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, 74 insertions, 0 deletions

diff --git a/net/llc/llc_output.c b/net/llc/llc_output.c
new file mode 100644
index 000000000..5a6466fc6
--- /dev/null
+++ b/net/llc/llc_output.c
@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * llc_output.c - LLC minimal output path
+ *
+ * Copyright (c) 1997 by Procom Technology, Inc.
+ * 		 2001-2003 by Arnaldo Carvalho de Melo <acme@conectiva.com.br>
+ */
+
+#include <linux/if_arp.h>
+#include <linux/netdevice.h>
+#include <linux/skbuff.h>
+#include <linux/export.h>
+#include <net/llc.h>
+#include <net/llc_pdu.h>
+
+/**
+ *	llc_mac_hdr_init - fills MAC header fields
+ *	@skb: Address of the frame to initialize its MAC header
+ *	@sa: The MAC source address
+ *	@da: The MAC destination address
+ *
+ *	Fills MAC header fields, depending on MAC type. Returns 0, If MAC type
+ *	is a valid type and initialization completes correctly 1, otherwise.
+ */
+int llc_mac_hdr_init(struct sk_buff *skb,
+		     const unsigned char *sa, const unsigned char *da)
+{
+	int rc = -EINVAL;
+
+	switch (skb->dev->type) {
+	case ARPHRD_ETHER:
+	case ARPHRD_LOOPBACK:
+		rc = dev_hard_header(skb, skb->dev, ETH_P_802_2, da, sa,
+				     skb->len);
+		if (rc > 0)
+			rc = 0;
+		break;
+	default:
+		break;
+	}
+	return rc;
+}
+
+/**
+ *	llc_build_and_send_ui_pkt - unitdata request interface for upper layers
+ *	@sap: sap to use
+ *	@skb: packet to send
+ *	@dmac: destination mac address
+ *	@dsap: destination sap
+ *
+ *	Upper layers calls this function when upper layer wants to send data
+ *	using connection-less mode communication (UI pdu).
+ *
+ *	Accept data frame from network layer to be sent using connection-
+ *	less mode communication; timeout/retries handled by network layer;
+ *	package primitive as an event and send to SAP event handler
+ */
+int llc_build_and_send_ui_pkt(struct llc_sap *sap, struct sk_buff *skb,
+			      const unsigned char *dmac, unsigned char dsap)
+{
+	int rc;
+	llc_pdu_header_init(skb, LLC_PDU_TYPE_U, sap->laddr.lsap,
+			    dsap, LLC_PDU_CMD);
+	llc_pdu_init_as_ui_cmd(skb);
+	rc = llc_mac_hdr_init(skb, skb->dev->dev_addr, dmac);
+	if (likely(!rc))
+		rc = dev_queue_xmit(skb);
+	else
+		kfree_skb(skb);
+	return rc;
+}
+
+EXPORT_SYMBOL(llc_mac_hdr_init);
+EXPORT_SYMBOL(llc_build_and_send_ui_pkt);