[PATCH v17 10/11] rust: support formatting of foreign types

Tamir Duberstein
2025-10-15T19:25:23Z

Introduce a `fmt!` macro which wraps all arguments in
`kernel::fmt::Adapter` and a `kernel::fmt::Display` trait. This enables
formatting of foreign types (like `core::ffi::CStr`) that do not
implement `core::fmt::Display` due to concerns around lossy conversions
which do not apply in the kernel.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/Custom.20formatting/with/516476467
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
rust/kernel/fmt.rs | 87 +++++++++++++++++++++++++++++++++++++++++++++-
rust/kernel/prelude.rs | 3 +-
rust/macros/fmt.rs | 94 ++++++++++++++++++++++++++++++++++++++++++++++++++
rust/macros/lib.rs | 19 ++++++++++
rust/macros/quote.rs | 7 ++++
5 files changed, 207 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/fmt.rs b/rust/kernel/fmt.rs
index 0306e8388968..84d634201d90 100644
--- a/rust/kernel/fmt.rs
+++ b/rust/kernel/fmt.rs
@@ -4,4 +4,89 @@
//!
//! This module is intended to be used in place of `core::fmt` in kernel code.

-pub use core::fmt::{Arguments, Debug, Display, Error, Formatter, Result, Write};
+pub use core::fmt::{Arguments, Debug, Error, Formatter, Result, Write};
+
+/// Internal adapter used to route allow implementations of formatting traits for foreign types.
+///
+/// It is inserted automatically by the [`fmt!`] macro and is not meant to be used directly.
+///
+/// [`fmt!`]: crate::prelude::fmt!
+#[doc(hidden)]
+pub struct Adapter<T>(pub T);
+
+macro_rules! impl_fmt_adapter_forward {
+ ($($trait:ident),* $(,)?) => {
+ $(
+ impl<T: $trait> $trait for Adapter<T> {
+ fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+ let Self(t) = self;
+ $trait::fmt(t, f)
+ }
+ }
+ )*
+ };
+}
+
+use core::fmt::{Binary, LowerExp, LowerHex, Octal, Pointer, UpperExp, UpperHex};
+impl_fmt_adapter_forward!(Debug, LowerHex, UpperHex, Octal, Binary, Pointer, LowerExp, UpperExp);
+
+/// A copy of [`core::fmt::Display`] that allows us to implement it for foreign types.
+///
+/// Types should implement this trait rather than [`core::fmt::Display`]. Together with the
+/// [`Adapter`] type and [`fmt!`] macro, it allows for formatting foreign types (e.g. types from
+/// core) which do not implement [`core::fmt::Display`] directly.
+///
+/// [`fmt!`]: crate::prelude::fmt!
+pub trait Display {
+ /// Same as [`core::fmt::Display::fmt`].
+ fn fmt(&self, f: &mut Formatter<'_>) -> Result;
+}
+
+impl<T: ?Sized + Display> Display for &T {
+ fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+ Display::fmt(*self, f)
+ }
+}
+
+impl<T: ?Sized + Display> core::fmt::Display for Adapter<&T> {
+ fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+ let Self(t) = self;
+ Display::fmt(t, f)
+ }
+}
+
+macro_rules! impl_display_forward {
+ ($(
+ $( { $($generics:tt)* } )? $ty:ty $( { where $($where:tt)* } )?
+ ),* $(,)?) => {
+ $(
+ impl$($($generics)*)? Display for $ty $(where $($where)*)? {
+ fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+ core::fmt::Display::fmt(self, f)
+ }
+ }
+ )*
+ };
+}
+
+impl_display_forward!(
+ bool,
+ char,
+ core::panic::PanicInfo<'_>,
+ Arguments<'_>,
+ i128,
+ i16,
+ i32,
+ i64,
+ i8,
+ isize,
+ str,
+ u128,
+ u16,
+ u32,
+ u64,
+ u8,
+ usize,
+ {<T: ?Sized>} crate::sync::Arc<T> {where crate::sync::Arc<T>: core::fmt::Display},
+ {<T: ?Sized>} crate::sync::UniqueArc<T> {where crate::sync::UniqueArc<T>: core::fmt::Display},
+);
diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
index 198d09a31449..26424ad7e989 100644
--- a/rust/kernel/prelude.rs
+++ b/rust/kernel/prelude.rs
@@ -25,7 +25,7 @@
pub use crate::alloc::{flags::*, Box, KBox, KVBox, KVVec, KVec, VBox, VVec, Vec};

#[doc(no_inline)]
-pub use macros::{export, kunit_tests, module, vtable};
+pub use macros::{export, fmt, kunit_tests, module, vtable};

pub use pin_init::{init, pin_data, pin_init, pinned_drop, InPlaceWrite, Init, PinInit, Zeroable};

@@ -36,7 +36,6 @@
pub use super::dbg;
pub use super::{dev_alert, dev_crit, dev_dbg, dev_emerg, dev_err, dev_info, dev_notice, dev_warn};
pub use super::{pr_alert, pr_crit, pr_debug, pr_emerg, pr_err, pr_info, pr_notice, pr_warn};
-pub use core::format_args as fmt;

pub use super::{try_init, try_pin_init};

diff --git a/rust/macros/fmt.rs b/rust/macros/fmt.rs
new file mode 100644
index 000000000000..2f4b9f6e2211
--- /dev/null
+++ b/rust/macros/fmt.rs
@@ -0,0 +1,94 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use proc_macro::{Ident, TokenStream, TokenTree};
+use std::collections::BTreeSet;
+
+/// Please see [`crate::fmt`] for documentation.
+pub(crate) fn fmt(input: TokenStream) -> TokenStream {
+ let mut input = input.into_iter();
+
+ let first_opt = input.next();
+ let first_owned_str;
+ let mut names = BTreeSet::new();
+ let first_span = {
+ let Some((mut first_str, first_span)) = (match first_opt.as_ref() {
+ Some(TokenTree::Literal(first_lit)) => {
+ first_owned_str = first_lit.to_string();
+ Some(first_owned_str.as_str()).and_then(|first| {
+ let first = first.strip_prefix('"')?;
+ let first = first.strip_suffix('"')?;
+ Some((first, first_lit.span()))
+ })
+ }
+ _ => None,
+ }) else {
+ return first_opt.into_iter().chain(input).collect();
+ };
+
+ // Parse `identifier`s from the format string.
+ //
+ // See https://doc.rust-lang.org/std/fmt/index.html#syntax.
+ while let Some((_, rest)) = first_str.split_once('{') {
+ first_str = rest;
+ if let Some(rest) = first_str.strip_prefix('{') {
+ first_str = rest;
+ continue;
+ }
+ if let Some((name, rest)) = first_str.split_once('}') {
+ first_str = rest;
+ let name = name.split_once(':').map_or(name, |(name, _)| name);
+ if !name.is_empty() && !name.chars().all(|c| c.is_ascii_digit()) {
+ names.insert(name);
+ }
+ }
+ }
+ first_span
+ };
+
+ let adapter = quote_spanned!(first_span => ::kernel::fmt::Adapter);
+
+ let mut args = TokenStream::from_iter(first_opt);
+ {
+ let mut flush = |args: &mut TokenStream, current: &mut TokenStream| {
+ let current = std::mem::take(current);
+ if !current.is_empty() {
+ let (lhs, rhs) = (|| {
+ let mut current = current.into_iter();
+ let mut acc = TokenStream::new();
+ while let Some(tt) = current.next() {
+ // Split on `=` only once to handle cases like `a = b = c`.
+ if matches!(&tt, TokenTree::Punct(p) if p.as_char() == '=') {
+ names.remove(acc.to_string().as_str());
+ // Include the `=` itself to keep the handling below uniform.
+ acc.extend([tt]);
+ return (Some(acc), current.collect::<TokenStream>());
+ }
+ acc.extend([tt]);
+ }
+ (None, acc)
+ })();
+ args.extend(quote_spanned!(first_span => #lhs #adapter(&#rhs)));
+ }
+ };
+
+ let mut current = TokenStream::new();
+ for tt in input {
+ match &tt {
+ TokenTree::Punct(p) if p.as_char() == ',' => {
+ flush(&mut args, &mut current);
+ &mut args
+ }
+ _ => &mut current,
+ }
+ .extend([tt]);
+ }
+ flush(&mut args, &mut current);
+ }
+
+ for name in names {
+ let name = Ident::new(name, first_span);
+ args.extend(quote_spanned!(first_span => , #name = #adapter(&#name)));
+ }
+
+ quote_spanned!(first_span => ::core::format_args!(#args))
+}
diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
index fa847cf3a9b5..793f712dbf7c 100644
--- a/rust/macros/lib.rs
+++ b/rust/macros/lib.rs
@@ -15,6 +15,7 @@
mod quote;
mod concat_idents;
mod export;
+mod fmt;
mod helpers;
mod kunit;
mod module;
@@ -201,6 +202,24 @@ pub fn export(attr: TokenStream, ts: TokenStream) -> TokenStream {
export::export(attr, ts)
}

+/// Like [`core::format_args!`], but automatically wraps arguments in [`kernel::fmt::Adapter`].
+///
+/// This macro allows generating `fmt::Arguments` while ensuring that each argument is wrapped with
+/// `::kernel::fmt::Adapter`, which customizes formatting behavior for kernel logging.
+///
+/// Named arguments used in the format string (e.g. `{foo}`) are detected and resolved from local
+/// bindings. All positional and named arguments are automatically wrapped.
+///
+/// This macro is an implementation detail of other kernel logging macros like [`pr_info!`] and
+/// should not typically be used directly.
+///
+/// [`kernel::fmt::Adapter`]: ../kernel/fmt/struct.Adapter.html
+/// [`pr_info!`]: ../kernel/macro.pr_info.html
+#[proc_macro]
+pub fn fmt(input: TokenStream) -> TokenStream {
+ fmt::fmt(input)
+}
+
/// Concatenate two identifiers.
///
/// This is useful in macros that need to declare or reference items with names
diff --git a/rust/macros/quote.rs b/rust/macros/quote.rs
index acc140c18653..ddfc21577539 100644
--- a/rust/macros/quote.rs
+++ b/rust/macros/quote.rs
@@ -48,6 +48,7 @@ macro_rules! quote_spanned {
($span:expr => $($tt:tt)*) => {{
let mut tokens = ::proc_macro::TokenStream::new();
{
+ #[allow(unused_variables)]
let span = $span;
quote_spanned!(@proc tokens span $($tt)*);
}
@@ -146,6 +147,12 @@ macro_rules! quote_spanned {
)]);
quote_spanned!(@proc $v $span $($tt)*);
};
+ (@proc $v:ident $span:ident & $($tt:tt)*) => {
+ $v.extend([::proc_macro::TokenTree::Punct(
+ ::proc_macro::Punct::new('&', ::proc_macro::Spacing::Alone),
+ )]);
+ quote_spanned!(@proc $v $span $($tt)*);
+ };
(@proc $v:ident $span:ident _ $($tt:tt)*) => {
$v.extend([::proc_macro::TokenTree::Ident(
::proc_macro::Ident::new("_", $span),

--
2.51.0