[RFC PATCH v2 20/30] rust: fs: introduce `FileSystem::statfs`

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

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

Allow Rust file systems to expose their stats. `overlayfs` requires that
this be implemented by all file systems that are part of an overlay.
The planned file systems need to be overlayed with overlayfs, so they
must be able to implement this.

Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
---
 rust/bindings/bindings_helper.h |  1 +
 rust/kernel/error.rs            |  1 +
 rust/kernel/fs.rs               | 47 ++++++++++++++++++++++++++++++++-
 3 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 2133f95e8be5..f4c7c3951dbe 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -20,6 +20,7 @@
 #include <linux/refcount.h>
 #include <linux/sched.h>
 #include <linux/slab.h>
+#include <linux/statfs.h>
 #include <linux/wait.h>
 #include <linux/workqueue.h>
 #include <linux/xattr.h>
diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index f40a2bdf28d4..edada157879a 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -79,6 +79,7 @@ macro_rules! declare_err {
     declare_err!(ENOGRACE, "NFS file lock reclaim refused.");
     declare_err!(ENODATA, "No data available.");
     declare_err!(EOPNOTSUPP, "Operation not supported on transport endpoint.");
+    declare_err!(ENOSYS, "Invalid system call number.");
     declare_err!(ESTALE, "Stale file handle.");
     declare_err!(EUCLEAN, "Structure needs cleaning.");
 }
diff --git a/rust/kernel/fs.rs b/rust/kernel/fs.rs
index 5b8f9c346767..51de73008857 100644
--- a/rust/kernel/fs.rs
+++ b/rust/kernel/fs.rs
@@ -61,6 +61,31 @@ fn read_xattr(
     ) -> Result<usize> {
         Err(EOPNOTSUPP)
     }
+
+    /// Get filesystem statistics.
+    fn statfs(_dentry: &DEntry<Self>) -> Result<Stat> {
+        Err(ENOSYS)
+    }
+}
+
+/// File system stats.
+///
+/// A subset of C's `kstatfs`.
+pub struct Stat {
+    /// Magic number of the file system.
+    pub magic: usize,
+
+    /// The maximum length of a file name.
+    pub namelen: isize,
+
+    /// Block size.
+    pub bsize: isize,
+
+    /// Number of files in the file system.
+    pub files: u64,
+
+    /// Number of blocks in the file system.
+    pub blocks: u64,
 }
 
 /// A file system that is unspecified.
@@ -213,7 +238,7 @@ impl<T: FileSystem + ?Sized> Tables<T> {
         freeze_fs: None,
         thaw_super: None,
         unfreeze_fs: None,
-        statfs: None,
+        statfs: Some(Self::statfs_callback),
         remount_fs: None,
         umount_begin: None,
         show_options: None,
@@ -231,6 +256,26 @@ impl<T: FileSystem + ?Sized> Tables<T> {
         shutdown: None,
     };
 
+    unsafe extern "C" fn statfs_callback(
+        dentry_ptr: *mut bindings::dentry,
+        buf: *mut bindings::kstatfs,
+    ) -> ffi::c_int {
+        from_result(|| {
+            // SAFETY: The C API guarantees that `dentry_ptr` is a valid dentry.
+            let dentry = unsafe { DEntry::from_raw(dentry_ptr) };
+            let s = T::statfs(dentry)?;
+
+            // SAFETY: The C API guarantees that `buf` is valid for read and write.
+            let buf = unsafe { &mut *buf };
+            buf.f_type = s.magic as ffi::c_long;
+            buf.f_namelen = s.namelen as ffi::c_long;
+            buf.f_bsize = s.bsize as ffi::c_long;
+            buf.f_files = s.files;
+            buf.f_blocks = s.blocks;
+            Ok(0)
+        })
+    }
+
     const XATTR_HANDLERS: [*const bindings::xattr_handler; 2] = [&Self::XATTR_HANDLER, ptr::null()];
 
     const XATTR_HANDLER: bindings::xattr_handler = bindings::xattr_handler {
-- 
2.34.1