linux-kernel - [PATCH 2/3] rust: file: add f_pos and set_f

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Message-ID: <20240926-b4-miscdevice-v1-2-7349c2b2837a@google.com>
Date: Thu, 26 Sep 2024 14:58:56 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Arnd Bergmann <arnd@...db.de>, 
	Miguel Ojeda <ojeda@...nel.org>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>
Cc: Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Alice Ryhl <aliceryhl@...gle.com>
Subject: [PATCH 2/3] rust: file: add f_pos and set_f_pos

Add accessors for the file position. Most of the time, you should not
use these methods directly, and you should instead use a guard for the
file position to prove that you hold the fpos lock. However, under
limited circumstances, files are allowed to choose a different locking
strategy for their file position. These accessors can be used to handle
that case.

For now, these accessors are the only way to access the file position
within the llseek and read_iter callbacks.

Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
---
 rust/kernel/fs/file.rs | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
index e03dbe14d62a..c896a3b1d182 100644
--- a/rust/kernel/fs/file.rs
+++ b/rust/kernel/fs/file.rs
@@ -333,6 +333,26 @@ pub fn flags(&self) -> u32 {
         // FIXME(read_once): Replace with `read_once` when available on the Rust side.
         unsafe { core::ptr::addr_of!((*self.as_ptr()).f_flags).read_volatile() }
     }
+
+    /// Read the file position.
+    ///
+    /// # Safety
+    ///
+    /// You must hold the fpos lock or otherwise ensure that no data race will happen.
+    #[inline]
+    pub unsafe fn f_pos(&self) -> i64 {
+        unsafe { (*self.as_ptr()).f_pos }
+    }
+
+    /// Set the file position.
+    ///
+    /// # Safety
+    ///
+    /// You must hold the fpos lock or otherwise ensure that no data race will happen.
+    #[inline]
+    pub unsafe fn set_f_pos(&self, value: i64) {
+        unsafe { (*self.as_ptr()).f_pos = value };
+    }
 }
 
 impl File {

-- 
2.46.0.792.g87dc391469-goog