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, 57 insertions, 0 deletions

diff --git a/include/kunit/skbuff.h b/include/kunit/skbuff.h
new file mode 100644
index 000000000..077846943
--- /dev/null
+++ b/include/kunit/skbuff.h
@@ -0,0 +1,57 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * KUnit resource management helpers for SKBs (skbuff).
+ *
+ * Copyright (C) 2023 Intel Corporation
+ */
+
+#ifndef _KUNIT_SKBUFF_H
+#define _KUNIT_SKBUFF_H
+
+#include <kunit/resource.h>
+#include <linux/skbuff.h>
+
+static void kunit_action_kfree_skb(void *p)
+{
+	kfree_skb((struct sk_buff *)p);
+}
+
+/**
+ * kunit_zalloc_skb() - Allocate and initialize a resource managed skb.
+ * @test: The test case to which the skb belongs
+ * @len: size to allocate
+ * @gfp: allocation flags
+ *
+ * Allocate a new struct sk_buff with gfp flags, zero fill the given length
+ * and add it as a resource to the kunit test for automatic cleanup.
+ *
+ * Returns: newly allocated SKB, or %NULL on error
+ */
+static inline struct sk_buff *kunit_zalloc_skb(struct kunit *test, int len,
+					       gfp_t gfp)
+{
+	struct sk_buff *res = alloc_skb(len, gfp);
+
+	if (!res || skb_pad(res, len))
+		return NULL;
+
+	if (kunit_add_action_or_reset(test, kunit_action_kfree_skb, res))
+		return NULL;
+
+	return res;
+}
+
+/**
+ * kunit_kfree_skb() - Like kfree_skb except for allocations managed by KUnit.
+ * @test: The test case to which the resource belongs.
+ * @skb: The SKB to free.
+ */
+static inline void kunit_kfree_skb(struct kunit *test, struct sk_buff *skb)
+{
+	if (!skb)
+		return;
+
+	kunit_release_action(test, kunit_action_kfree_skb, (void *)skb);
+}
+
+#endif /* _KUNIT_SKBUFF_H */