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>

3 files changed, 165 insertions, 0 deletions

diff --git a/net/nsh/Kconfig b/net/nsh/Kconfig
new file mode 100644
index 000000000..84f2a2823
--- /dev/null
+++ b/net/nsh/Kconfig
@@ -0,0 +1,10 @@
+# SPDX-License-Identifier: GPL-2.0-only
+menuconfig NET_NSH
+	tristate "Network Service Header (NSH) protocol"
+	default n
+	help
+	  Network Service Header is an implementation of Service Function
+	  Chaining (RFC 7665). The current implementation in Linux supports
+	  only MD type 1 and only with the openvswitch module.
+
+	  If unsure, say N.
diff --git a/net/nsh/Makefile b/net/nsh/Makefile
new file mode 100644
index 000000000..de34a615a
--- /dev/null
+++ b/net/nsh/Makefile
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+obj-$(CONFIG_NET_NSH) += nsh.o
diff --git a/net/nsh/nsh.c b/net/nsh/nsh.c
new file mode 100644
index 000000000..bfb775806
--- /dev/null
+++ b/net/nsh/nsh.c
@@ -0,0 +1,153 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Network Service Header
+ *
+ * Copyright (c) 2017 Red Hat, Inc. -- Jiri Benc <jbenc@redhat.com>
+ */
+
+#include <linux/module.h>
+#include <linux/netdevice.h>
+#include <linux/skbuff.h>
+#include <net/gso.h>
+#include <net/nsh.h>
+#include <net/tun_proto.h>
+
+int nsh_push(struct sk_buff *skb, const struct nshhdr *pushed_nh)
+{
+	struct nshhdr *nh;
+	size_t length = nsh_hdr_len(pushed_nh);
+	u8 next_proto;
+
+	if (skb->mac_len) {
+		next_proto = TUN_P_ETHERNET;
+	} else {
+		next_proto = tun_p_from_eth_p(skb->protocol);
+		if (!next_proto)
+			return -EAFNOSUPPORT;
+	}
+
+	/* Add the NSH header */
+	if (skb_cow_head(skb, length) < 0)
+		return -ENOMEM;
+
+	skb_push(skb, length);
+	nh = (struct nshhdr *)(skb->data);
+	memcpy(nh, pushed_nh, length);
+	nh->np = next_proto;
+	skb_postpush_rcsum(skb, nh, length);
+
+	skb->protocol = htons(ETH_P_NSH);
+	skb_reset_mac_header(skb);
+	skb_reset_network_header(skb);
+	skb_reset_mac_len(skb);
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(nsh_push);
+
+int nsh_pop(struct sk_buff *skb)
+{
+	struct nshhdr *nh;
+	size_t length;
+	__be16 inner_proto;
+
+	if (!pskb_may_pull(skb, NSH_BASE_HDR_LEN))
+		return -ENOMEM;
+	nh = (struct nshhdr *)(skb->data);
+	length = nsh_hdr_len(nh);
+	if (length < NSH_BASE_HDR_LEN)
+		return -EINVAL;
+	inner_proto = tun_p_to_eth_p(nh->np);
+	if (!pskb_may_pull(skb, length))
+		return -ENOMEM;
+
+	if (!inner_proto)
+		return -EAFNOSUPPORT;
+
+	skb_pull_rcsum(skb, length);
+	skb_reset_mac_header(skb);
+	skb_reset_network_header(skb);
+	skb_reset_mac_len(skb);
+	skb->protocol = inner_proto;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(nsh_pop);
+
+static struct sk_buff *nsh_gso_segment(struct sk_buff *skb,
+				       netdev_features_t features)
+{
+	unsigned int outer_hlen, mac_len, nsh_len;
+	struct sk_buff *segs = ERR_PTR(-EINVAL);
+	u16 mac_offset = skb->mac_header;
+	__be16 outer_proto, proto;
+
+	skb_reset_network_header(skb);
+
+	outer_proto = skb->protocol;
+	outer_hlen = skb_mac_header_len(skb);
+	mac_len = skb->mac_len;
+
+	if (unlikely(!pskb_may_pull(skb, NSH_BASE_HDR_LEN)))
+		goto out;
+	nsh_len = nsh_hdr_len(nsh_hdr(skb));
+	if (nsh_len < NSH_BASE_HDR_LEN)
+		goto out;
+	if (unlikely(!pskb_may_pull(skb, nsh_len)))
+		goto out;
+
+	proto = tun_p_to_eth_p(nsh_hdr(skb)->np);
+	if (!proto)
+		goto out;
+
+	__skb_pull(skb, nsh_len);
+
+	skb_reset_mac_header(skb);
+	skb->mac_len = proto == htons(ETH_P_TEB) ? ETH_HLEN : 0;
+	skb->protocol = proto;
+
+	features &= NETIF_F_SG;
+	segs = skb_mac_gso_segment(skb, features);
+	if (IS_ERR_OR_NULL(segs)) {
+		skb_gso_error_unwind(skb, htons(ETH_P_NSH), nsh_len,
+				     mac_offset, mac_len);
+		goto out;
+	}
+
+	for (skb = segs; skb; skb = skb->next) {
+		skb->protocol = outer_proto;
+		__skb_push(skb, nsh_len + outer_hlen);
+		skb_reset_mac_header(skb);
+		skb_set_network_header(skb, outer_hlen);
+		skb->mac_len = mac_len;
+	}
+
+out:
+	return segs;
+}
+
+static struct packet_offload nsh_packet_offload __read_mostly = {
+	.type = htons(ETH_P_NSH),
+	.priority = 15,
+	.callbacks = {
+		.gso_segment = nsh_gso_segment,
+	},
+};
+
+static int __init nsh_init_module(void)
+{
+	dev_add_offload(&nsh_packet_offload);
+	return 0;
+}
+
+static void __exit nsh_cleanup_module(void)
+{
+	dev_remove_offload(&nsh_packet_offload);
+}
+
+module_init(nsh_init_module);
+module_exit(nsh_cleanup_module);
+
+MODULE_AUTHOR("Jiri Benc <jbenc@redhat.com>");
+MODULE_DESCRIPTION("NSH protocol");
+MODULE_LICENSE("GPL v2");