kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 469 insertions, 0 deletions

diff --git a/rust/syn/error.rs b/rust/syn/error.rs
new file mode 100644
index 000000000..6fa0faf7f
--- /dev/null
+++ b/rust/syn/error.rs
@@ -0,0 +1,469 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+#[cfg(feature = "parsing")]
+use crate::buffer::Cursor;
+use crate::thread::ThreadBound;
+use proc_macro2::{
+    Delimiter, Group, Ident, LexError, Literal, Punct, Spacing, Span, TokenStream, TokenTree,
+};
+#[cfg(feature = "printing")]
+use quote::ToTokens;
+use std::fmt::{self, Debug, Display};
+use std::slice;
+use std::vec;
+
+/// The result of a Syn parser.
+pub type Result<T> = std::result::Result<T, Error>;
+
+/// Error returned when a Syn parser cannot parse the input tokens.
+///
+/// # Error reporting in proc macros
+///
+/// The correct way to report errors back to the compiler from a procedural
+/// macro is by emitting an appropriately spanned invocation of
+/// [`compile_error!`] in the generated code. This produces a better diagnostic
+/// message than simply panicking the macro.
+///
+/// [`compile_error!`]: std::compile_error!
+///
+/// When parsing macro input, the [`parse_macro_input!`] macro handles the
+/// conversion to `compile_error!` automatically.
+///
+/// [`parse_macro_input!`]: crate::parse_macro_input!
+///
+/// ```
+/// # extern crate proc_macro;
+/// #
+/// use proc_macro::TokenStream;
+/// use syn::parse::{Parse, ParseStream, Result};
+/// use syn::{parse_macro_input, ItemFn};
+///
+/// # const IGNORE: &str = stringify! {
+/// #[proc_macro_attribute]
+/// # };
+/// pub fn my_attr(args: TokenStream, input: TokenStream) -> TokenStream {
+///     let args = parse_macro_input!(args as MyAttrArgs);
+///     let input = parse_macro_input!(input as ItemFn);
+///
+///     /* ... */
+///     # TokenStream::new()
+/// }
+///
+/// struct MyAttrArgs {
+///     # _k: [(); { stringify! {
+///     ...
+///     # }; 0 }]
+/// }
+///
+/// impl Parse for MyAttrArgs {
+///     fn parse(input: ParseStream) -> Result<Self> {
+///         # stringify! {
+///         ...
+///         # };
+///         # unimplemented!()
+///     }
+/// }
+/// ```
+///
+/// For errors that arise later than the initial parsing stage, the
+/// [`.to_compile_error()`] or [`.into_compile_error()`] methods can be used to
+/// perform an explicit conversion to `compile_error!`.
+///
+/// [`.to_compile_error()`]: Error::to_compile_error
+/// [`.into_compile_error()`]: Error::into_compile_error
+///
+/// ```
+/// # extern crate proc_macro;
+/// #
+/// # use proc_macro::TokenStream;
+/// # use syn::{parse_macro_input, DeriveInput};
+/// #
+/// # const IGNORE: &str = stringify! {
+/// #[proc_macro_derive(MyDerive)]
+/// # };
+/// pub fn my_derive(input: TokenStream) -> TokenStream {
+///     let input = parse_macro_input!(input as DeriveInput);
+///
+///     // fn(DeriveInput) -> syn::Result<proc_macro2::TokenStream>
+///     expand::my_derive(input)
+///         .unwrap_or_else(syn::Error::into_compile_error)
+///         .into()
+/// }
+/// #
+/// # mod expand {
+/// #     use proc_macro2::TokenStream;
+/// #     use syn::{DeriveInput, Result};
+/// #
+/// #     pub fn my_derive(input: DeriveInput) -> Result<TokenStream> {
+/// #         unimplemented!()
+/// #     }
+/// # }
+/// ```
+pub struct Error {
+    messages: Vec<ErrorMessage>,
+}
+
+struct ErrorMessage {
+    // Span is implemented as an index into a thread-local interner to keep the
+    // size small. It is not safe to access from a different thread. We want
+    // errors to be Send and Sync to play nicely with ecosystem crates for error
+    // handling, so pin the span we're given to its original thread and assume
+    // it is Span::call_site if accessed from any other thread.
+    span: ThreadBound<SpanRange>,
+    message: String,
+}
+
+// Cannot use std::ops::Range<Span> because that does not implement Copy,
+// whereas ThreadBound<T> requires a Copy impl as a way to ensure no Drop impls
+// are involved.
+struct SpanRange {
+    start: Span,
+    end: Span,
+}
+
+#[cfg(test)]
+struct _Test
+where
+    Error: Send + Sync;
+
+impl Error {
+    /// Usually the [`ParseStream::error`] method will be used instead, which
+    /// automatically uses the correct span from the current position of the
+    /// parse stream.
+    ///
+    /// Use `Error::new` when the error needs to be triggered on some span other
+    /// than where the parse stream is currently positioned.
+    ///
+    /// [`ParseStream::error`]: crate::parse::ParseBuffer::error
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use syn::{Error, Ident, LitStr, Result, Token};
+    /// use syn::parse::ParseStream;
+    ///
+    /// // Parses input that looks like `name = "string"` where the key must be
+    /// // the identifier `name` and the value may be any string literal.
+    /// // Returns the string literal.
+    /// fn parse_name(input: ParseStream) -> Result<LitStr> {
+    ///     let name_token: Ident = input.parse()?;
+    ///     if name_token != "name" {
+    ///         // Trigger an error not on the current position of the stream,
+    ///         // but on the position of the unexpected identifier.
+    ///         return Err(Error::new(name_token.span(), "expected `name`"));
+    ///     }
+    ///     input.parse::<Token![=]>()?;
+    ///     let s: LitStr = input.parse()?;
+    ///     Ok(s)
+    /// }
+    /// ```
+    pub fn new<T: Display>(span: Span, message: T) -> Self {
+        return new(span, message.to_string());
+
+        fn new(span: Span, message: String) -> Error {
+            Error {
+                messages: vec![ErrorMessage {
+                    span: ThreadBound::new(SpanRange {
+                        start: span,
+                        end: span,
+                    }),
+                    message,
+                }],
+            }
+        }
+    }
+
+    /// Creates an error with the specified message spanning the given syntax
+    /// tree node.
+    ///
+    /// Unlike the `Error::new` constructor, this constructor takes an argument
+    /// `tokens` which is a syntax tree node. This allows the resulting `Error`
+    /// to attempt to span all tokens inside of `tokens`. While you would
+    /// typically be able to use the `Spanned` trait with the above `Error::new`
+    /// constructor, implementation limitations today mean that
+    /// `Error::new_spanned` may provide a higher-quality error message on
+    /// stable Rust.
+    ///
+    /// When in doubt it's recommended to stick to `Error::new` (or
+    /// `ParseStream::error`)!
+    #[cfg(feature = "printing")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
+    pub fn new_spanned<T: ToTokens, U: Display>(tokens: T, message: U) -> Self {
+        return new_spanned(tokens.into_token_stream(), message.to_string());
+
+        fn new_spanned(tokens: TokenStream, message: String) -> Error {
+            let mut iter = tokens.into_iter();
+            let start = iter.next().map_or_else(Span::call_site, |t| t.span());
+            let end = iter.last().map_or(start, |t| t.span());
+            Error {
+                messages: vec![ErrorMessage {
+                    span: ThreadBound::new(SpanRange { start, end }),
+                    message,
+                }],
+            }
+        }
+    }
+
+    /// The source location of the error.
+    ///
+    /// Spans are not thread-safe so this function returns `Span::call_site()`
+    /// if called from a different thread than the one on which the `Error` was
+    /// originally created.
+    pub fn span(&self) -> Span {
+        let SpanRange { start, end } = match self.messages[0].span.get() {
+            Some(span) => *span,
+            None => return Span::call_site(),
+        };
+        start.join(end).unwrap_or(start)
+    }
+
+    /// Render the error as an invocation of [`compile_error!`].
+    ///
+    /// The [`parse_macro_input!`] macro provides a convenient way to invoke
+    /// this method correctly in a procedural macro.
+    ///
+    /// [`compile_error!`]: std::compile_error!
+    /// [`parse_macro_input!`]: crate::parse_macro_input!
+    pub fn to_compile_error(&self) -> TokenStream {
+        self.messages
+            .iter()
+            .map(ErrorMessage::to_compile_error)
+            .collect()
+    }
+
+    /// Render the error as an invocation of [`compile_error!`].
+    ///
+    /// [`compile_error!`]: std::compile_error!
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # extern crate proc_macro;
+    /// #
+    /// use proc_macro::TokenStream;
+    /// use syn::{parse_macro_input, DeriveInput, Error};
+    ///
+    /// # const _: &str = stringify! {
+    /// #[proc_macro_derive(MyTrait)]
+    /// # };
+    /// pub fn derive_my_trait(input: TokenStream) -> TokenStream {
+    ///     let input = parse_macro_input!(input as DeriveInput);
+    ///     my_trait::expand(input)
+    ///         .unwrap_or_else(Error::into_compile_error)
+    ///         .into()
+    /// }
+    ///
+    /// mod my_trait {
+    ///     use proc_macro2::TokenStream;
+    ///     use syn::{DeriveInput, Result};
+    ///
+    ///     pub(crate) fn expand(input: DeriveInput) -> Result<TokenStream> {
+    ///         /* ... */
+    ///         # unimplemented!()
+    ///     }
+    /// }
+    /// ```
+    pub fn into_compile_error(self) -> TokenStream {
+        self.to_compile_error()
+    }
+
+    /// Add another error message to self such that when `to_compile_error()` is
+    /// called, both errors will be emitted together.
+    pub fn combine(&mut self, another: Error) {
+        self.messages.extend(another.messages);
+    }
+}
+
+impl ErrorMessage {
+    fn to_compile_error(&self) -> TokenStream {
+        let (start, end) = match self.span.get() {
+            Some(range) => (range.start, range.end),
+            None => (Span::call_site(), Span::call_site()),
+        };
+
+        // ::core::compile_error!($message)
+        TokenStream::from_iter([
+            TokenTree::Punct({
+                let mut punct = Punct::new(':', Spacing::Joint);
+                punct.set_span(start);
+                punct
+            }),
+            TokenTree::Punct({
+                let mut punct = Punct::new(':', Spacing::Alone);
+                punct.set_span(start);
+                punct
+            }),
+            TokenTree::Ident(Ident::new("core", start)),
+            TokenTree::Punct({
+                let mut punct = Punct::new(':', Spacing::Joint);
+                punct.set_span(start);
+                punct
+            }),
+            TokenTree::Punct({
+                let mut punct = Punct::new(':', Spacing::Alone);
+                punct.set_span(start);
+                punct
+            }),
+            TokenTree::Ident(Ident::new("compile_error", start)),
+            TokenTree::Punct({
+                let mut punct = Punct::new('!', Spacing::Alone);
+                punct.set_span(start);
+                punct
+            }),
+            TokenTree::Group({
+                let mut group = Group::new(Delimiter::Brace, {
+                    TokenStream::from_iter([TokenTree::Literal({
+                        let mut string = Literal::string(&self.message);
+                        string.set_span(end);
+                        string
+                    })])
+                });
+                group.set_span(end);
+                group
+            }),
+        ])
+    }
+}
+
+#[cfg(feature = "parsing")]
+pub(crate) fn new_at<T: Display>(scope: Span, cursor: Cursor, message: T) -> Error {
+    if cursor.eof() {
+        Error::new(scope, format!("unexpected end of input, {}", message))
+    } else {
+        let span = crate::buffer::open_span_of_group(cursor);
+        Error::new(span, message)
+    }
+}
+
+#[cfg(all(feature = "parsing", any(feature = "full", feature = "derive")))]
+pub(crate) fn new2<T: Display>(start: Span, end: Span, message: T) -> Error {
+    return new2(start, end, message.to_string());
+
+    fn new2(start: Span, end: Span, message: String) -> Error {
+        Error {
+            messages: vec![ErrorMessage {
+                span: ThreadBound::new(SpanRange { start, end }),
+                message,
+            }],
+        }
+    }
+}
+
+impl Debug for Error {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        if self.messages.len() == 1 {
+            formatter
+                .debug_tuple("Error")
+                .field(&self.messages[0])
+                .finish()
+        } else {
+            formatter
+                .debug_tuple("Error")
+                .field(&self.messages)
+                .finish()
+        }
+    }
+}
+
+impl Debug for ErrorMessage {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        Debug::fmt(&self.message, formatter)
+    }
+}
+
+impl Display for Error {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str(&self.messages[0].message)
+    }
+}
+
+impl Clone for Error {
+    fn clone(&self) -> Self {
+        Error {
+            messages: self.messages.clone(),
+        }
+    }
+}
+
+impl Clone for ErrorMessage {
+    fn clone(&self) -> Self {
+        ErrorMessage {
+            span: self.span,
+            message: self.message.clone(),
+        }
+    }
+}
+
+impl Clone for SpanRange {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl Copy for SpanRange {}
+
+impl std::error::Error for Error {}
+
+impl From<LexError> for Error {
+    fn from(err: LexError) -> Self {
+        Error::new(err.span(), err)
+    }
+}
+
+impl IntoIterator for Error {
+    type Item = Error;
+    type IntoIter = IntoIter;
+
+    fn into_iter(self) -> Self::IntoIter {
+        IntoIter {
+            messages: self.messages.into_iter(),
+        }
+    }
+}
+
+pub struct IntoIter {
+    messages: vec::IntoIter<ErrorMessage>,
+}
+
+impl Iterator for IntoIter {
+    type Item = Error;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        Some(Error {
+            messages: vec![self.messages.next()?],
+        })
+    }
+}
+
+impl<'a> IntoIterator for &'a Error {
+    type Item = Error;
+    type IntoIter = Iter<'a>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        Iter {
+            messages: self.messages.iter(),
+        }
+    }
+}
+
+pub struct Iter<'a> {
+    messages: slice::Iter<'a, ErrorMessage>,
+}
+
+impl<'a> Iterator for Iter<'a> {
+    type Item = Error;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        Some(Error {
+            messages: vec![self.messages.next()?.clone()],
+        })
+    }
+}
+
+impl Extend<Error> for Error {
+    fn extend<T: IntoIterator<Item = Error>>(&mut self, iter: T) {
+        for err in iter {
+            self.combine(err);
+        }
+    }
+}