Merge tag 'rust-fixes-6.12' of https://github.com/Rust-for-Linux/linux

Pull Rust fixes from Miguel Ojeda:
 "Toolchain and infrastructure:

   - Fix/improve a couple 'depends on' on the newly added CFI/KASAN
     suppport to avoid build errors/warnings

   - Fix ARCH_SLAB_MINALIGN multiple definition error for RISC-V under
     !CONFIG_MMU

   - Clean upcoming (Rust 1.83.0) Clippy warnings

  'kernel' crate:

   - 'sync' module: fix soundness issue by requiring 'T: Sync' for
     'LockedBy::access'; and fix helpers build error under PREEMPT_RT

   - Fix trivial sorting issue ('rustfmtcheck') on the v6.12 Rust merge"

* tag 'rust-fixes-6.12' of https://github.com/Rust-for-Linux/linux:
  rust: kunit: use C-string literals to clean warning
  cfi: encode cfi normalized integers + kasan/gcov bug in Kconfig
  rust: KASAN+RETHUNK requires rustc 1.83.0
  rust: cfi: fix `patchable-function-entry` starting version
  rust: mutex: fix __mutex_init() usage in case of PREEMPT_RT
  rust: fix `ARCH_SLAB_MINALIGN` multiple definition error
  rust: sync: require `T: Sync` for `LockedBy::access`
  rust: kernel: sort Rust modules

3 files changed, 17 insertions, 7 deletions

diff --git a/rust/kernel/kunit.rs b/rust/kernel/kunit.rs
index 0ba77276ae7ef..824da0e9738a0 100644
--- a/rust/kernel/kunit.rs
+++ b/rust/kernel/kunit.rs
@@ -18,7 +18,7 @@ pub fn err(args: fmt::Arguments<'_>) {
     #[cfg(CONFIG_PRINTK)]
     unsafe {
         bindings::_printk(
-            b"\x013%pA\0".as_ptr() as _,
+            c"\x013%pA".as_ptr() as _,
             &args as *const _ as *const c_void,
         );
     }
@@ -34,7 +34,7 @@ pub fn info(args: fmt::Arguments<'_>) {
     #[cfg(CONFIG_PRINTK)]
     unsafe {
         bindings::_printk(
-            b"\x016%pA\0".as_ptr() as _,
+            c"\x016%pA".as_ptr() as _,
             &args as *const _ as *const c_void,
         );
     }
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 22a3bfa5a9e96..b5f4b3ce6b482 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -44,8 +44,8 @@ pub mod net;
 pub mod page;
 pub mod prelude;
 pub mod print;
-pub mod sizes;
 pub mod rbtree;
+pub mod sizes;
 mod static_assert;
 #[doc(hidden)]
 pub mod std_vendor;
diff --git a/rust/kernel/sync/locked_by.rs b/rust/kernel/sync/locked_by.rs
index babc731bd5f62..ce2ee8d878658 100644
--- a/rust/kernel/sync/locked_by.rs
+++ b/rust/kernel/sync/locked_by.rs
@@ -83,8 +83,12 @@ pub struct LockedBy<T: ?Sized, U: ?Sized> {
 // SAFETY: `LockedBy` can be transferred across thread boundaries iff the data it protects can.
 unsafe impl<T: ?Sized + Send, U: ?Sized> Send for LockedBy<T, U> {}
 
-// SAFETY: `LockedBy` serialises the interior mutability it provides, so it is `Sync` as long as the
-// data it protects is `Send`.
+// SAFETY: If `T` is not `Sync`, then parallel shared access to this `LockedBy` allows you to use
+// `access_mut` to hand out `&mut T` on one thread at the time. The requirement that `T: Send` is
+// sufficient to allow that.
+//
+// If `T` is `Sync`, then the `access` method also becomes available, which allows you to obtain
+// several `&T` from several threads at once. However, this is okay as `T` is `Sync`.
 unsafe impl<T: ?Sized + Send, U: ?Sized> Sync for LockedBy<T, U> {}
 
 impl<T, U> LockedBy<T, U> {
@@ -118,7 +122,10 @@ impl<T: ?Sized, U> LockedBy<T, U> {
     ///
     /// Panics if `owner` is different from the data protected by the lock used in
     /// [`new`](LockedBy::new).
-    pub fn access<'a>(&'a self, owner: &'a U) -> &'a T {
+    pub fn access<'a>(&'a self, owner: &'a U) -> &'a T
+    where
+        T: Sync,
+    {
         build_assert!(
             size_of::<U>() > 0,
             "`U` cannot be a ZST because `owner` wouldn't be unique"
@@ -127,7 +134,10 @@ impl<T: ?Sized, U> LockedBy<T, U> {
             panic!("mismatched owners");
         }
 
-        // SAFETY: `owner` is evidence that the owner is locked.
+        // SAFETY: `owner` is evidence that there are only shared references to the owner for the
+        // duration of 'a, so it's not possible to use `Self::access_mut` to obtain a mutable
+        // reference to the inner value that aliases with this shared reference. The type is `Sync`
+        // so there are no other requirements.
         unsafe { &*self.data.get() }
     }