Add UserSliceWriter::write_slice_file(), which is the same as
UserSliceWriter::write_slice_partial() but updates the given
file::Offset by the number of bytes written.

This is equivalent to C's `simple_read_from_buffer()` and useful when
dealing with file offsets from file operations.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
[ Replace saturating_add() with the raw operator and a corresponding
  OVERFLOW comment. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

The existing write_slice() method is a wrapper around copy_to_user() and
expects the user buffer to be larger than the source buffer.

However, userspace may split up reads in multiple partial operations
providing an offset into the source buffer and a smaller user buffer.

In order to support this common case, provide a helper for partial
writes.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Matthew Maurer <mmaurer@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
[ Replace map_or() with let-else; use saturating_add(). - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Add UserSliceReader::read_slice_file(), which is the same as
UserSliceReader::read_slice_partial() but updates the given file::Offset
by the number of bytes read.

This is equivalent to C's `simple_write_to_buffer()` and useful when
dealing with file offsets from file operations.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
[ Replace saturating_add() with the raw operator and a corresponding
  OVERFLOW comment. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

The existing read_slice() method is a wrapper around copy_from_user()
and expects the user buffer to be larger than the destination buffer.

However, userspace may split up writes in multiple partial operations
providing an offset into the destination buffer and a smaller user
buffer.

In order to support this common case, provide a helper for partial
reads.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Matthew Maurer <mmaurer@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
[ Replace map_or() with let-else; use saturating_add(). - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Currently, Rust code uses a typedef for unsigned long to represent
userspace addresses. This is unfortunate because it means that userspace
addresses could accidentally be mixed up with other integers. To
alleviate that, we introduce a new UserPtr struct that wraps a raw
pointer to represent a userspace address. By using a struct, type
checking enforces that userspace addresses cannot be mixed up with
anything else.

This is similar to the __user annotation in C that detects cases where
user pointers are mixed with non-user pointers.

Note that unlike __user pointers in C, this type is just a pointer
without a target type. This means that it can't detect cases such as
mixing up which struct this user pointer references. However, that is
okay due to the way this is intended to be used - generally, you create
a UserPtr in your ioctl callback from the provided usize *before*
dispatching on which ioctl is in use, and then after dispatching on the
ioctl you pass the UserPtr into a UserSliceReader or UserSliceWriter;
selecting the target type does not happen until you have obtained the
UserSliceReader/Writer.

The UserPtr type is not marked with #[derive(Debug)], which means that
it's not possible to print values of this type. This avoids ASLR
leakage.

The type is added to the prelude as it is a fairly fundamental type
similar to c_int. The wrapping_add() method is renamed to
wrapping_byte_add() for consistency with the method name found on raw
pointers.

Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250616-userptr-newtype-v3-1-5ff7b2d18d9e@google.com
[ Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

This patch adds a more convenient method for reading C strings from
userspace. Logic is added to NUL-terminate the buffer when necessary so
that a &CStr can be returned.

Note that we treat attempts to read past `self.length` as a fault, so
this returns EFAULT if that limit is exceeded before `buf.len()` is
reached.

Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250616-strncpy-from-user-v5-2-2d3fb0e1f5af@google.com
[ Use `from_mut` to clean `clippy::ref_as_ptr` lint. Reworded
  title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

This patch adds a direct wrapper around the C function of the same name.
It's not really intended for direct use by Rust code since
strncpy_from_user has a somewhat unfortunate API where it only
nul-terminates the buffer if there's space for the nul-terminator. This
means that a direct Rust wrapper around it could not return a &CStr
since the buffer may not be a cstring. However, we still add the method
to build more convenient APIs on top of it, which will happen in
subsequent patches.

Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250616-strncpy-from-user-v5-1-2d3fb0e1f5af@google.com
[ Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

In Rust 1.78.0, Clippy introduced the `ref_as_ptr` lint [1]:

> Using `as` casts may result in silently changing mutability or type.

While this doesn't 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 -- no functional change intended.

Link: https://rust-lang.github.io/rust-clippy/master/index.html#ref_as_ptr [1]
Suggested-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/all/D8PGG7NTWB6U.3SS3A5LN4XWMN@proton.me/
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250615-ptr-as-ptr-v12-6-f43b024581e8@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Pull alloc updates from Danilo Krummrich:
 "Box:

   - Support for type coercion, e.g. 'Box<T>' to 'Box<dyn U>' if T
     implements U

  Vec:

   - Implement new methods (prerequisites for nova-core and binder)
      - Vec::truncate()
      - Vec::resize()
      - Vec::clear()
      - Vec::pop()
      - Vec::push_within_capacity()
         - New error type: PushError
      - Vec::drain_all()
      - Vec::retain()
      - Vec::remove()
         - New error type: RemoveError
      - Vec::insert_within_capacity
         - New error type: InsertError

   - Simplify Vec::push() using Vec::spare_capacity_mut()

   - Split Vec::set_len() into Vec::inc_len() and Vec::dec_len()
      - Add type invariant Vec::len() <= Vec::capacity
      - Simplify Vec::truncate() using Vec::dec_len()"

* tag 'alloc-next-v6.16-2025-05-13' of https://github.com/Rust-for-Linux/linux:
  rust: alloc: add Vec::insert_within_capacity
  rust: alloc: add Vec::remove
  rust: alloc: add Vec::retain
  rust: alloc: add Vec::drain_all
  rust: alloc: add Vec::push_within_capacity
  rust: alloc: add Vec::pop
  rust: alloc: add Vec::clear
  rust: alloc: replace `Vec::set_len` with `inc_len`
  rust: alloc: refactor `Vec::truncate` using `dec_len`
  rust: alloc: add `Vec::dec_len`
  rust: alloc: add Vec::len() <= Vec::capacity invariant
  rust: alloc: allow coercion from `Box<T>` to `Box<dyn U>` if T implements U
  rust: alloc: use `spare_capacity_mut` to reduce unsafe
  rust: alloc: add Vec::resize method
  rust: alloc: add Vec::truncate method
  rust: alloc: add missing invariant in Vec::set_len()

The `kernel` prelude brings `Result` and the error codes; and the prelude
itself is already available in the examples automatically.

In addition, `Result` already defaults to `T = ()`.

Thus simplify.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250429151445.438977-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>