[RFC PATCH v2 12/30] rust: fs: introduce `file::Operations::seek`

[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 seek to a different file location, which may also be used when
reading directory entries.

Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
---
 rust/kernel/fs/file.rs    | 73 ++++++++++++++++++++++++++++++++++++++-
 samples/rust/rust_rofs.rs |  6 +++-
 2 files changed, 77 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
index 6d61723f440d..77eb6d230568 100644
--- a/rust/kernel/fs/file.rs
+++ b/rust/kernel/fs/file.rs
@@ -270,12 +270,65 @@ fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
     }
 }
 
+/// Indicates how to interpret the `offset` argument in [`Operations::seek`].
+#[repr(u32)]
+pub enum Whence {
+    /// `offset` bytes from the start of the file.
+    Set = bindings::SEEK_SET,
+
+    /// `offset` bytes from the end of the file.
+    End = bindings::SEEK_END,
+
+    /// `offset` bytes from the current location.
+    Cur = bindings::SEEK_CUR,
+
+    /// The next location greater than or equal to `offset` that contains data.
+    Data = bindings::SEEK_DATA,
+
+    /// The next location greater than or equal to `offset` that contains a hole.
+    Hole = bindings::SEEK_HOLE,
+}
+
+impl TryFrom<i32> for Whence {
+    type Error = crate::error::Error;
+
+    fn try_from(v: i32) -> Result<Self> {
+        match v {
+            v if v == Self::Set as i32 => Ok(Self::Set),
+            v if v == Self::End as i32 => Ok(Self::End),
+            v if v == Self::Cur as i32 => Ok(Self::Cur),
+            v if v == Self::Data as i32 => Ok(Self::Data),
+            v if v == Self::Hole as i32 => Ok(Self::Hole),
+            _ => Err(EDOM),
+        }
+    }
+}
+
+/// Generic implementation of [`Operations::seek`].
+pub fn generic_seek(
+    file: &File<impl FileSystem + ?Sized>,
+    offset: Offset,
+    whence: Whence,
+) -> Result<Offset> {
+    let n = unsafe { bindings::generic_file_llseek(file.0.get(), offset, whence as i32) };
+    if n < 0 {
+        Err(Error::from_errno(n.try_into()?))
+    } else {
+        Ok(n)
+    }
+}
+
 /// Operations implemented by files.
 #[vtable]
 pub trait Operations {
     /// File system that these operations are compatible with.
     type FileSystem: FileSystem + ?Sized;
 
+    /// Seeks the file to the given offset.
+    fn seek(_file: &File<Self::FileSystem>, _offset: Offset, _whence: Whence) -> Result<Offset> {
+        Err(EINVAL)
+    }
+
     /// Reads directory entries from directory files.
     ///
     /// [`DirEmitter::pos`] holds the current position of the directory reader.
@@ -298,7 +351,11 @@ pub const fn new<U: Operations<FileSystem = T> + ?Sized>() -> Self {
         impl<T: Operations + ?Sized> Table<T> {
             const TABLE: bindings::file_operations = bindings::file_operations {
                 owner: ptr::null_mut(),
-                llseek: None,
+                llseek: if T::HAS_SEEK {
+                    Some(Self::seek_callback)
+                } else {
+                    None
+                },
                 read: None,
                 write: None,
                 read_iter: None,
@@ -336,6 +393,20 @@ impl<T: Operations + ?Sized> Table<T> {
                 uring_cmd_iopoll: None,
             };
 
+            unsafe extern "C" fn seek_callback(
+                file_ptr: *mut bindings::file,
+                offset: bindings::loff_t,
+                whence: i32,
+            ) -> bindings::loff_t {
+                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) };
+                    T::seek(file, offset, whence.try_into()?)
+                })
+            }
+
             unsafe extern "C" fn read_dir_callback(
                 file_ptr: *mut bindings::file,
                 ctx_ptr: *mut bindings::dir_context,
diff --git a/samples/rust/rust_rofs.rs b/samples/rust/rust_rofs.rs
index 9da01346d8f8..abec084360da 100644
--- a/samples/rust/rust_rofs.rs
+++ b/samples/rust/rust_rofs.rs
@@ -2,7 +2,7 @@
 
 //! Rust read-only file system sample.
 
-use kernel::fs::{dentry, file, file::File, inode, inode::INode, sb::SuperBlock};
+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};
 
@@ -76,6 +76,10 @@ fn init_root(sb: &SuperBlock<Self>) -> Result<dentry::Root<Self>> {
 impl file::Operations for RoFs {
     type FileSystem = Self;
 
+    fn seek(file: &File<Self>, offset: Offset, whence: file::Whence) -> Result<Offset> {
+        file::generic_seek(file, offset, whence)
+    }
+
     fn read_dir(
         _file: &File<Self>,
         inode: &Locked<&INode<Self>, inode::ReadSem>,
-- 
2.34.1