rust: fs: introduce FileSystem::statfs

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>

Author: wedsonaf

rust/bindings/bindings_helper.h

@@ -11,6 +11,7 @@
 #include <linux/fs.h>
 #include <linux/fs_context.h>
 #include <linux/slab.h>
+#include <linux/statfs.h>
 #include <linux/pagemap.h>
 #include <linux/refcount.h>
 #include <linux/wait.h>

rust/kernel/error.rs

@@ -83,6 +83,7 @@ pub mod code {
     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.");
 }
 
 /// Generic integer kernel error.

rust/kernel/fs.rs

@@ -50,6 +50,31 @@ pub trait FileSystem {
     fn read_xattr(_inode: &INode<Self>, _name: &CStr, _outbuf: &mut [u8]) -> Result<usize> {
         Err(EOPNOTSUPP)
     }
+
+    /// Get filesystem statistics.
+    fn statfs(_sb: &SuperBlock<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: u32,
+
+    /// The maximum length of a file name.
+    pub namelen: i64,
+
+    /// Block size.
+    pub bsize: i64,
+
+    /// Number of files in the file system.
+    pub files: u64,
+
+    /// Number of blocks in the file system.
+    pub blocks: u64,
 }
 
 /// The types of directory entries reported by [`FileSystem::read_dir`].
@@ -478,7 +503,7 @@ 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,
@@ -496,6 +521,31 @@ impl<T: FileSystem + ?Sized> Tables<T> {
         shutdown: None,
     };
 
+    unsafe extern "C" fn statfs_callback(
+        dentry: *mut bindings::dentry,
+        buf: *mut bindings::kstatfs,
+    ) -> core::ffi::c_int {
+        from_result(|| {
+            // SAFETY: The C API guarantees that `dentry` is valid for read. `d_sb` is
+            // immutable, so it's safe to read it. The superblock is guaranteed to be valid dor
+            // the duration of the call.
+            let sb = unsafe { &*(*dentry).d_sb.cast::<SuperBlock<T>>() };
+            let s = T::statfs(sb)?;
+
+            // SAFETY: The C API guarantees that `buf` is valid for read and write.
+            let buf = unsafe { &mut *buf };
+            buf.f_type = s.magic.into();
+            buf.f_namelen = s.namelen;
+            buf.f_bsize = s.bsize;
+            buf.f_files = s.files;
+            buf.f_blocks = s.blocks;
+            buf.f_bfree = 0;
+            buf.f_bavail = 0;
+            buf.f_ffree = 0;
+            Ok(0)
+        })
+    }
+
     const XATTR_HANDLERS: [*const bindings::xattr_handler; 2] = [&Self::XATTR_HANDLER, ptr::null()];
 
     const XATTR_HANDLER: bindings::xattr_handler = bindings::xattr_handler {