref().actor.unwrap()(
+                ctx.as_ptr(),
+                dir.name.as_ptr().cast(),
+                dir.name.len() as i32,
+                pos as i64,
+                dir.desc.nid as u64,
+                dir.desc.file_type as u32,
+            );
+            ctx.as_mut().pos = pos as i64;
+        }) {
+        Ok(_) => {
+            unsafe { ctx.as_mut().pos = erofs_inode.info().file_size() as i64 }
+            0
+        }
+        Err(e) => (i32::from(e)) as c_int,
+    }
+}
diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index 1d9dfae285d5..6f57bb866637 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -444,6 +444,7 @@ int erofs_iget5_set(struct inode *inode, void *opaque);
 #define erofs_iget erofs_iget_rust
 #define erofs_get_parent erofs_get_parent_rust
 #define erofs_lookup erofs_lookup_rust
+#define erofs_readdir erofs_readdir_rust
 #else
 struct inode *erofs_iget(struct super_block *sb, erofs_nid_t nid);
 #endif
diff --git a/fs/erofs/rust_bindings.h b/fs/erofs/rust_bindings.h
index b35014aa5cae..8b71d65e2c0b 100644
--- a/fs/erofs/rust_bindings.h
+++ b/fs/erofs/rust_bindings.h
@@ -23,4 +23,5 @@ extern struct inode *erofs_iget_rust(struct super_block *sb, erofs_nid_t nid);
 extern struct dentry *erofs_lookup_rust(struct inode *inode, struct dentry *dentry,
 			      unsigned int flags);
 extern struct dentry *erofs_get_parent_rust(struct dentry *dentry);
+extern int erofs_readdir_rust(struct file *file, struct dir_context *ctx);
 #endif
-- 
2.46.0