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, 35 insertions, 0 deletions

diff --git a/include/net/hwbm.h b/include/net/hwbm.h
new file mode 100644
index 000000000..bdbe91c60
--- /dev/null
+++ b/include/net/hwbm.h
@@ -0,0 +1,35 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _HWBM_H
+#define _HWBM_H
+
+#include <linux/mutex.h>
+
+struct hwbm_pool {
+	/* Capacity of the pool */
+	int size;
+	/* Size of the buffers managed */
+	int frag_size;
+	/* Number of buffers currently used by this pool */
+	int buf_num;
+	/* constructor called during allocation */
+	int (*construct)(struct hwbm_pool *bm_pool, void *buf);
+	/* protect access to the buffer counter*/
+	struct mutex buf_lock;
+	/* private data */
+	void *priv;
+};
+#ifdef CONFIG_HWBM
+void hwbm_buf_free(struct hwbm_pool *bm_pool, void *buf);
+int hwbm_pool_refill(struct hwbm_pool *bm_pool, gfp_t gfp);
+int hwbm_pool_add(struct hwbm_pool *bm_pool, unsigned int buf_num);
+#else
+static inline void hwbm_buf_free(struct hwbm_pool *bm_pool, void *buf) {}
+
+static inline int hwbm_pool_refill(struct hwbm_pool *bm_pool, gfp_t gfp)
+{ return 0; }
+
+static inline int hwbm_pool_add(struct hwbm_pool *bm_pool,
+				unsigned int buf_num)
+{ return 0; }
+#endif /* CONFIG_HWBM */
+#endif /* _HWBM_H */