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, 224 insertions, 0 deletions

diff --git a/net/ipv6/ipcomp6.c b/net/ipv6/ipcomp6.c
new file mode 100644
index 000000000..b340d67eb
--- /dev/null
+++ b/net/ipv6/ipcomp6.c
@@ -0,0 +1,224 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * IP Payload Compression Protocol (IPComp) for IPv6 - RFC3173
+ *
+ * Copyright (C)2003 USAGI/WIDE Project
+ *
+ * Author	Mitsuru KANDA  <mk@linux-ipv6.org>
+ */
+/*
+ * [Memo]
+ *
+ * Outbound:
+ *  The compression of IP datagram MUST be done before AH/ESP processing,
+ *  fragmentation, and the addition of Hop-by-Hop/Routing header.
+ *
+ * Inbound:
+ *  The decompression of IP datagram MUST be done after the reassembly,
+ *  AH/ESP processing.
+ */
+
+#define pr_fmt(fmt) "IPv6: " fmt
+
+#include <linux/module.h>
+#include <net/ip.h>
+#include <net/xfrm.h>
+#include <net/ipcomp.h>
+#include <linux/crypto.h>
+#include <linux/err.h>
+#include <linux/pfkeyv2.h>
+#include <linux/random.h>
+#include <linux/percpu.h>
+#include <linux/smp.h>
+#include <linux/list.h>
+#include <linux/vmalloc.h>
+#include <linux/rtnetlink.h>
+#include <net/ip6_route.h>
+#include <net/icmp.h>
+#include <net/ipv6.h>
+#include <net/protocol.h>
+#include <linux/ipv6.h>
+#include <linux/icmpv6.h>
+#include <linux/mutex.h>
+
+static int ipcomp6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
+				u8 type, u8 code, int offset, __be32 info)
+{
+	struct net *net = dev_net(skb->dev);
+	__be32 spi;
+	const struct ipv6hdr *iph = (const struct ipv6hdr *)skb->data;
+	struct ip_comp_hdr *ipcomph =
+		(struct ip_comp_hdr *)(skb->data + offset);
+	struct xfrm_state *x;
+
+	if (type != ICMPV6_PKT_TOOBIG &&
+	    type != NDISC_REDIRECT)
+		return 0;
+
+	spi = htonl(ntohs(ipcomph->cpi));
+	x = xfrm_state_lookup(net, skb->mark, (const xfrm_address_t *)&iph->daddr,
+			      spi, IPPROTO_COMP, AF_INET6);
+	if (!x)
+		return 0;
+
+	if (type == NDISC_REDIRECT)
+		ip6_redirect(skb, net, skb->dev->ifindex, 0,
+			     sock_net_uid(net, NULL));
+	else
+		ip6_update_pmtu(skb, net, info, 0, 0, sock_net_uid(net, NULL));
+	xfrm_state_put(x);
+
+	return 0;
+}
+
+static struct lock_class_key xfrm_state_lock_key;
+static struct xfrm_state *ipcomp6_tunnel_create(struct xfrm_state *x)
+{
+	struct net *net = xs_net(x);
+	struct xfrm_state *t = NULL;
+
+	t = xfrm_state_alloc(net);
+	if (!t)
+		goto out;
+	lockdep_set_class(&t->lock, &xfrm_state_lock_key);
+
+	t->id.proto = IPPROTO_IPV6;
+	t->id.spi = xfrm6_tunnel_alloc_spi(net, (xfrm_address_t *)&x->props.saddr);
+	if (!t->id.spi)
+		goto error;
+
+	memcpy(t->id.daddr.a6, x->id.daddr.a6, sizeof(struct in6_addr));
+	memcpy(&t->sel, &x->sel, sizeof(t->sel));
+	t->props.family = AF_INET6;
+	t->props.mode = x->props.mode;
+	memcpy(t->props.saddr.a6, x->props.saddr.a6, sizeof(struct in6_addr));
+	memcpy(&t->mark, &x->mark, sizeof(t->mark));
+	t->if_id = x->if_id;
+
+	if (xfrm_init_state(t, NULL))
+		goto error;
+
+	atomic_set(&t->tunnel_users, 1);
+
+out:
+	return t;
+
+error:
+	t->km.state = XFRM_STATE_DEAD;
+	xfrm_state_put(t);
+	t = NULL;
+	goto out;
+}
+
+static int ipcomp6_tunnel_attach(struct xfrm_state *x)
+{
+	struct net *net = xs_net(x);
+	int err = 0;
+	struct xfrm_state *t = NULL;
+	__be32 spi;
+	u32 mark = x->mark.m & x->mark.v;
+
+	spi = xfrm6_tunnel_spi_lookup(net, (xfrm_address_t *)&x->props.saddr);
+	if (spi)
+		t = xfrm_state_lookup(net, mark, (xfrm_address_t *)&x->id.daddr,
+					      spi, IPPROTO_IPV6, AF_INET6);
+	if (!t) {
+		t = ipcomp6_tunnel_create(x);
+		if (!t) {
+			err = -EINVAL;
+			goto out;
+		}
+		xfrm_state_insert(t);
+		xfrm_state_hold(t);
+	}
+	x->tunnel = t;
+	atomic_inc(&t->tunnel_users);
+
+out:
+	return err;
+}
+
+static int ipcomp6_init_state(struct xfrm_state *x,
+			      struct netlink_ext_ack *extack)
+{
+	int err = -EINVAL;
+
+	x->props.header_len = 0;
+	switch (x->props.mode) {
+	case XFRM_MODE_TRANSPORT:
+		break;
+	case XFRM_MODE_TUNNEL:
+		x->props.header_len += sizeof(struct ipv6hdr);
+		break;
+	default:
+		NL_SET_ERR_MSG(extack, "Unsupported XFRM mode for IPcomp");
+		goto out;
+	}
+
+	err = ipcomp_init_state(x, extack);
+	if (err)
+		goto out;
+
+	if (x->props.mode == XFRM_MODE_TUNNEL) {
+		err = ipcomp6_tunnel_attach(x);
+		if (err) {
+			NL_SET_ERR_MSG(extack, "Kernel error: failed to initialize the associated state");
+			goto out;
+		}
+	}
+
+	err = 0;
+out:
+	return err;
+}
+
+static int ipcomp6_rcv_cb(struct sk_buff *skb, int err)
+{
+	return 0;
+}
+
+static const struct xfrm_type ipcomp6_type = {
+	.owner		= THIS_MODULE,
+	.proto		= IPPROTO_COMP,
+	.init_state	= ipcomp6_init_state,
+	.destructor	= ipcomp_destroy,
+	.input		= ipcomp_input,
+	.output		= ipcomp_output,
+};
+
+static struct xfrm6_protocol ipcomp6_protocol = {
+	.handler	= xfrm6_rcv,
+	.input_handler	= xfrm_input,
+	.cb_handler	= ipcomp6_rcv_cb,
+	.err_handler	= ipcomp6_err,
+	.priority	= 0,
+};
+
+static int __init ipcomp6_init(void)
+{
+	if (xfrm_register_type(&ipcomp6_type, AF_INET6) < 0) {
+		pr_info("%s: can't add xfrm type\n", __func__);
+		return -EAGAIN;
+	}
+	if (xfrm6_protocol_register(&ipcomp6_protocol, IPPROTO_COMP) < 0) {
+		pr_info("%s: can't add protocol\n", __func__);
+		xfrm_unregister_type(&ipcomp6_type, AF_INET6);
+		return -EAGAIN;
+	}
+	return 0;
+}
+
+static void __exit ipcomp6_fini(void)
+{
+	if (xfrm6_protocol_deregister(&ipcomp6_protocol, IPPROTO_COMP) < 0)
+		pr_info("%s: can't remove protocol\n", __func__);
+	xfrm_unregister_type(&ipcomp6_type, AF_INET6);
+}
+
+module_init(ipcomp6_init);
+module_exit(ipcomp6_fini);
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("IP Payload Compression Protocol (IPComp) for IPv6 - RFC3173");
+MODULE_AUTHOR("Mitsuru KANDA <mk@linux-ipv6.org>");
+
+MODULE_ALIAS_XFRM_TYPE(AF_INET6, XFRM_PROTO_COMP);