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, 226 insertions, 0 deletions

diff --git a/net/ipv6/fou6.c b/net/ipv6/fou6.c
new file mode 100644
index 000000000..157765259
--- /dev/null
+++ b/net/ipv6/fou6.c
@@ -0,0 +1,226 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/module.h>
+#include <linux/errno.h>
+#include <linux/socket.h>
+#include <linux/skbuff.h>
+#include <linux/ip.h>
+#include <linux/udp.h>
+#include <linux/icmpv6.h>
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <net/fou.h>
+#include <net/ip.h>
+#include <net/ip6_tunnel.h>
+#include <net/ip6_checksum.h>
+#include <net/protocol.h>
+#include <net/udp.h>
+#include <net/udp_tunnel.h>
+
+#if IS_ENABLED(CONFIG_IPV6_FOU_TUNNEL)
+
+static void fou6_build_udp(struct sk_buff *skb, struct ip_tunnel_encap *e,
+			   struct flowi6 *fl6, u8 *protocol, __be16 sport)
+{
+	struct udphdr *uh;
+
+	skb_push(skb, sizeof(struct udphdr));
+	skb_reset_transport_header(skb);
+
+	uh = udp_hdr(skb);
+
+	uh->dest = e->dport;
+	uh->source = sport;
+	uh->len = htons(skb->len);
+	udp6_set_csum(!(e->flags & TUNNEL_ENCAP_FLAG_CSUM6), skb,
+		      &fl6->saddr, &fl6->daddr, skb->len);
+
+	*protocol = IPPROTO_UDP;
+}
+
+static int fou6_build_header(struct sk_buff *skb, struct ip_tunnel_encap *e,
+			     u8 *protocol, struct flowi6 *fl6)
+{
+	__be16 sport;
+	int err;
+	int type = e->flags & TUNNEL_ENCAP_FLAG_CSUM6 ?
+		SKB_GSO_UDP_TUNNEL_CSUM : SKB_GSO_UDP_TUNNEL;
+
+	err = __fou_build_header(skb, e, protocol, &sport, type);
+	if (err)
+		return err;
+
+	fou6_build_udp(skb, e, fl6, protocol, sport);
+
+	return 0;
+}
+
+static int gue6_build_header(struct sk_buff *skb, struct ip_tunnel_encap *e,
+			     u8 *protocol, struct flowi6 *fl6)
+{
+	__be16 sport;
+	int err;
+	int type = e->flags & TUNNEL_ENCAP_FLAG_CSUM6 ?
+		SKB_GSO_UDP_TUNNEL_CSUM : SKB_GSO_UDP_TUNNEL;
+
+	err = __gue_build_header(skb, e, protocol, &sport, type);
+	if (err)
+		return err;
+
+	fou6_build_udp(skb, e, fl6, protocol, sport);
+
+	return 0;
+}
+
+static int gue6_err_proto_handler(int proto, struct sk_buff *skb,
+				  struct inet6_skb_parm *opt,
+				  u8 type, u8 code, int offset, __be32 info)
+{
+	const struct inet6_protocol *ipprot;
+
+	ipprot = rcu_dereference(inet6_protos[proto]);
+	if (ipprot && ipprot->err_handler) {
+		if (!ipprot->err_handler(skb, opt, type, code, offset, info))
+			return 0;
+	}
+
+	return -ENOENT;
+}
+
+static int gue6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
+		    u8 type, u8 code, int offset, __be32 info)
+{
+	int transport_offset = skb_transport_offset(skb);
+	struct guehdr *guehdr;
+	size_t len, optlen;
+	int ret;
+
+	len = sizeof(struct udphdr) + sizeof(struct guehdr);
+	if (!pskb_may_pull(skb, transport_offset + len))
+		return -EINVAL;
+
+	guehdr = (struct guehdr *)&udp_hdr(skb)[1];
+
+	switch (guehdr->version) {
+	case 0: /* Full GUE header present */
+		break;
+	case 1: {
+		/* Direct encasulation of IPv4 or IPv6 */
+		skb_set_transport_header(skb, -(int)sizeof(struct icmp6hdr));
+
+		switch (((struct iphdr *)guehdr)->version) {
+		case 4:
+			ret = gue6_err_proto_handler(IPPROTO_IPIP, skb, opt,
+						     type, code, offset, info);
+			goto out;
+		case 6:
+			ret = gue6_err_proto_handler(IPPROTO_IPV6, skb, opt,
+						     type, code, offset, info);
+			goto out;
+		default:
+			ret = -EOPNOTSUPP;
+			goto out;
+		}
+	}
+	default: /* Undefined version */
+		return -EOPNOTSUPP;
+	}
+
+	if (guehdr->control)
+		return -ENOENT;
+
+	optlen = guehdr->hlen << 2;
+
+	if (!pskb_may_pull(skb, transport_offset + len + optlen))
+		return -EINVAL;
+
+	guehdr = (struct guehdr *)&udp_hdr(skb)[1];
+	if (validate_gue_flags(guehdr, optlen))
+		return -EINVAL;
+
+	/* Handling exceptions for direct UDP encapsulation in GUE would lead to
+	 * recursion. Besides, this kind of encapsulation can't even be
+	 * configured currently. Discard this.
+	 */
+	if (guehdr->proto_ctype == IPPROTO_UDP)
+		return -EOPNOTSUPP;
+
+	skb_set_transport_header(skb, -(int)sizeof(struct icmp6hdr));
+	ret = gue6_err_proto_handler(guehdr->proto_ctype, skb,
+				     opt, type, code, offset, info);
+
+out:
+	skb_set_transport_header(skb, transport_offset);
+	return ret;
+}
+
+
+static const struct ip6_tnl_encap_ops fou_ip6tun_ops = {
+	.encap_hlen = fou_encap_hlen,
+	.build_header = fou6_build_header,
+	.err_handler = gue6_err,
+};
+
+static const struct ip6_tnl_encap_ops gue_ip6tun_ops = {
+	.encap_hlen = gue_encap_hlen,
+	.build_header = gue6_build_header,
+	.err_handler = gue6_err,
+};
+
+static int ip6_tnl_encap_add_fou_ops(void)
+{
+	int ret;
+
+	ret = ip6_tnl_encap_add_ops(&fou_ip6tun_ops, TUNNEL_ENCAP_FOU);
+	if (ret < 0) {
+		pr_err("can't add fou6 ops\n");
+		return ret;
+	}
+
+	ret = ip6_tnl_encap_add_ops(&gue_ip6tun_ops, TUNNEL_ENCAP_GUE);
+	if (ret < 0) {
+		pr_err("can't add gue6 ops\n");
+		ip6_tnl_encap_del_ops(&fou_ip6tun_ops, TUNNEL_ENCAP_FOU);
+		return ret;
+	}
+
+	return 0;
+}
+
+static void ip6_tnl_encap_del_fou_ops(void)
+{
+	ip6_tnl_encap_del_ops(&fou_ip6tun_ops, TUNNEL_ENCAP_FOU);
+	ip6_tnl_encap_del_ops(&gue_ip6tun_ops, TUNNEL_ENCAP_GUE);
+}
+
+#else
+
+static int ip6_tnl_encap_add_fou_ops(void)
+{
+	return 0;
+}
+
+static void ip6_tnl_encap_del_fou_ops(void)
+{
+}
+
+#endif
+
+static int __init fou6_init(void)
+{
+	int ret;
+
+	ret = ip6_tnl_encap_add_fou_ops();
+
+	return ret;
+}
+
+static void __exit fou6_fini(void)
+{
+	ip6_tnl_encap_del_fou_ops();
+}
+
+module_init(fou6_init);
+module_exit(fou6_fini);
+MODULE_AUTHOR("Tom Herbert <therbert@google.com>");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Foo over UDP (IPv6)");