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, 221 insertions, 0 deletions

diff --git a/rust/syn/op.rs b/rust/syn/op.rs
new file mode 100644
index 000000000..73dada3fa
--- /dev/null
+++ b/rust/syn/op.rs
@@ -0,0 +1,221 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+ast_enum! {
+    /// A binary operator: `+`, `+=`, `&`.
+    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
+    #[non_exhaustive]
+    pub enum BinOp {
+        /// The `+` operator (addition)
+        Add(Token![+]),
+        /// The `-` operator (subtraction)
+        Sub(Token![-]),
+        /// The `*` operator (multiplication)
+        Mul(Token![*]),
+        /// The `/` operator (division)
+        Div(Token![/]),
+        /// The `%` operator (modulus)
+        Rem(Token![%]),
+        /// The `&&` operator (logical and)
+        And(Token![&&]),
+        /// The `||` operator (logical or)
+        Or(Token![||]),
+        /// The `^` operator (bitwise xor)
+        BitXor(Token![^]),
+        /// The `&` operator (bitwise and)
+        BitAnd(Token![&]),
+        /// The `|` operator (bitwise or)
+        BitOr(Token![|]),
+        /// The `<<` operator (shift left)
+        Shl(Token![<<]),
+        /// The `>>` operator (shift right)
+        Shr(Token![>>]),
+        /// The `==` operator (equality)
+        Eq(Token![==]),
+        /// The `<` operator (less than)
+        Lt(Token![<]),
+        /// The `<=` operator (less than or equal to)
+        Le(Token![<=]),
+        /// The `!=` operator (not equal to)
+        Ne(Token![!=]),
+        /// The `>=` operator (greater than or equal to)
+        Ge(Token![>=]),
+        /// The `>` operator (greater than)
+        Gt(Token![>]),
+        /// The `+=` operator
+        AddAssign(Token![+=]),
+        /// The `-=` operator
+        SubAssign(Token![-=]),
+        /// The `*=` operator
+        MulAssign(Token![*=]),
+        /// The `/=` operator
+        DivAssign(Token![/=]),
+        /// The `%=` operator
+        RemAssign(Token![%=]),
+        /// The `^=` operator
+        BitXorAssign(Token![^=]),
+        /// The `&=` operator
+        BitAndAssign(Token![&=]),
+        /// The `|=` operator
+        BitOrAssign(Token![|=]),
+        /// The `<<=` operator
+        ShlAssign(Token![<<=]),
+        /// The `>>=` operator
+        ShrAssign(Token![>>=]),
+    }
+}
+
+ast_enum! {
+    /// A unary operator: `*`, `!`, `-`.
+    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
+    #[non_exhaustive]
+    pub enum UnOp {
+        /// The `*` operator for dereferencing
+        Deref(Token![*]),
+        /// The `!` operator for logical inversion
+        Not(Token![!]),
+        /// The `-` operator for negation
+        Neg(Token![-]),
+    }
+}
+
+#[cfg(feature = "parsing")]
+pub(crate) mod parsing {
+    use crate::error::Result;
+    use crate::op::{BinOp, UnOp};
+    use crate::parse::{Parse, ParseStream};
+
+    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
+    impl Parse for BinOp {
+        fn parse(input: ParseStream) -> Result<Self> {
+            if input.peek(Token![+=]) {
+                input.parse().map(BinOp::AddAssign)
+            } else if input.peek(Token![-=]) {
+                input.parse().map(BinOp::SubAssign)
+            } else if input.peek(Token![*=]) {
+                input.parse().map(BinOp::MulAssign)
+            } else if input.peek(Token![/=]) {
+                input.parse().map(BinOp::DivAssign)
+            } else if input.peek(Token![%=]) {
+                input.parse().map(BinOp::RemAssign)
+            } else if input.peek(Token![^=]) {
+                input.parse().map(BinOp::BitXorAssign)
+            } else if input.peek(Token![&=]) {
+                input.parse().map(BinOp::BitAndAssign)
+            } else if input.peek(Token![|=]) {
+                input.parse().map(BinOp::BitOrAssign)
+            } else if input.peek(Token![<<=]) {
+                input.parse().map(BinOp::ShlAssign)
+            } else if input.peek(Token![>>=]) {
+                input.parse().map(BinOp::ShrAssign)
+            } else if input.peek(Token![&&]) {
+                input.parse().map(BinOp::And)
+            } else if input.peek(Token![||]) {
+                input.parse().map(BinOp::Or)
+            } else if input.peek(Token![<<]) {
+                input.parse().map(BinOp::Shl)
+            } else if input.peek(Token![>>]) {
+                input.parse().map(BinOp::Shr)
+            } else if input.peek(Token![==]) {
+                input.parse().map(BinOp::Eq)
+            } else if input.peek(Token![<=]) {
+                input.parse().map(BinOp::Le)
+            } else if input.peek(Token![!=]) {
+                input.parse().map(BinOp::Ne)
+            } else if input.peek(Token![>=]) {
+                input.parse().map(BinOp::Ge)
+            } else if input.peek(Token![+]) {
+                input.parse().map(BinOp::Add)
+            } else if input.peek(Token![-]) {
+                input.parse().map(BinOp::Sub)
+            } else if input.peek(Token![*]) {
+                input.parse().map(BinOp::Mul)
+            } else if input.peek(Token![/]) {
+                input.parse().map(BinOp::Div)
+            } else if input.peek(Token![%]) {
+                input.parse().map(BinOp::Rem)
+            } else if input.peek(Token![^]) {
+                input.parse().map(BinOp::BitXor)
+            } else if input.peek(Token![&]) {
+                input.parse().map(BinOp::BitAnd)
+            } else if input.peek(Token![|]) {
+                input.parse().map(BinOp::BitOr)
+            } else if input.peek(Token![<]) {
+                input.parse().map(BinOp::Lt)
+            } else if input.peek(Token![>]) {
+                input.parse().map(BinOp::Gt)
+            } else {
+                Err(input.error("expected binary operator"))
+            }
+        }
+    }
+
+    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
+    impl Parse for UnOp {
+        fn parse(input: ParseStream) -> Result<Self> {
+            let lookahead = input.lookahead1();
+            if lookahead.peek(Token![*]) {
+                input.parse().map(UnOp::Deref)
+            } else if lookahead.peek(Token![!]) {
+                input.parse().map(UnOp::Not)
+            } else if lookahead.peek(Token![-]) {
+                input.parse().map(UnOp::Neg)
+            } else {
+                Err(lookahead.error())
+            }
+        }
+    }
+}
+
+#[cfg(feature = "printing")]
+mod printing {
+    use crate::op::{BinOp, UnOp};
+    use proc_macro2::TokenStream;
+    use quote::ToTokens;
+
+    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
+    impl ToTokens for BinOp {
+        fn to_tokens(&self, tokens: &mut TokenStream) {
+            match self {
+                BinOp::Add(t) => t.to_tokens(tokens),
+                BinOp::Sub(t) => t.to_tokens(tokens),
+                BinOp::Mul(t) => t.to_tokens(tokens),
+                BinOp::Div(t) => t.to_tokens(tokens),
+                BinOp::Rem(t) => t.to_tokens(tokens),
+                BinOp::And(t) => t.to_tokens(tokens),
+                BinOp::Or(t) => t.to_tokens(tokens),
+                BinOp::BitXor(t) => t.to_tokens(tokens),
+                BinOp::BitAnd(t) => t.to_tokens(tokens),
+                BinOp::BitOr(t) => t.to_tokens(tokens),
+                BinOp::Shl(t) => t.to_tokens(tokens),
+                BinOp::Shr(t) => t.to_tokens(tokens),
+                BinOp::Eq(t) => t.to_tokens(tokens),
+                BinOp::Lt(t) => t.to_tokens(tokens),
+                BinOp::Le(t) => t.to_tokens(tokens),
+                BinOp::Ne(t) => t.to_tokens(tokens),
+                BinOp::Ge(t) => t.to_tokens(tokens),
+                BinOp::Gt(t) => t.to_tokens(tokens),
+                BinOp::AddAssign(t) => t.to_tokens(tokens),
+                BinOp::SubAssign(t) => t.to_tokens(tokens),
+                BinOp::MulAssign(t) => t.to_tokens(tokens),
+                BinOp::DivAssign(t) => t.to_tokens(tokens),
+                BinOp::RemAssign(t) => t.to_tokens(tokens),
+                BinOp::BitXorAssign(t) => t.to_tokens(tokens),
+                BinOp::BitAndAssign(t) => t.to_tokens(tokens),
+                BinOp::BitOrAssign(t) => t.to_tokens(tokens),
+                BinOp::ShlAssign(t) => t.to_tokens(tokens),
+                BinOp::ShrAssign(t) => t.to_tokens(tokens),
+            }
+        }
+    }
+
+    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
+    impl ToTokens for UnOp {
+        fn to_tokens(&self, tokens: &mut TokenStream) {
+            match self {
+                UnOp::Deref(t) => t.to_tokens(tokens),
+                UnOp::Not(t) => t.to_tokens(tokens),
+                UnOp::Neg(t) => t.to_tokens(tokens),
+            }
+        }
+    }
+}