[PATCH v12 2/5] rust: support formatting of foreign types

Tamir Duberstein
2025-06-19T15:06:37Z

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.

Replace all direct calls to `format_args!` with `fmt!`.

Replace all implementations of `core::fmt::Display` with implementations
of `kernel::fmt::Display`.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/Custom.20formatting/with/516476467
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
drivers/block/rnull.rs | 2 +-
drivers/gpu/nova-core/gpu.rs | 4 +-
rust/kernel/block/mq.rs | 2 +-
rust/kernel/device.rs | 2 +-
rust/kernel/fmt.rs | 89 +++++++++++++++++++++++++++++++++++++++
rust/kernel/kunit.rs | 6 +--
rust/kernel/lib.rs | 1 +
rust/kernel/prelude.rs | 3 +-
rust/kernel/print.rs | 4 +-
rust/kernel/seq_file.rs | 2 +-
rust/kernel/str.rs | 22 ++++------
rust/macros/fmt.rs | 99 ++++++++++++++++++++++++++++++++++++++++++++
rust/macros/lib.rs | 19 +++++++++
rust/macros/quote.rs | 7 ++++
scripts/rustdoc_test_gen.rs | 2 +-
15 files changed, 236 insertions(+), 28 deletions(-)

diff --git a/drivers/block/rnull.rs b/drivers/block/rnull.rs
index d07e76ae2c13..6366da12c5a5 100644
--- a/drivers/block/rnull.rs
+++ b/drivers/block/rnull.rs
@@ -51,7 +51,7 @@ fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
.logical_block_size(4096)?
.physical_block_size(4096)?
.rotational(false)
- .build(format_args!("rnullb{}", 0), tagset)
+ .build(fmt!("rnullb{}", 0), tagset)
})();

try_pin_init!(Self {
diff --git a/drivers/gpu/nova-core/gpu.rs b/drivers/gpu/nova-core/gpu.rs
index 60b86f370284..e6b208175ff9 100644
--- a/drivers/gpu/nova-core/gpu.rs
+++ b/drivers/gpu/nova-core/gpu.rs
@@ -91,7 +91,7 @@ pub(crate) fn arch(&self) -> Architecture {
// Hence, replace with something like strum_macros derive(Display).
//
// For now, redirect to fmt::Debug for convenience.
-impl fmt::Display for Chipset {
+impl kernel::fmt::Display for Chipset {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "{self:?}")
}
@@ -132,7 +132,7 @@ fn from_boot0(boot0: regs::NV_PMC_BOOT_0) -> Self {
}
}

-impl fmt::Display for Revision {
+impl kernel::fmt::Display for Revision {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "{:x}.{:x}", self.major, self.minor)
}
diff --git a/rust/kernel/block/mq.rs b/rust/kernel/block/mq.rs
index fb0f393c1cea..842be88aa1cf 100644
--- a/rust/kernel/block/mq.rs
+++ b/rust/kernel/block/mq.rs
@@ -82,7 +82,7 @@
//! Arc::pin_init(TagSet::new(1, 256, 1), flags::GFP_KERNEL)?;
//! let mut disk = gen_disk::GenDiskBuilder::new()
//! .capacity_sectors(4096)
-//! .build(format_args!("myblk"), tagset)?;
+//! .build(fmt!("myblk"), tagset)?;
//!
//! # Ok::<(), kernel::error::Error>(())
//! ```
diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
index dea06b79ecb5..cdc6c8161e24 100644
--- a/rust/kernel/device.rs
+++ b/rust/kernel/device.rs
@@ -345,7 +345,7 @@ macro_rules! impl_device_context_into_aref {
macro_rules! dev_printk {
($method:ident, $dev:expr, $($f:tt)*) => {
{
- ($dev).$method(::core::format_args!($($f)*));
+ ($dev).$method($crate::prelude::fmt!($($f)*));
}
}
}
diff --git a/rust/kernel/fmt.rs b/rust/kernel/fmt.rs
new file mode 100644
index 000000000000..348d16987de6
--- /dev/null
+++ b/rust/kernel/fmt.rs
@@ -0,0 +1,89 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Formatting utilities.
+
+use core::fmt;
+
+/// Internal adapter used to route allow implementations of formatting traits for foreign types.
+///
+/// It is inserted automatically by the [`fmt!`] macro and is not meant to be used directly.
+///
+/// [`fmt!`]: crate::prelude::fmt!
+#[doc(hidden)]
+pub struct Adapter<T>(pub T);
+
+macro_rules! impl_fmt_adapter_forward {
+ ($($trait:ident),* $(,)?) => {
+ $(
+ impl<T: fmt::$trait> fmt::$trait for Adapter<T> {
+ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+ let Self(t) = self;
+ fmt::$trait::fmt(t, f)
+ }
+ }
+ )*
+ };
+}
+
+impl_fmt_adapter_forward!(Debug, LowerHex, UpperHex, Octal, Binary, Pointer, LowerExp, UpperExp);
+
+/// A copy of [`fmt::Display`] that allows us to implement it for foreign types.
+///
+/// Types should implement this trait rather than [`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 [`fmt::Display`] directly.
+///
+/// [`fmt!`]: crate::prelude::fmt!
+pub trait Display {
+ /// Same as [`fmt::Display::fmt`].
+ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result;
+}
+
+impl<T: ?Sized + Display> Display for &T {
+ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+ Display::fmt(*self, f)
+ }
+}
+
+impl<T: ?Sized + Display> fmt::Display for Adapter<&T> {
+ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::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 fmt::Formatter<'_>) -> fmt::Result {
+ fmt::Display::fmt(self, f)
+ }
+ }
+ )*
+ };
+}
+
+impl_display_forward!(
+ bool,
+ char,
+ core::panic::PanicInfo<'_>,
+ fmt::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>: fmt::Display},
+ {<T: ?Sized>} crate::sync::UniqueArc<T> {where crate::sync::UniqueArc<T>: fmt::Display},
+);
diff --git a/rust/kernel/kunit.rs b/rust/kernel/kunit.rs
index 4b8cdcb21e77..20ae65c2a18e 100644
--- a/rust/kernel/kunit.rs
+++ b/rust/kernel/kunit.rs
@@ -71,14 +71,14 @@ macro_rules! kunit_assert {
// mistake (it is hidden to prevent that).
//
// This mimics KUnit's failed assertion format.
- $crate::kunit::err(format_args!(
+ $crate::kunit::err($crate::prelude::fmt!(
" # {}: ASSERTION FAILED at {FILE}:{LINE}\n",
$name
));
- $crate::kunit::err(format_args!(
+ $crate::kunit::err($crate::prelude::fmt!(
" Expected {CONDITION} to be true, but is false\n"
));
- $crate::kunit::err(format_args!(
+ $crate::kunit::err($crate::prelude::fmt!(
" Failure not reported to KUnit since this is a non-KUnit task\n"
));
break 'out;
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 6b4774b2b1c3..aadcfaa5c759 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -77,6 +77,7 @@
pub mod faux;
#[cfg(CONFIG_RUST_FW_LOADER_ABSTRACTIONS)]
pub mod firmware;
+pub mod fmt;
pub mod fs;
pub mod init;
pub mod io;
diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
index 2f30a398dddd..41cebd906c4c 100644
--- a/rust/kernel/prelude.rs
+++ b/rust/kernel/prelude.rs
@@ -22,7 +22,7 @@
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};

@@ -31,7 +31,6 @@
// `super::std_vendor` is hidden, which makes the macro inline for some reason.
#[doc(no_inline)]
pub use super::dbg;
-pub use super::fmt;
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};

diff --git a/rust/kernel/print.rs b/rust/kernel/print.rs
index 9783d960a97a..0f5e15128005 100644
--- a/rust/kernel/print.rs
+++ b/rust/kernel/print.rs
@@ -149,7 +149,7 @@ macro_rules! print_macro (
// takes borrows on the arguments, but does not extend the scope of temporaries.
// Therefore, a `match` expression is used to keep them around, since
// the scrutinee is kept until the end of the `match`.
- match format_args!($($arg)+) {
+ match $crate::prelude::fmt!($($arg)+) {
// SAFETY: This hidden macro should only be called by the documented
// printing macros which ensure the format string is one of the fixed
// ones. All `__LOG_PREFIX`s are null-terminated as they are generated
@@ -168,7 +168,7 @@ macro_rules! print_macro (
// The `CONT` case.
($format_string:path, true, $($arg:tt)+) => (
$crate::print::call_printk_cont(
- format_args!($($arg)+),
+ $crate::prelude::fmt!($($arg)+),
);
);
);
diff --git a/rust/kernel/seq_file.rs b/rust/kernel/seq_file.rs
index 7a9403eb6e5b..627bc2f7b3d2 100644
--- a/rust/kernel/seq_file.rs
+++ b/rust/kernel/seq_file.rs
@@ -47,7 +47,7 @@ pub fn call_printf(&self, args: core::fmt::Arguments<'_>) {
#[macro_export]
macro_rules! seq_print {
($m:expr, $($arg:tt)+) => (
- $m.call_printf(format_args!($($arg)+))
+ $m.call_printf($crate::prelude::fmt!($($arg)+))
);
}
pub use seq_print;
diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
index a927db8e079c..3ef99ba2e93c 100644
--- a/rust/kernel/str.rs
+++ b/rust/kernel/str.rs
@@ -50,11 +50,11 @@ pub fn strip_prefix(&self, pattern: impl AsRef<Self>) -> Option<&BStr> {
}
}

-impl fmt::Display for BStr {
+impl crate::fmt::Display for BStr {
/// Formats printable ASCII characters, escaping the rest.
///
/// ```
- /// # use kernel::{fmt, b_str, str::{BStr, CString}};
+ /// # use kernel::{prelude::fmt, b_str, str::{BStr, CString}};
/// let ascii = b_str!("Hello, BStr!");
/// let s = CString::try_from_fmt(fmt!("{}", ascii))?;
/// assert_eq!(s.as_bytes(), "Hello, BStr!".as_bytes());
@@ -85,7 +85,7 @@ impl fmt::Debug for BStr {
/// escaping the rest.
///
/// ```
- /// # use kernel::{fmt, b_str, str::{BStr, CString}};
+ /// # use kernel::{prelude::fmt, b_str, str::{BStr, CString}};
/// // Embedded double quotes are escaped.
/// let ascii = b_str!("Hello, \"BStr\"!");
/// let s = CString::try_from_fmt(fmt!("{:?}", ascii))?;
@@ -424,12 +424,12 @@ pub fn to_ascii_uppercase(&self) -> Result<CString, AllocError> {
}
}

-impl fmt::Display for CStr {
+impl crate::fmt::Display for CStr {
/// Formats printable ASCII characters, escaping the rest.
///
/// ```
/// # use kernel::c_str;
- /// # use kernel::fmt;
+ /// # use kernel::prelude::fmt;
/// # use kernel::str::CStr;
/// # use kernel::str::CString;
/// let penguin = c_str!("🐧");
@@ -459,7 +459,7 @@ impl fmt::Debug for CStr {
///
/// ```
/// # use kernel::c_str;
- /// # use kernel::fmt;
+ /// # use kernel::prelude::fmt;
/// # use kernel::str::CStr;
/// # use kernel::str::CString;
/// let penguin = c_str!("🐧");
@@ -578,7 +578,7 @@ mod tests {

macro_rules! format {
($($f:tt)*) => ({
- CString::try_from_fmt(::kernel::fmt!($($f)*))?.to_str()?
+ CString::try_from_fmt(crate::prelude::fmt!($($f)*))?.to_str()?
})
}

@@ -840,7 +840,7 @@ fn write_str(&mut self, s: &str) -> fmt::Result {
/// # Examples
///
/// ```
-/// use kernel::{str::CString, fmt};
+/// use kernel::{str::CString, prelude::fmt};
///
/// let s = CString::try_from_fmt(fmt!("{}{}{}", "abc", 10, 20))?;
/// assert_eq!(s.as_bytes_with_nul(), "abc1020\0".as_bytes());
@@ -930,9 +930,3 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
-
-/// A convenience alias for [`core::format_args`].
-#[macro_export]
-macro_rules! fmt {
- ($($f:tt)*) => ( ::core::format_args!($($f)*) )
-}
diff --git a/rust/macros/fmt.rs b/rust/macros/fmt.rs
new file mode 100644
index 000000000000..edc37c220a89
--- /dev/null
+++ b/rust/macros/fmt.rs
@@ -0,0 +1,99 @@
+// 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_lit = {
+ let Some((mut first_str, first_lit)) = (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))
+ })
+ }
+ _ => None,
+ }) else {
+ return first_opt.into_iter().chain(input).collect();
+ };
+ 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_lit
+ };
+
+ let first_span = first_lit.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..980e70d253e4 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 `core::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),
diff --git a/scripts/rustdoc_test_gen.rs b/scripts/rustdoc_test_gen.rs
index 1ca253594d38..507d36875196 100644
--- a/scripts/rustdoc_test_gen.rs
+++ b/scripts/rustdoc_test_gen.rs
@@ -201,7 +201,7 @@ macro_rules! assert_eq {{
// This follows the syntax for declaring test metadata in the proposed KTAP v2 spec, which may
// be used for the proposed KUnit test attributes API. Thus hopefully this will make migration
// easier later on.
- ::kernel::kunit::info(format_args!(" # {kunit_name}.location: {real_path}:{line}\n"));
+ ::kernel::kunit::info(fmt!(" # {kunit_name}.location: {real_path}:{line}\n"));

/// The anchor where the test code body starts.
#[allow(unused)]

--
2.50.0