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, 56 insertions, 0 deletions

diff --git a/net/batman-adv/bitarray.h b/net/batman-adv/bitarray.h
new file mode 100644
index 000000000..2b9f5dbb3
--- /dev/null
+++ b/net/batman-adv/bitarray.h
@@ -0,0 +1,56 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (C) B.A.T.M.A.N. contributors:
+ *
+ * Simon Wunderlich, Marek Lindner
+ */
+
+#ifndef _NET_BATMAN_ADV_BITARRAY_H_
+#define _NET_BATMAN_ADV_BITARRAY_H_
+
+#include "main.h"
+
+#include <linux/bitops.h>
+#include <linux/compiler.h>
+#include <linux/stddef.h>
+#include <linux/types.h>
+
+/**
+ * batadv_test_bit() - check if bit is set in the current window
+ *
+ * @seq_bits: pointer to the sequence number bitmap of received packets
+ * @last_seqno: latest sequence number in seq_bits
+ * @curr_seqno: sequence number to test for
+ *
+ * Return: true if the corresponding bit in the given seq_bits indicates true
+ * and curr_seqno is within range of last_seqno. Otherwise returns false.
+ */
+static inline bool batadv_test_bit(const unsigned long *seq_bits,
+				   u32 last_seqno, u32 curr_seqno)
+{
+	s32 diff;
+
+	diff = last_seqno - curr_seqno;
+	if (diff < 0 || diff >= BATADV_TQ_LOCAL_WINDOW_SIZE)
+		return false;
+	return test_bit(diff, seq_bits) != 0;
+}
+
+/**
+ * batadv_set_bit() - Turn corresponding bit on, so we can remember that we got
+ *  the packet
+ * @seq_bits: bitmap of the packet receive window
+ * @n: relative sequence number of newly received packet
+ */
+static inline void batadv_set_bit(unsigned long *seq_bits, s32 n)
+{
+	/* if too old, just drop it */
+	if (n < 0 || n >= BATADV_TQ_LOCAL_WINDOW_SIZE)
+		return;
+
+	set_bit(n, seq_bits); /* turn the position on */
+}
+
+bool batadv_bit_get_packet(void *priv, unsigned long *seq_bits,
+			   s32 seq_num_diff, int set_mark);
+
+#endif /* _NET_BATMAN_ADV_BITARRAY_H_ */