Message-ID: <CAGSQo02=B86jKhSZoLdADbuKro3vQaq1V6CZGwSa8hRMhvKHtg@mail.gmail.com>
Date: Fri, 12 Dec 2025 16:34:47 -0800
From: Matthew Maurer <mmaurer@...gle.com>
To: Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] rust: Support deriving `AsBytes`/`FromBytes` on
 bindgen types

On Fri, Dec 12, 2025 at 3:42 PM Matthew Maurer <mmaurer@...gle.com> wrote:
>
> To support this, we need to move the `transmute` module into a separate
> crate to allow the `bindings` crate to depend on it. Most user code is
> still expected to address the module as `kernel::transmute`, which is a
> re-export. `traits::transmute` is now available for use in `bindings`.
>
> Signed-off-by: Matthew Maurer <mmaurer@...gle.com>
> ---
>  rust/Makefile                        | 19 +++++++++++++++----
>  rust/bindgen_parameters              |  8 ++++++++
>  rust/bindings/lib.rs                 |  1 +
>  rust/kernel/lib.rs                   |  3 ++-
>  rust/macros/lib.rs                   | 24 ++++++++++++++++++++++--
>  rust/macros/transmute.rs             | 12 +++++++-----
>  rust/traits/lib.rs                   |  6 ++++++
>  rust/{kernel => traits}/transmute.rs |  0
>  rust/uapi/lib.rs                     |  1 +
>  9 files changed, 62 insertions(+), 12 deletions(-)
>
> diff --git a/rust/Makefile b/rust/Makefile
> index 5d357dce1704d15e43effc528be8f5a4d74d3d8d..bbede4b5b4faa5edeb1d33bdce338a7b76915d07 100644
> --- a/rust/Makefile
> +++ b/rust/Makefile
> @@ -660,26 +660,37 @@ $(obj)/ffi.o: private skip_gendwarfksyms = 1
>  $(obj)/ffi.o: $(src)/ffi.rs $(obj)/compiler_builtins.o FORCE
>         +$(call if_changed_rule,rustc_library)
>
> -$(obj)/bindings.o: private rustc_target_flags = --extern ffi --extern pin_init
> +$(obj)/bindings.o: private rustc_target_flags = --extern ffi --extern pin_init \
> +       --extern traits --extern macros
>  $(obj)/bindings.o: $(src)/bindings/lib.rs \
>      $(obj)/ffi.o \
>      $(obj)/pin_init.o \
> +    $(obj)/traits.o \
> +    $(obj)/$(libmacros_name) \
>      $(obj)/bindings/bindings_generated.rs \
>      $(obj)/bindings/bindings_helpers_generated.rs FORCE
>         +$(call if_changed_rule,rustc_library)
>
> -$(obj)/uapi.o: private rustc_target_flags = --extern ffi --extern pin_init
> +$(obj)/traits.o: private skip_gendwarfksyms = 1
> +$(obj)/traits.o: $(src)/traits/lib.rs FORCE

I've noticed that with high parallelism on a clean build this build
rule will sometimes fail because `compiler_builtins.o` is not listed
as a dependency (I had kind of expected `rustc_library` to imply
that). I'll fix that in a v2, but figured I'd note that for anyone
trying it out in the meantime.

> +       +$(call if_changed_rule,rustc_library)
> +
> +
> +$(obj)/uapi.o: private rustc_target_flags = --extern ffi --extern pin_init \
> +       --extern traits --extern macros
>  $(obj)/uapi.o: private skip_gendwarfksyms = 1
>  $(obj)/uapi.o: $(src)/uapi/lib.rs \
>      $(obj)/ffi.o \
>      $(obj)/pin_init.o \
> +    $(obj)/traits.o \
> +    $(obj)/$(libmacros_name) \
>      $(obj)/uapi/uapi_generated.rs FORCE
>         +$(call if_changed_rule,rustc_library)
>
>  $(obj)/kernel.o: private rustc_target_flags = --extern ffi --extern pin_init \
> -    --extern build_error --extern macros --extern bindings --extern uapi
> +    --extern build_error --extern macros --extern bindings --extern uapi --extern traits
>  $(obj)/kernel.o: $(src)/kernel/lib.rs $(obj)/build_error.o $(obj)/pin_init.o \
> -    $(obj)/$(libmacros_name) $(obj)/bindings.o $(obj)/uapi.o FORCE
> +    $(obj)/$(libmacros_name) $(obj)/bindings.o $(obj)/uapi.o $(obj)/traits.o FORCE
>         +$(call if_changed_rule,rustc_library)
>
>  ifdef CONFIG_JUMP_LABEL
> diff --git a/rust/bindgen_parameters b/rust/bindgen_parameters
> index fd2fd1c3cb9a51ea46fcd721907783b457aa1378..60910ba8731180516b4980b9e0dc5fb0c297da55 100644
> --- a/rust/bindgen_parameters
> +++ b/rust/bindgen_parameters
> @@ -64,3 +64,11 @@
>  # Structs should implement `Zeroable` when all of their fields do.
>  --with-derive-custom-struct .*=MaybeZeroable
>  --with-derive-custom-union .*=MaybeZeroable
> +
> +# Every C struct can try to derive FromBytes. If they have unmet requirements,
> +# the impl will just be a no-op due to the where clause.
> +--with-derive-custom-struct .*=FromBytesTrait
> +
> +# We can't auto-derive AsBytes, as we need a const-time check to see if there
> +# is padding involved. Add it explicitly when you expect no padding.
> +--with-derive-custom-struct cpumask=AsBytesTrait
> diff --git a/rust/bindings/lib.rs b/rust/bindings/lib.rs
> index 0c57cf9b4004f176997c59ecc58a9a9ac76163d9..07932ab130c0a8e56e35d49ace77ae1becc09ae6 100644
> --- a/rust/bindings/lib.rs
> +++ b/rust/bindings/lib.rs
> @@ -31,6 +31,7 @@
>  #[allow(clippy::undocumented_unsafe_blocks)]
>  #[cfg_attr(CONFIG_RUSTC_HAS_UNNECESSARY_TRANSMUTES, allow(unnecessary_transmutes))]
>  mod bindings_raw {
> +    use macros::{AsBytesTrait, FromBytesTrait};
>      use pin_init::{MaybeZeroable, Zeroable};
>
>      // Manual definition for blocklisted types.
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index f812cf12004286962985a068665443dc22c389a2..8c605f4905223077f5d824f27220c4c24e71d5f4 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -146,7 +146,6 @@
>  pub mod task;
>  pub mod time;
>  pub mod tracepoint;
> -pub mod transmute;
>  pub mod types;
>  pub mod uaccess;
>  #[cfg(CONFIG_USB = "y")]
> @@ -159,6 +158,8 @@
>  pub use macros;
>  pub use uapi;
>
> +pub use traits::*;
> +
>  /// Prefix to appear before log messages printed from within the `kernel` crate.
>  const __LOG_PREFIX: &[u8] = b"rust_kernel\0";
>
> diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
> index cec8bd3a742b3f6b3680c4286f73c8fa550a58a0..862de56339bbb18cd30a260444dc19f24b13a0d0 100644
> --- a/rust/macros/lib.rs
> +++ b/rust/macros/lib.rs
> @@ -503,7 +503,7 @@ pub fn kunit_tests(attr: TokenStream, ts: TokenStream) -> TokenStream {
>  #[proc_macro_derive(FromBytes)]
>  pub fn derive_from_bytes(tokens: TokenStream) -> TokenStream {
>      let input = parse_macro_input!(tokens as DeriveInput);
> -    transmute::from_bytes(input).into()
> +    transmute::from_bytes("kernel", input).into()
>  }
>
>  /// Implements `AsBytes` for a struct.
> @@ -533,5 +533,25 @@ pub fn derive_from_bytes(tokens: TokenStream) -> TokenStream {
>  #[proc_macro_derive(AsBytes)]
>  pub fn derive_as_bytes(tokens: TokenStream) -> TokenStream {
>      let input = parse_macro_input!(tokens as DeriveInput);
> -    transmute::as_bytes(input).into()
> +    transmute::as_bytes("kernel", input).into()
> +}
> +
> +#[doc(hidden)]
> +#[proc_macro_derive(FromBytesTrait)]
> +/// This is equivalent to `FromBytes`, but uses the `trait` crate as the trait definition site
> +/// instead of `kernel`. This is intended for use inside `bindings`. Everyone else can refer to the
> +/// trait through `kernel`.
> +pub fn derive_from_bytes_trait(tokens: TokenStream) -> TokenStream {
> +    let input = parse_macro_input!(tokens as DeriveInput);
> +    transmute::from_bytes("traits", input).into()
> +}
> +
> +#[doc(hidden)]
> +#[proc_macro_derive(AsBytesTrait)]
> +/// This is equivalent to `AsBytes`, but uses the `trait` crate as the trait definition site
> +/// instead of `kernel`. This is intended for use inside `bindings`. Everyone else can refer to the
> +/// trait through `kernel`.
> +pub fn derive_as_bytes_trait(tokens: TokenStream) -> TokenStream {
> +    let input = parse_macro_input!(tokens as DeriveInput);
> +    transmute::as_bytes("traits", input).into()
>  }
> diff --git a/rust/macros/transmute.rs b/rust/macros/transmute.rs
> index 43cf36a1334f1fed23c0e777026392f987f78d8d..9bd6d279675fb1d58cdceff1f4dde0dcf33fbf99 100644
> --- a/rust/macros/transmute.rs
> +++ b/rust/macros/transmute.rs
> @@ -1,6 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0
>
> -use proc_macro2::TokenStream;
> +use proc_macro2::{Span, TokenStream};
>  use syn::{parse_quote, DeriveInput, Fields, Ident, ItemConst, Path, WhereClause};
>
>  fn all_fields_impl(fields: &Fields, trait_: &Path) -> WhereClause {
> @@ -19,7 +19,8 @@ fn struct_padding_check(fields: &Fields, name: &Ident) -> ItemConst {
>      }
>  }
>
> -pub(crate) fn as_bytes(input: DeriveInput) -> TokenStream {
> +pub(crate) fn as_bytes(crate_: &str, input: DeriveInput) -> TokenStream {
> +    let crate_ = Ident::new(crate_, Span::call_site());
>      if !input.generics.params.is_empty() {
>          return quote::quote! { compile_error!("#[derive(AsBytes)] does not support generics") };
>      }
> @@ -27,7 +28,7 @@ pub(crate) fn as_bytes(input: DeriveInput) -> TokenStream {
>          return quote::quote! { compile_error!("#[derive(AsBytes)] only supports structs") };
>      };
>      let name = input.ident;
> -    let trait_ = parse_quote! { ::kernel::transmute::AsBytes };
> +    let trait_ = parse_quote! { ::#crate_::transmute::AsBytes };
>      let where_clause = all_fields_impl(&ds.fields, &trait_);
>      let padding_check = struct_padding_check(&ds.fields, &name);
>      quote::quote! {
> @@ -37,13 +38,14 @@ unsafe impl #trait_ for #name #where_clause {}
>      }
>  }
>
> -pub(crate) fn from_bytes(input: DeriveInput) -> TokenStream {
> +pub(crate) fn from_bytes(crate_: &str, input: DeriveInput) -> TokenStream {
> +    let crate_ = Ident::new(crate_, Span::call_site());
>      let syn::Data::Struct(ref ds) = &input.data else {
>          return quote::quote! { compile_error!("#[derive(FromBytes)] only supports structs") };
>      };
>      let (impl_generics, ty_generics, base_where_clause) = input.generics.split_for_impl();
>      let name = input.ident;
> -    let trait_ = parse_quote! { ::kernel::transmute::FromBytes };
> +    let trait_ = parse_quote! { ::#crate_::transmute::FromBytes };
>      let mut where_clause = all_fields_impl(&ds.fields, &trait_);
>      if let Some(base_clause) = base_where_clause {
>          where_clause
> diff --git a/rust/traits/lib.rs b/rust/traits/lib.rs
> new file mode 100644
> index 0000000000000000000000000000000000000000..33020a17333521b3bee71bd3d003a759ac97fedd
> --- /dev/null
> +++ b/rust/traits/lib.rs
> @@ -0,0 +1,6 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Support crate containing traits and other definitions that need to be available without the
> +//! kernel crate. The usual case for this is code that needs to be available in the bindings crate.
> +#![no_std]
> +pub mod transmute;
> diff --git a/rust/kernel/transmute.rs b/rust/traits/transmute.rs
> similarity index 100%
> rename from rust/kernel/transmute.rs
> rename to rust/traits/transmute.rs
> diff --git a/rust/uapi/lib.rs b/rust/uapi/lib.rs
> index 1d5fd9efb93e9db97fec84fca2bae37b500c20c5..928b4b2eb035250399e3150f20b638edbf04dac1 100644
> --- a/rust/uapi/lib.rs
> +++ b/rust/uapi/lib.rs
> @@ -34,6 +34,7 @@
>  type __kernel_ssize_t = isize;
>  type __kernel_ptrdiff_t = isize;
>
> +use macros::{AsBytesTrait, FromBytesTrait};
>  use pin_init::MaybeZeroable;
>
>  include!(concat!(env!("OBJTREE"), "/rust/uapi/uapi_generated.rs"));
>
> --
> 2.52.0.305.g3fc767764a-goog
>

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