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, 242 insertions, 0 deletions

diff --git a/rust/syn/parse_quote.rs b/rust/syn/parse_quote.rs
new file mode 100644
index 000000000..7cc7b9b8f
--- /dev/null
+++ b/rust/syn/parse_quote.rs
@@ -0,0 +1,242 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+/// Quasi-quotation macro that accepts input like the [`quote!`] macro but uses
+/// type inference to figure out a return type for those tokens.
+///
+/// [`quote!`]: https://docs.rs/quote/1.0/quote/index.html
+///
+/// The return type can be any syntax tree node that implements the [`Parse`]
+/// trait.
+///
+/// [`Parse`]: crate::parse::Parse
+///
+/// ```
+/// use quote::quote;
+/// use syn::{parse_quote, Stmt};
+///
+/// fn main() {
+///     let name = quote!(v);
+///     let ty = quote!(u8);
+///
+///     let stmt: Stmt = parse_quote! {
+///         let #name: #ty = Default::default();
+///     };
+///
+///     println!("{:#?}", stmt);
+/// }
+/// ```
+///
+/// *This macro is available only if Syn is built with both the `"parsing"` and
+/// `"printing"` features.*
+///
+/// # Example
+///
+/// The following helper function adds a bound `T: HeapSize` to every type
+/// parameter `T` in the input generics.
+///
+/// ```
+/// use syn::{parse_quote, Generics, GenericParam};
+///
+/// // Add a bound `T: HeapSize` to every type parameter T.
+/// fn add_trait_bounds(mut generics: Generics) -> Generics {
+///     for param in &mut generics.params {
+///         if let GenericParam::Type(type_param) = param {
+///             type_param.bounds.push(parse_quote!(HeapSize));
+///         }
+///     }
+///     generics
+/// }
+/// ```
+///
+/// # Special cases
+///
+/// This macro can parse the following additional types as a special case even
+/// though they do not implement the `Parse` trait.
+///
+/// - [`Attribute`] — parses one attribute, allowing either outer like `#[...]`
+///   or inner like `#![...]`
+/// - [`Vec<Attribute>`] — parses multiple attributes, including mixed kinds in
+///   any order
+/// - [`Punctuated<T, P>`] — parses zero or more `T` separated by punctuation
+///   `P` with optional trailing punctuation
+/// - [`Vec<Arm>`] — parses arms separated by optional commas according to the
+///   same grammar as the inside of a `match` expression
+/// - [`Vec<Stmt>`] — parses the same as `Block::parse_within`
+/// - [`Pat`], [`Box<Pat>`] — parses the same as
+///   `Pat::parse_multi_with_leading_vert`
+/// - [`Field`] — parses a named or unnamed struct field
+///
+/// [`Vec<Attribute>`]: Attribute
+/// [`Vec<Arm>`]: Arm
+/// [`Vec<Stmt>`]: Block::parse_within
+/// [`Pat`]: Pat::parse_multi_with_leading_vert
+/// [`Box<Pat>`]: Pat::parse_multi_with_leading_vert
+///
+/// # Panics
+///
+/// Panics if the tokens fail to parse as the expected syntax tree type. The
+/// caller is responsible for ensuring that the input tokens are syntactically
+/// valid.
+#[cfg_attr(docsrs, doc(cfg(all(feature = "parsing", feature = "printing"))))]
+#[macro_export]
+macro_rules! parse_quote {
+    ($($tt:tt)*) => {
+        $crate::__private::parse_quote($crate::__private::quote::quote!($($tt)*))
+    };
+}
+
+/// This macro is [`parse_quote!`] + [`quote_spanned!`][quote::quote_spanned].
+///
+/// Please refer to each of their documentation.
+///
+/// # Example
+///
+/// ```
+/// use quote::{quote, quote_spanned};
+/// use syn::spanned::Spanned;
+/// use syn::{parse_quote_spanned, ReturnType, Signature};
+///
+/// // Changes `fn()` to `fn() -> Pin<Box<dyn Future<Output = ()>>>`,
+/// // and `fn() -> T` to `fn() -> Pin<Box<dyn Future<Output = T>>>`,
+/// // without introducing any call_site() spans.
+/// fn make_ret_pinned_future(sig: &mut Signature) {
+///     let ret = match &sig.output {
+///         ReturnType::Default => quote_spanned!(sig.paren_token.span=> ()),
+///         ReturnType::Type(_, ret) => quote!(#ret),
+///     };
+///     sig.output = parse_quote_spanned! {ret.span()=>
+///         -> ::std::pin::Pin<::std::boxed::Box<dyn ::std::future::Future<Output = #ret>>>
+///     };
+/// }
+/// ```
+#[cfg_attr(docsrs, doc(cfg(all(feature = "parsing", feature = "printing"))))]
+#[macro_export]
+macro_rules! parse_quote_spanned {
+    ($span:expr=> $($tt:tt)*) => {
+        $crate::__private::parse_quote($crate::__private::quote::quote_spanned!($span=> $($tt)*))
+    };
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Can parse any type that implements Parse.
+
+use crate::error::Result;
+use crate::parse::{Parse, ParseStream, Parser};
+use proc_macro2::TokenStream;
+
+// Not public API.
+#[doc(hidden)]
+#[track_caller]
+pub fn parse<T: ParseQuote>(token_stream: TokenStream) -> T {
+    let parser = T::parse;
+    match parser.parse2(token_stream) {
+        Ok(t) => t,
+        Err(err) => panic!("{}", err),
+    }
+}
+
+#[doc(hidden)]
+pub trait ParseQuote: Sized {
+    fn parse(input: ParseStream) -> Result<Self>;
+}
+
+impl<T: Parse> ParseQuote for T {
+    fn parse(input: ParseStream) -> Result<Self> {
+        <T as Parse>::parse(input)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Any other types that we want `parse_quote!` to be able to parse.
+
+use crate::punctuated::Punctuated;
+#[cfg(any(feature = "full", feature = "derive"))]
+use crate::{attr, Attribute, Field, FieldMutability, Ident, Type, Visibility};
+#[cfg(feature = "full")]
+use crate::{Arm, Block, Pat, Stmt};
+
+#[cfg(any(feature = "full", feature = "derive"))]
+impl ParseQuote for Attribute {
+    fn parse(input: ParseStream) -> Result<Self> {
+        if input.peek(Token![#]) && input.peek2(Token![!]) {
+            attr::parsing::single_parse_inner(input)
+        } else {
+            attr::parsing::single_parse_outer(input)
+        }
+    }
+}
+
+#[cfg(any(feature = "full", feature = "derive"))]
+impl ParseQuote for Vec<Attribute> {
+    fn parse(input: ParseStream) -> Result<Self> {
+        let mut attrs = Vec::new();
+        while !input.is_empty() {
+            attrs.push(ParseQuote::parse(input)?);
+        }
+        Ok(attrs)
+    }
+}
+
+#[cfg(any(feature = "full", feature = "derive"))]
+impl ParseQuote for Field {
+    fn parse(input: ParseStream) -> Result<Self> {
+        let attrs = input.call(Attribute::parse_outer)?;
+        let vis: Visibility = input.parse()?;
+
+        let ident: Option<Ident>;
+        let colon_token: Option<Token![:]>;
+        let is_named = input.peek(Ident) && input.peek2(Token![:]) && !input.peek2(Token![::]);
+        if is_named {
+            ident = Some(input.parse()?);
+            colon_token = Some(input.parse()?);
+        } else {
+            ident = None;
+            colon_token = None;
+        }
+
+        let ty: Type = input.parse()?;
+
+        Ok(Field {
+            attrs,
+            vis,
+            mutability: FieldMutability::None,
+            ident,
+            colon_token,
+            ty,
+        })
+    }
+}
+
+#[cfg(feature = "full")]
+impl ParseQuote for Pat {
+    fn parse(input: ParseStream) -> Result<Self> {
+        Pat::parse_multi_with_leading_vert(input)
+    }
+}
+
+#[cfg(feature = "full")]
+impl ParseQuote for Box<Pat> {
+    fn parse(input: ParseStream) -> Result<Self> {
+        <Pat as ParseQuote>::parse(input).map(Box::new)
+    }
+}
+
+impl<T: Parse, P: Parse> ParseQuote for Punctuated<T, P> {
+    fn parse(input: ParseStream) -> Result<Self> {
+        Self::parse_terminated(input)
+    }
+}
+
+#[cfg(feature = "full")]
+impl ParseQuote for Vec<Stmt> {
+    fn parse(input: ParseStream) -> Result<Self> {
+        Block::parse_within(input)
+    }
+}
+
+#[cfg(feature = "full")]
+impl ParseQuote for Vec<Arm> {
+    fn parse(input: ParseStream) -> Result<Self> {
+        Arm::parse_multiple(input)
+    }
+}