rust: enable clippy::undocumented_unsafe_blocks lint

Checking that we are not missing any `// SAFETY` comments in our `unsafe`
blocks is something we have wanted to do for a long time, as well as
cleaning up the remaining cases that were not documented [1].

Back when Rust for Linux started, this was something that could have
been done via a script, like Rust's `tidy`. Soon after, in Rust 1.58.0,
Clippy implemented the `undocumented_unsafe_blocks` lint [2].

Even though the lint has a few false positives, e.g. in some cases where
attributes appear between the comment and the `unsafe` block [3], there
are workarounds and the lint seems quite usable already.

Thus enable the lint now.

We still have a few cases to clean up, so just allow those for the moment
by writing a `TODO` comment -- some of those may be good candidates for
new contributors.

Link: #351 [1]
Link: https://rust-lang.github.io/rust-clippy/master/#/undocumented_unsafe_blocks [2]
Link: rust-lang/rust-clippy#13189 [3]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Tested-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240904204347.168520-5-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Makefile

@@ -457,6 +457,7 @@ export rust_common_flags := --edition=2021 \
 			    -Wclippy::needless_bitwise_bool \
 			    -Wclippy::needless_continue \
 			    -Wclippy::no_mangle_with_rust_abi \
+			    -Wclippy::undocumented_unsafe_blocks \
 			    -Wrustdoc::missing_crate_level_docs
 
 KBUILD_HOSTCFLAGS   := $(KBUILD_USERHOSTCFLAGS) $(HOST_LFS_CFLAGS) \

mm/kasan/kasan_test_rust.rs

@@ -17,5 +17,6 @@ pub extern "C" fn kasan_test_rust_uaf() -> u8 {
     }
     let ptr: *mut u8 = addr_of_mut!(v[2048]);
     drop(v);
+    // SAFETY: Incorrect, on purpose.
     unsafe { *ptr }
 }

rust/bindings/lib.rs

@@ -25,6 +25,7 @@
 )]
 
 #[allow(dead_code)]
+#[allow(clippy::undocumented_unsafe_blocks)]
 mod bindings_raw {
     // Use glob import here to expose all helpers.
     // Symbols defined within the module will take precedence to the glob import.

rust/kernel/alloc/allocator.rs

@@ -31,6 +31,7 @@ pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: F
     unsafe { bindings::krealloc(ptr as *const core::ffi::c_void, size, flags.0) as *mut u8 }
 }
 
+// SAFETY: TODO.
 unsafe impl GlobalAlloc for KernelAllocator {
     unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
         // SAFETY: `ptr::null_mut()` is null and `layout` has a non-zero size by the function safety
@@ -39,6 +40,7 @@ unsafe impl GlobalAlloc for KernelAllocator {
     }
 
     unsafe fn dealloc(&self, ptr: *mut u8, _layout: Layout) {
+        // SAFETY: TODO.
         unsafe {
             bindings::kfree(ptr as *const core::ffi::c_void);
         }

rust/kernel/error.rs

@@ -171,9 +171,11 @@ impl fmt::Debug for Error {
         match self.name() {
             // Print out number if no name can be found.
             None => f.debug_tuple("Error").field(&-self.0).finish(),
-            // SAFETY: These strings are ASCII-only.
             Some(name) => f
-                .debug_tuple(unsafe { core::str::from_utf8_unchecked(name) })
+                .debug_tuple(
+                    // SAFETY: These strings are ASCII-only.
+                    unsafe { core::str::from_utf8_unchecked(name) },
+                )
                 .finish(),
         }
     }
@@ -277,6 +279,8 @@ pub(crate) fn from_err_ptr<T>(ptr: *mut T) -> Result<*mut T> {
     if unsafe { bindings::IS_ERR(const_ptr) } {
         // SAFETY: The FFI function does not deref the pointer.
         let err = unsafe { bindings::PTR_ERR(const_ptr) };
+
+        #[allow(clippy::unnecessary_cast)]
         // CAST: If `IS_ERR()` returns `true`,
         // then `PTR_ERR()` is guaranteed to return a
         // negative value greater-or-equal to `-bindings::MAX_ERRNO`,
@@ -286,7 +290,6 @@ pub(crate) fn from_err_ptr<T>(ptr: *mut T) -> Result<*mut T> {
         //
         // SAFETY: `IS_ERR()` ensures `err` is a
         // negative value greater-or-equal to `-bindings::MAX_ERRNO`.
-        #[allow(clippy::unnecessary_cast)]
         return Err(unsafe { Error::from_errno_unchecked(err as core::ffi::c_int) });
     }
     Ok(ptr)

rust/kernel/init.rs

@@ -541,6 +541,7 @@ macro_rules! stack_try_pin_init {
 /// }
 /// pin_init!(&this in Buf {
 ///     buf: [0; 64],
+///     // SAFETY: TODO.
 ///     ptr: unsafe { addr_of_mut!((*this.as_ptr()).buf).cast() },
 ///     pin: PhantomPinned,
 /// });
@@ -875,6 +876,7 @@ pub unsafe trait PinInit<T: ?Sized, E = Infallible>: Sized {
     /// }
     ///
     /// let foo = pin_init!(Foo {
+    ///     // SAFETY: TODO.
     ///     raw <- unsafe {
     ///         Opaque::ffi_init(|s| {
     ///             init_foo(s);
@@ -1162,6 +1164,7 @@ where
 // SAFETY: Every type can be initialized by-value.
 unsafe impl<T, E> Init<T, E> for T {
     unsafe fn __init(self, slot: *mut T) -> Result<(), E> {
+        // SAFETY: TODO.
         unsafe { slot.write(self) };
         Ok(())
     }
@@ -1170,6 +1173,7 @@ unsafe impl<T, E> Init<T, E> for T {
 // SAFETY: Every type can be initialized by-value. `__pinned_init` calls `__init`.
 unsafe impl<T, E> PinInit<T, E> for T {
     unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
+        // SAFETY: TODO.
         unsafe { self.__init(slot) }
     }
 }
@@ -1411,6 +1415,7 @@ pub fn zeroed<T: Zeroable>() -> impl Init<T> {
 
 macro_rules! impl_zeroable {
     ($($({$($generics:tt)*})? $t:ty, )*) => {
+        // SAFETY: Safety comments written in the macro invocation.
         $(unsafe impl$($($generics)*)? Zeroable for $t {})*
     };
 }

rust/kernel/init/__internal.rs

@@ -112,10 +112,12 @@ impl<T: ?Sized> Clone for AllData<T> {
 
 impl<T: ?Sized> Copy for AllData<T> {}
 
+// SAFETY: TODO.
 unsafe impl<T: ?Sized> InitData for AllData<T> {
     type Datee = T;
 }
 
+// SAFETY: TODO.
 unsafe impl<T: ?Sized> HasInitData for T {
     type InitData = AllData<T>;
 

rust/kernel/init/macros.rs

@@ -513,6 +513,7 @@ macro_rules! __pinned_drop {
             }
         ),
     ) => {
+        // SAFETY: TODO.
         unsafe $($impl_sig)* {
             // Inherit all attributes and the type/ident tokens for the signature.
             $(#[$($attr)*])*
@@ -872,6 +873,7 @@ macro_rules! __pin_data {
                 }
             }
 
+            // SAFETY: TODO.
             unsafe impl<$($impl_generics)*>
                 $crate::init::__internal::PinData for __ThePinData<$($ty_generics)*>
             where $($whr)*
@@ -997,6 +999,7 @@ macro_rules! __pin_data {
                     slot: *mut $p_type,
                     init: impl $crate::init::PinInit<$p_type, E>,
                 ) -> ::core::result::Result<(), E> {
+                    // SAFETY: TODO.
                     unsafe { $crate::init::PinInit::__pinned_init(init, slot) }
                 }
             )*
@@ -1007,6 +1010,7 @@ macro_rules! __pin_data {
                     slot: *mut $type,
                     init: impl $crate::init::Init<$type, E>,
                 ) -> ::core::result::Result<(), E> {
+                    // SAFETY: TODO.
                     unsafe { $crate::init::Init::__init(init, slot) }
                 }
             )*
@@ -1121,6 +1125,8 @@ macro_rules! __init_internal {
         // no possibility of returning without `unsafe`.
         struct __InitOk;
         // Get the data about fields from the supplied type.
+        //
+        // SAFETY: TODO.
         let data = unsafe {
             use $crate::init::__internal::$has_data;
             // Here we abuse `paste!` to retokenize `$t`. Declarative macros have some internal
@@ -1176,6 +1182,7 @@ macro_rules! __init_internal {
         let init = move |slot| -> ::core::result::Result<(), $err> {
             init(slot).map(|__InitOk| ())
         };
+        // SAFETY: TODO.
         let init = unsafe { $crate::init::$construct_closure::<_, $err>(init) };
         init
     }};
@@ -1324,6 +1331,8 @@ macro_rules! __init_internal {
         // Endpoint, nothing more to munch, create the initializer.
         // Since we are in the closure that is never called, this will never get executed.
         // We abuse `slot` to get the correct type inference here:
+        //
+        // SAFETY: TODO.
         unsafe {
             // Here we abuse `paste!` to retokenize `$t`. Declarative macros have some internal
             // information that is associated to already parsed fragments, so a path fragment

rust/kernel/list.rs

@@ -354,6 +354,7 @@ impl<T: ?Sized + ListItem<ID>, const ID: u64> List<T, ID> {
     ///
     /// `item` must not be in a different linked list (with the same id).
     pub unsafe fn remove(&mut self, item: &T) -> Option<ListArc<T, ID>> {
+        // SAFETY: TODO.
         let mut item = unsafe { ListLinks::fields(T::view_links(item)) };
         // SAFETY: The user provided a reference, and reference are never dangling.
         //

rust/kernel/print.rs

@@ -23,6 +23,7 @@ unsafe extern "C" fn rust_fmt_argument(
     use fmt::Write;
     // SAFETY: The C contract guarantees that `buf` is valid if it's less than `end`.
     let mut w = unsafe { RawFormatter::from_ptrs(buf.cast(), end.cast()) };
+    // SAFETY: TODO.
     let _ = w.write_fmt(unsafe { *(ptr as *const fmt::Arguments<'_>) });
     w.pos().cast()
 }
@@ -102,6 +103,7 @@ pub unsafe fn call_printk(
 ) {
     // `_printk` does not seem to fail in any path.
     #[cfg(CONFIG_PRINTK)]
+    // SAFETY: TODO.
     unsafe {
         bindings::_printk(
             format_string.as_ptr() as _,

rust/kernel/str.rs

@@ -162,10 +162,10 @@ impl CStr {
     /// Returns the length of this string with `NUL`.
     #[inline]
     pub const fn len_with_nul(&self) -> usize {
-        // SAFETY: This is one of the invariant of `CStr`.
-        // We add a `unreachable_unchecked` here to hint the optimizer that
-        // the value returned from this function is non-zero.
         if self.0.is_empty() {
+            // SAFETY: This is one of the invariant of `CStr`.
+            // We add a `unreachable_unchecked` here to hint the optimizer that
+            // the value returned from this function is non-zero.
             unsafe { core::hint::unreachable_unchecked() };
         }
         self.0.len()
@@ -301,6 +301,7 @@ impl CStr {
     /// ```
     #[inline]
     pub unsafe fn as_str_unchecked(&self) -> &str {
+        // SAFETY: TODO.
         unsafe { core::str::from_utf8_unchecked(self.as_bytes()) }
     }
 

rust/kernel/sync/condvar.rs

@@ -92,8 +92,8 @@ pub struct CondVar {
     _pin: PhantomPinned,
 }
 
-// SAFETY: `CondVar` only uses a `struct wait_queue_head`, which is safe to use on any thread.
 #[allow(clippy::non_send_fields_in_send_ty)]
+// SAFETY: `CondVar` only uses a `struct wait_queue_head`, which is safe to use on any thread.
 unsafe impl Send for CondVar {}
 
 // SAFETY: `CondVar` only uses a `struct wait_queue_head`, which is safe to use on multiple threads

rust/kernel/sync/lock.rs

@@ -150,9 +150,9 @@ impl<T: ?Sized, B: Backend> Guard<'_, T, B> {
         // SAFETY: The caller owns the lock, so it is safe to unlock it.
         unsafe { B::unlock(self.lock.state.get(), &self.state) };
 
-        // SAFETY: The lock was just unlocked above and is being relocked now.
-        let _relock =
-            ScopeGuard::new(|| unsafe { B::relock(self.lock.state.get(), &mut self.state) });
+        let _relock = ScopeGuard::new(||
+                // SAFETY: The lock was just unlocked above and is being relocked now.
+                unsafe { B::relock(self.lock.state.get(), &mut self.state) });
 
         cb()
     }

rust/kernel/types.rs

@@ -410,13 +410,15 @@ impl<T: AlwaysRefCounted> ARef<T> {
     ///
     /// struct Empty {}
     ///
+    /// # // SAFETY: TODO.
     /// unsafe impl AlwaysRefCounted for Empty {
     ///     fn inc_ref(&self) {}
     ///     unsafe fn dec_ref(_obj: NonNull<Self>) {}
     /// }
     ///
     /// let mut data = Empty {};
     /// let ptr = NonNull::<Empty>::new(&mut data as *mut _).unwrap();
+    /// # // SAFETY: TODO.
     /// let data_ref: ARef<Empty> = unsafe { ARef::from_raw(ptr) };
     /// let raw_ptr: NonNull<Empty> = ARef::into_raw(data_ref);
     ///
@@ -492,6 +494,7 @@ pub unsafe trait FromBytes {}
 
 macro_rules! impl_frombytes {
     ($($({$($generics:tt)*})? $t:ty, )*) => {
+        // SAFETY: Safety comments written in the macro invocation.
         $(unsafe impl$($($generics)*)? FromBytes for $t {})*
     };
 }
@@ -526,6 +529,7 @@ pub unsafe trait AsBytes {}
 
 macro_rules! impl_asbytes {
     ($($({$($generics:tt)*})? $t:ty, )*) => {
+        // SAFETY: Safety comments written in the macro invocation.
         $(unsafe impl$($($generics)*)? AsBytes for $t {})*
     };
 }

rust/kernel/workqueue.rs

@@ -519,6 +519,7 @@ impl_has_work! {
     impl{T} HasWork<Self> for ClosureWork<T> { self.work }
 }
 
+// SAFETY: TODO.
 unsafe impl<T, const ID: u64> WorkItemPointer<ID> for Arc<T>
 where
     T: WorkItem<ID, Pointer = Self>,
@@ -536,6 +537,7 @@ where
     }
 }
 
+// SAFETY: TODO.
 unsafe impl<T, const ID: u64> RawWorkItem<ID> for Arc<T>
 where
     T: WorkItem<ID, Pointer = Self>,
@@ -564,6 +566,7 @@ where
     }
 }
 
+// SAFETY: TODO.
 unsafe impl<T, const ID: u64> WorkItemPointer<ID> for Pin<Box<T>>
 where
     T: WorkItem<ID, Pointer = Self>,
@@ -583,6 +586,7 @@ where
     }
 }
 
+// SAFETY: TODO.
 unsafe impl<T, const ID: u64> RawWorkItem<ID> for Pin<Box<T>>
 where
     T: WorkItem<ID, Pointer = Self>,

rust/uapi/lib.rs

@@ -14,6 +14,7 @@
 #![cfg_attr(test, allow(unsafe_op_in_unsafe_fn))]
 #![allow(
     clippy::all,
+    clippy::undocumented_unsafe_blocks,
     dead_code,
     missing_docs,
     non_camel_case_types,