[RFC PATCH 19/24] erofs: introduce namei alternative to C

[Yiyang Wu <toolmanp@tlmp.cc>]

This patch introduces erofs_lookup_rust and erofs_get_parent_rust
written in Rust as an alternative to the original namei.c.

Signed-off-by: Yiyang Wu <toolmanp@tlmp.cc>
---
 fs/erofs/Makefile        |  2 +-
 fs/erofs/internal.h      |  2 ++
 fs/erofs/namei.c         |  2 ++
 fs/erofs/namei_rs.rs     | 56 ++++++++++++++++++++++++++++++++++++++++
 fs/erofs/rust_bindings.h |  4 ++-
 fs/erofs/super.c         |  2 ++
 6 files changed, 66 insertions(+), 2 deletions(-)
 create mode 100644 fs/erofs/namei_rs.rs

diff --git a/fs/erofs/Makefile b/fs/erofs/Makefile
index 46de6f490ca2..0f748f3e0ff6 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-$(CONFIG_EROFS_FS_RUST) += super_rs.o inode_rs.o rust_helpers.o
+erofs-$(CONFIG_EROFS_FS_RUST) += super_rs.o inode_rs.o namei_rs.o rust_helpers.o
diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index 42ce84783be7..1d9dfae285d5 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -442,6 +442,8 @@ int erofs_iget5_eq(struct inode *inode, void *opaque);
 int erofs_iget5_set(struct inode *inode, void *opaque);
 #ifdef CONFIG_EROFS_FS_RUST
 #define erofs_iget erofs_iget_rust
+#define erofs_get_parent erofs_get_parent_rust
+#define erofs_lookup erofs_lookup_rust
 #else
 struct inode *erofs_iget(struct super_block *sb, erofs_nid_t nid);
 #endif
diff --git a/fs/erofs/namei.c b/fs/erofs/namei.c
index c94d0c1608a8..f657d475c4a1 100644
--- a/fs/erofs/namei.c
+++ b/fs/erofs/namei.c
@@ -7,6 +7,7 @@
 #include "xattr.h"
 #include <trace/events/erofs.h>
 
+#ifndef CONFIG_EROFS_FS_RUST
 struct erofs_qstr {
 	const unsigned char *name;
 	const unsigned char *end;
@@ -214,6 +215,7 @@ static struct dentry *erofs_lookup(struct inode *dir, struct dentry *dentry,
 		inode = erofs_iget(dir->i_sb, nid);
 	return d_splice_alias(inode, dentry);
 }
+#endif
 
 const struct inode_operations erofs_dir_iops = {
 	.lookup = erofs_lookup,
diff --git a/fs/erofs/namei_rs.rs b/fs/erofs/namei_rs.rs
new file mode 100644
index 000000000000..d73a0a7bee1e
--- /dev/null
+++ b/fs/erofs/namei_rs.rs
@@ -0,0 +1,56 @@
+// 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::{d_obtain_alias, d_splice_alias, dentry, inode};
+use kernel::container_of;
+
+use rust::{erofs_sys::operations::*, kinode::*, ksuperblock::*};
+
+/// Lookup function for dentry-inode lookup replacement.
+#[no_mangle]
+pub unsafe extern "C" fn erofs_lookup_rust(
+    k_inode: NonNull<inode>,
+    dentry: NonNull<dentry>,
+    _flags: c_uint,
+) -> *mut c_void {
+    // SAFETY: We are sure that the inode is a Kernel Inode since alloc_inode is called
+    let erofs_inode = unsafe { &*container_of!(k_inode.as_ptr(), KernelInode, k_inode) };
+    // SAFETY: The super_block is initialized when the erofs_alloc_sbi_rust is called.
+    let sbi = erofs_sbi(unsafe { NonNull::new(k_inode.as_ref().i_sb).unwrap() });
+    // SAFETY: this is backed by qstr which is c representation of a valid slice.
+    let name = unsafe {
+        core::str::from_utf8_unchecked(core::slice::from_raw_parts(
+            dentry.as_ref().d_name.name,
+            dentry.as_ref().d_name.__bindgen_anon_1.__bindgen_anon_1.len as usize,
+        ))
+    };
+    let k_inode: *mut inode =
+        dir_lookup(sbi.filesystem.as_ref(), &mut sbi.inodes, erofs_inode, name)
+            .map_or(core::ptr::null_mut(), |result| result.k_inode.as_mut_ptr());
+
+    // SAFETY: We are sure that the inner k_inode has already been initialized.
+    unsafe { d_splice_alias(k_inode, dentry.as_ptr()).cast() }
+}
+
+/// Exported as a replacement of erofs_get_parent.
+#[no_mangle]
+pub unsafe extern "C" fn erofs_get_parent_rust(child: NonNull<dentry>) -> *mut c_void {
+    // SAFETY: We are sure that the inode is a Kernel Inode since alloc_inode is called
+    let k_inode = unsafe { child.as_ref().d_inode };
+    // SAFETY: The super_block is initialized when the erofs_alloc_sbi_rust is called.
+    let sbi = erofs_sbi(unsafe { NonNull::new((*k_inode).i_sb).unwrap() }); // SAFETY: We are sure that the inode is a Kernel Inode since alloc_inode is called
+    let inode = unsafe { &*container_of!(k_inode, KernelInode, k_inode) };
+    let k_inode: *mut inode = dir_lookup(sbi.filesystem.as_ref(), &mut sbi.inodes, inode, "..")
+        .map_or(core::ptr::null_mut(), |result| result.k_inode.as_mut_ptr());
+    // SAFETY: We are sure that the inner k_inode has already been initialized
+    unsafe { d_obtain_alias(k_inode).cast() }
+}
diff --git a/fs/erofs/rust_bindings.h b/fs/erofs/rust_bindings.h
index 657f109dd6e7..b35014aa5cae 100644
--- a/fs/erofs/rust_bindings.h
+++ b/fs/erofs/rust_bindings.h
@@ -6,7 +6,6 @@
 
 #include <linux/fs.h>
 
-
 typedef u64 erofs_nid_t;
 typedef u64 erofs_off_t;
 /* data type for filesystem-wide blocks number */
@@ -21,4 +20,7 @@ extern void *erofs_alloc_sbi_rust(struct super_block *sb);
 extern void *erofs_free_sbi_rust(struct super_block *sb);
 extern int erofs_iget5_eq_rust(struct inode *inode, void *opaque);
 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);
 #endif
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 659502bdf5fe..d49c804acf3d 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -554,6 +554,7 @@ static struct dentry *erofs_fh_to_parent(struct super_block *sb,
 				    erofs_nfs_get_inode);
 }
 
+#ifndef CONFIG_EROFS_FS_RUST
 static struct dentry *erofs_get_parent(struct dentry *child)
 {
 	erofs_nid_t nid;
@@ -565,6 +566,7 @@ static struct dentry *erofs_get_parent(struct dentry *child)
 		return ERR_PTR(err);
 	return d_obtain_alias(erofs_iget(child->d_sb, nid));
 }
+#endif
 
 static const struct export_operations erofs_export_ops = {
 	.encode_fh = generic_encode_ino32_fh,
-- 
2.46.0