linux-kernel - Re: [PATCH v3 3/5] rust: scatterlist: Add abstraction for sg

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Message-Id: <DCCFL57R0B4B.D289IWKTOTOE@kernel.org>
Date: Tue, 26 Aug 2025 16:32:24 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: <akpm@...ux-foundation.org>, <ojeda@...nel.org>,
 <alex.gaynor@...il.com>, <boqun.feng@...il.com>, <gary@...yguo.net>,
 <bjorn3_gh@...tonmail.com>, <lossin@...nel.org>, <a.hindborg@...nel.org>,
 <tmgross@...ch.edu>, <abdiel.janulgue@...il.com>, <acourbot@...dia.com>,
 <jgg@...pe.ca>, <lyude@...hat.com>, <robin.murphy@....com>,
 <daniel.almeida@...labora.com>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 3/5] rust: scatterlist: Add abstraction for sg_table

On Tue Aug 26, 2025 at 4:16 PM CEST, Alice Ryhl wrote:
> On Mon, Aug 25, 2025 at 03:24:42PM +0200, Danilo Krummrich wrote:
> Overall LGTM. With comments addressed:
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
>
>> +impl RawSGTable {
>> +    fn new(
>> +        pages: &mut [*mut bindings::page],
>
> This should probably be unsafe due to the raw pointer. Or could we pass
> any pointer here?

Good catch, we should indeed make this unsafe and add the corresponding safety
requirements:

diff --git a/rust/kernel/scatterlist.rs b/rust/kernel/scatterlist.rs
index e76e5c2cbdc7..a562c0360842 100644
--- a/rust/kernel/scatterlist.rs
+++ b/rust/kernel/scatterlist.rs
@@ -251,7 +251,12 @@ unsafe impl Send for RawSGTable {}
 unsafe impl Sync for RawSGTable {}
 
 impl RawSGTable {
-    fn new(
+    /// # Safety
+    ///
+    /// - `pages` must be a slice of valid `struct page *`.
+    /// - The pages pointed to by `pages` must remain valid for the entire lifetime of the returned
+    ///   [`RawSGTable`].
+    unsafe fn new(
         pages: &mut [*mut bindings::page],
         size: usize,
         max_segment: u32,
@@ -355,7 +360,11 @@ fn new(
         };
 
         Ok(try_pin_init!(&this in Self {
-            sgt <- RawSGTable::new(&mut page_vec, size, max_segment, flags),
+            // SAFETY:
+            // - `page_vec` is a `KVec` of valid `struct page *` obtained from `pages`.
+            // - The pages contained in `pages` remain valid for the entire lifetime of the
+            //   `RawSGTable`.
+            sgt <- unsafe { RawSGTable::new(&mut page_vec, size, max_segment, flags) },
             dma <- {
                 // SAFETY: `this` is a valid pointer to uninitialized memory.
                 let sgt = unsafe { &raw mut (*this.as_ptr()).sgt }.cast();