Re: [PATCH 5/6] rust: binder: enable `clippy::as_underscore`

[Tamir Duberstein]

On Tue, May 26, 2026 at 8:44 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>
> On Fri, May 22, 2026 at 07:12:50PM +0200, Tamir Duberstein wrote:
> > In Rust 1.63.0, Clippy introduced the `as_underscore` lint [1]:
> >
> > > The conversion might include lossy conversion or a dangerous cast that
> > > might go undetected due to the type being inferred.
> > >
> > > The lint is allowed by default as using `_` is less wordy than always
> > > specifying the type.
> >
> > Always specifying the type is especially helpful in function call
> > contexts where the inferred type may change at a distance. Specifying
> > the type also allows Clippy to spot more cases of `useless_conversion`.
> >
> > While this does not eliminate unchecked `as` conversions, it makes such
> > conversions easier to scrutinize. It also has the slight benefit of
> > removing a degree of freedom on which to bikeshed. Thus apply the
> > changes and enable the lint in the Binder Rust driver -- no functional
> > change intended.
> >
> > Link: https://rust-lang.github.io/rust-clippy/master/index.html#as_underscore [1]
> > Signed-off-by: Tamir Duberstein <tamird@xxxxxxxxxx>
> > ---
> >  drivers/android/binder/allocation.rs       |  4 ++--
> >  drivers/android/binder/defs.rs             |  2 +-
> >  drivers/android/binder/node.rs             | 10 +++++-----
> >  drivers/android/binder/node/wrapper.rs     |  2 +-
> >  drivers/android/binder/process.rs          |  6 +++---
> >  drivers/android/binder/rust_binder_main.rs |  4 ++--
> >  drivers/android/binder/thread.rs           | 24 +++++++++++++-----------
> >  drivers/android/binder/transaction.rs      | 12 ++++++------
> >  8 files changed, 33 insertions(+), 31 deletions(-)
> >
> > diff --git a/drivers/android/binder/allocation.rs b/drivers/android/binder/allocation.rs
> > index 0cab959e4b7e..4ac42bb7216b 100644
> > --- a/drivers/android/binder/allocation.rs
> > +++ b/drivers/android/binder/allocation.rs
> > @@ -376,8 +376,8 @@ pub(crate) fn transfer_binder_object(
> >                  BINDER_TYPE_WEAK_BINDER
> >              };
> >              newobj.flags = obj.flags;
> > -            newobj.__bindgen_anon_1.binder = ptr as _;
> > -            newobj.cookie = cookie as _;
> > +            newobj.__bindgen_anon_1.binder = ptr;
> > +            newobj.cookie = cookie;
>
> Are you sure these casts are unnecessary on all platforms?
>
> >              self.write(offset, &newobj)?;
> >              // Increment the user ref count on the node. It will be decremented as part of the
> >              // destruction of the buffer, when we see a binder or weak-binder object.
> > diff --git a/drivers/android/binder/defs.rs b/drivers/android/binder/defs.rs
> > index fd0ef3f9ebd1..a9682eb6d984 100644
> > --- a/drivers/android/binder/defs.rs
> > +++ b/drivers/android/binder/defs.rs
> > @@ -146,7 +146,7 @@ fn default() -> Self {
> >  impl BinderVersion {
> >      pub(crate) fn current() -> Self {
> >          Self(MaybeUninit::new(uapi::binder_version {
> > -            protocol_version: BINDER_CURRENT_PROTOCOL_VERSION as _,
> > +            protocol_version: BINDER_CURRENT_PROTOCOL_VERSION as i32,
>
> Are you sure i32 is the right type on all platforms?

Yes, this is __s32 in C.

>
> >          }))
> >      }
> >  }
> > diff --git a/drivers/android/binder/node.rs b/drivers/android/binder/node.rs
> > index a4a3ddda6ac9..8f5307c03b1a 100644
> > --- a/drivers/android/binder/node.rs
> > +++ b/drivers/android/binder/node.rs
> > @@ -464,7 +464,7 @@ pub(crate) fn incr_refcount_allow_zero2one_with_wrapper(
> >          owner_inner: &mut ProcessInner,
> >      ) -> Option<DLArc<dyn DeliverToRead>> {
> >          match self.incr_refcount_allow_zero2one(strong, owner_inner) {
> > -            Ok(Some(node)) => Some(node as _),
> > +            Ok(Some(node)) => Some(node as DLArc<dyn DeliverToRead>),
> >              Ok(None) => None,
> >              Err(CouldNotDeliverCriticalIncrement) => {
> >                  assert!(strong);
> > @@ -489,8 +489,8 @@ pub(crate) fn populate_counts(
> >          guard: &Guard<'_, ProcessInner, SpinLockBackend>,
> >      ) {
> >          let inner = self.inner.access(guard);
> > -        out.strong_count = inner.strong.count as _;
> > -        out.weak_count = inner.weak.count as _;
> > +        out.strong_count = inner.strong.count as u32;
> > +        out.weak_count = inner.weak.count as u32;
>
> Are you sure u32 is the right type on all platforms?

Yes, these fields are __u32 in C.

>
> >      }
> >
> >      pub(crate) fn populate_debug_info(
> > @@ -498,8 +498,8 @@ pub(crate) fn populate_debug_info(
> >          out: &mut BinderNodeDebugInfo,
> >          guard: &Guard<'_, ProcessInner, SpinLockBackend>,
> >      ) {
> > -        out.ptr = self.ptr as _;
> > -        out.cookie = self.cookie as _;
> > +        out.ptr = self.ptr;
> > +        out.cookie = self.cookie;
> >          let inner = self.inner.access(guard);
> >          if inner.strong.has_count {
> >              out.has_strong_ref = 1;
> > diff --git a/drivers/android/binder/node/wrapper.rs b/drivers/android/binder/node/wrapper.rs
> > index 43294c050502..6e4ca01c941a 100644
> > --- a/drivers/android/binder/node/wrapper.rs
> > +++ b/drivers/android/binder/node/wrapper.rs
> > @@ -21,7 +21,7 @@ pub(crate) fn new() -> Result<Self> {
> >
> >      pub(super) fn init(self, node: DArc<Node>) -> DLArc<dyn DeliverToRead> {
> >          match self.inner.pin_init_with(DTRWrap::new(NodeWrapper { node })) {
> > -            Ok(initialized) => ListArc::from(initialized) as _,
> > +            Ok(initialized) => ListArc::from(initialized) as DLArc<dyn DeliverToRead>,
> >              Err(err) => match err {},
> >          }
> >      }
> > diff --git a/drivers/android/binder/process.rs b/drivers/android/binder/process.rs
> > index 820cbd541435..3a22260eb9b4 100644
> > --- a/drivers/android/binder/process.rs
> > +++ b/drivers/android/binder/process.rs
> > @@ -259,7 +259,7 @@ pub(crate) fn new_node_ref_with_thread(
> >          let push = match wrapper {
> >              None => node
> >                  .incr_refcount_allow_zero2one(strong, self)?
> > -                .map(|node| node as _),
> > +                .map(|node| node as DLArc<dyn DeliverToRead>),
> >              Some(wrapper) => node.incr_refcount_allow_zero2one_with_wrapper(strong, wrapper, self),
> >          };
> >          if let Some(node) = push {
> > @@ -741,7 +741,7 @@ fn set_as_manager(
> >          } else {
> >              (0, 0, 0)
> >          };
> > -        let node_ref = self.get_node(ptr, cookie, flags as _, true, thread)?;
> > +        let node_ref = self.get_node(ptr, cookie, flags, true, thread)?;
>
> Are you sure there are no platforms that require a cast here?
>
> >          let node = node_ref.node.clone();
> >          self.ctx.set_manager_node(node_ref)?;
> >          self.inner.lock().is_manager = true;
> > @@ -1517,7 +1517,7 @@ fn get_frozen_status(data: UserSlice) -> Result {
> >
> >      for ctx in crate::context::get_all_contexts()? {
> >          ctx.for_each_proc(|proc| {
> > -            if proc.task.pid() == info.pid as _ {
> > +            if proc.task.pid() == info.pid as i32 {
>
> Should use the Pid type alias.

Agreed, fixed in the next spin.

>
> >                  found = true;
> >                  let inner = proc.inner.lock();
> >                  let txns_pending = inner.txns_pending_locked();
> > diff --git a/drivers/android/binder/rust_binder_main.rs b/drivers/android/binder/rust_binder_main.rs
> > index 88da29413e16..2c10a8cd3d88 100644
> > --- a/drivers/android/binder/rust_binder_main.rs
> > +++ b/drivers/android/binder/rust_binder_main.rs
> > @@ -6,7 +6,7 @@
> >
> >  #![crate_name = "rust_binder"]
> >  #![recursion_limit = "256"]
> > -#![allow(clippy::as_underscore, clippy::cast_lossless)]
> > +#![allow(clippy::cast_lossless)]
> >
> >  use kernel::{
> >      bindings::{self, seq_file},
> > @@ -412,7 +412,7 @@ unsafe impl<T> Sync for AssertSync<T> {}
> >      // SAFETY: We previously set `private_data` in `rust_binder_open`.
> >      let f = unsafe { Arc::<Process>::borrow((*file).private_data) };
> >      // SAFETY: The caller ensures that the file is valid.
> > -    match Process::ioctl(f, unsafe { File::from_raw_file(file) }, cmd as _, arg as _) {
> > +    match Process::ioctl(f, unsafe { File::from_raw_file(file) }, cmd, arg) {
>
> Several more cases below where it's unclear whether these changes are
> correct on all platforms including 32-bit targets.

Restored a few instances of cast-to-typedef below on the next spin.

>
> >          Ok(()) => 0,
> >          Err(err) => err.to_errno() as isize,
> >      }
> > diff --git a/drivers/android/binder/thread.rs b/drivers/android/binder/thread.rs
> > index 97d5f31e8fe3..87298a8c597d 100644
> > --- a/drivers/android/binder/thread.rs
> > +++ b/drivers/android/binder/thread.rs
> > @@ -666,9 +666,9 @@ fn translate_object(
> >                  let strong = obj.hdr.type_ == BINDER_TYPE_BINDER;
> >                  // SAFETY: `binder` is a `binder_uintptr_t`; any bit pattern is a valid
> >                  // representation.
> > -                let ptr = unsafe { obj.__bindgen_anon_1.binder } as _;
> > -                let cookie = obj.cookie as _;
> > -                let flags = obj.flags as _;
> > +                let ptr = unsafe { obj.__bindgen_anon_1.binder };
> > +                let cookie = obj.cookie;
> > +                let flags = obj.flags;
> >                  let node = self
> >                      .process
> >                      .as_arc_borrow()
> > @@ -679,7 +679,7 @@ fn translate_object(
> >              BinderObjectRef::Handle(obj) => {
> >                  let strong = obj.hdr.type_ == BINDER_TYPE_HANDLE;
> >                  // SAFETY: `handle` is a `u32`; any bit pattern is a valid representation.
> > -                let handle = unsafe { obj.__bindgen_anon_1.handle } as _;
> > +                let handle = unsafe { obj.__bindgen_anon_1.handle };
> >                  let node = self.process.get_node_from_handle(handle, strong)?;
> >                  security::binder_transfer_binder(&self.process.cred, &view.alloc.process.cred)?;
> >                  view.transfer_binder_object(offset, obj, strong, node)?;
> > @@ -736,7 +736,7 @@ fn translate_object(
> >                      ScatterGatherEntry {
> >                          obj_index,
> >                          offset: alloc_offset,
> > -                        sender_uaddr: obj.buffer as _,
> > +                        sender_uaddr: obj.buffer as usize,
> >                          length: obj_length,
> >                          pointer_fixups: KVec::new(),
> >                          fixup_min_offset: 0,
> > @@ -843,7 +843,7 @@ fn translate_object(
> >                      .ok_or(EINVAL)?;
> >
> >                  let mut fda_bytes = KVec::new();
> > -                UserSlice::new(UserPtr::from_addr(fda_uaddr as _), fds_len)
> > +                UserSlice::new(UserPtr::from_addr(fda_uaddr as usize), fds_len)
> >                      .read_all(&mut fda_bytes, GFP_KERNEL)?;
> >
> >                  if fds_len != fda_bytes.len() {
> > @@ -1365,7 +1365,7 @@ fn write(self: &Arc<Self>, req: &mut BinderWriteRead) -> Result {
> >          let write_start = req.write_buffer.wrapping_add(req.write_consumed);
> >          let write_len = req.write_size.saturating_sub(req.write_consumed);
> >          let mut reader =
> > -            UserSlice::new(UserPtr::from_addr(write_start as _), write_len as _).reader();
> > +            UserSlice::new(UserPtr::from_addr(write_start as usize), write_len as usize).reader();
> >
> >          while reader.len() >= size_of::<u32>() && self.inner.lock().return_work.is_unused() {
> >              let before = reader.len();
> > @@ -1436,7 +1436,7 @@ fn read(self: &Arc<Self>, req: &mut BinderWriteRead, wait: bool) -> Result {
> >          let read_start = req.read_buffer.wrapping_add(req.read_consumed);
> >          let read_len = req.read_size.saturating_sub(req.read_consumed);
> >          let mut writer = BinderReturnWriter::new(
> > -            UserSlice::new(UserPtr::from_addr(read_start as _), read_len as _).writer(),
> > +            UserSlice::new(UserPtr::from_addr(read_start as usize), read_len as usize).writer(),
> >              self,
> >          );
> >          let (in_pool, has_transaction, thread_todo, use_proc_queue) = {
> > @@ -1500,9 +1500,11 @@ fn read(self: &Arc<Self>, req: &mut BinderWriteRead, wait: bool) -> Result {
> >
> >          // Write BR_SPAWN_LOOPER if the process needs more threads for its pool.
> >          if has_noop_placeholder && in_pool && self.process.needs_thread() {
> > -            let mut writer =
> > -                UserSlice::new(UserPtr::from_addr(req.read_buffer as _), req.read_size as _)
> > -                    .writer();
> > +            let mut writer = UserSlice::new(
> > +                UserPtr::from_addr(req.read_buffer as usize),
> > +                req.read_size as usize,
> > +            )
> > +            .writer();
> >              writer.write(&BR_SPAWN_LOOPER)?;
> >          }
> >          Ok(())
> > diff --git a/drivers/android/binder/transaction.rs b/drivers/android/binder/transaction.rs
> > index 47d5e4d88b07..6edd19461785 100644
> > --- a/drivers/android/binder/transaction.rs
> > +++ b/drivers/android/binder/transaction.rs
> > @@ -402,16 +402,16 @@ fn do_work(
> >          let tr = tr_sec.tr_data();
> >          if let Some(target_node) = &self.target_node {
> >              let (ptr, cookie) = target_node.get_id();
> > -            tr.target.ptr = ptr as _;
> > -            tr.cookie = cookie as _;
> > +            tr.target.ptr = ptr;
> > +            tr.cookie = cookie;
> >          };
> >          tr.code = self.code;
> >          tr.flags = self.flags;
> > -        tr.data_size = self.data_size as _;
> > -        tr.data.ptr.buffer = self.data_address as _;
> > -        tr.offsets_size = self.offsets_size as _;
> > +        tr.data_size = self.data_size as u64;
> > +        tr.data.ptr.buffer = self.data_address as u64;
> > +        tr.offsets_size = self.offsets_size as u64;
> >          if tr.offsets_size > 0 {
> > -            tr.data.ptr.offsets = (self.data_address + ptr_align(self.data_size).unwrap()) as _;
> > +            tr.data.ptr.offsets = (self.data_address + ptr_align(self.data_size).unwrap()) as u64;
> >          }
> >          tr.sender_euid = self.sender_euid.into_uid_in_current_ns();
> >          tr.sender_pid = 0;
> >
> > --
> > 2.54.0
> >

Thanks for the review!