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, 283 insertions, 0 deletions

diff --git a/include/net/dst_metadata.h b/include/net/dst_metadata.h
new file mode 100644
index 000000000..f45d1e316
--- /dev/null
+++ b/include/net/dst_metadata.h
@@ -0,0 +1,283 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __NET_DST_METADATA_H
+#define __NET_DST_METADATA_H 1
+
+#include <linux/skbuff.h>
+#include <net/ip.h>
+#include <net/ip_tunnels.h>
+#include <net/macsec.h>
+#include <net/dst.h>
+
+enum metadata_type {
+	METADATA_IP_TUNNEL,
+	METADATA_HW_PORT_MUX,
+	METADATA_MACSEC,
+	METADATA_XFRM,
+};
+
+struct hw_port_info {
+	struct net_device *lower_dev;
+	u32 port_id;
+};
+
+struct macsec_info {
+	sci_t sci;
+};
+
+struct xfrm_md_info {
+	u32 if_id;
+	int link;
+	struct dst_entry *dst_orig;
+};
+
+struct metadata_dst {
+	struct dst_entry		dst;
+	enum metadata_type		type;
+	union {
+		struct ip_tunnel_info	tun_info;
+		struct hw_port_info	port_info;
+		struct macsec_info	macsec_info;
+		struct xfrm_md_info	xfrm_info;
+	} u;
+};
+
+static inline struct metadata_dst *skb_metadata_dst(const struct sk_buff *skb)
+{
+	struct metadata_dst *md_dst = (struct metadata_dst *) skb_dst(skb);
+
+	if (md_dst && md_dst->dst.flags & DST_METADATA)
+		return md_dst;
+
+	return NULL;
+}
+
+static inline struct ip_tunnel_info *
+skb_tunnel_info(const struct sk_buff *skb)
+{
+	struct metadata_dst *md_dst = skb_metadata_dst(skb);
+	struct dst_entry *dst;
+
+	if (md_dst && md_dst->type == METADATA_IP_TUNNEL)
+		return &md_dst->u.tun_info;
+
+	dst = skb_dst(skb);
+	if (dst && dst->lwtstate &&
+	    (dst->lwtstate->type == LWTUNNEL_ENCAP_IP ||
+	     dst->lwtstate->type == LWTUNNEL_ENCAP_IP6))
+		return lwt_tun_info(dst->lwtstate);
+
+	return NULL;
+}
+
+static inline struct xfrm_md_info *lwt_xfrm_info(struct lwtunnel_state *lwt)
+{
+	return (struct xfrm_md_info *)lwt->data;
+}
+
+static inline struct xfrm_md_info *skb_xfrm_md_info(const struct sk_buff *skb)
+{
+	struct metadata_dst *md_dst = skb_metadata_dst(skb);
+	struct dst_entry *dst;
+
+	if (md_dst && md_dst->type == METADATA_XFRM)
+		return &md_dst->u.xfrm_info;
+
+	dst = skb_dst(skb);
+	if (dst && dst->lwtstate &&
+	    dst->lwtstate->type == LWTUNNEL_ENCAP_XFRM)
+		return lwt_xfrm_info(dst->lwtstate);
+
+	return NULL;
+}
+
+static inline bool skb_valid_dst(const struct sk_buff *skb)
+{
+	struct dst_entry *dst = skb_dst(skb);
+
+	return dst && !(dst->flags & DST_METADATA);
+}
+
+static inline int skb_metadata_dst_cmp(const struct sk_buff *skb_a,
+				       const struct sk_buff *skb_b)
+{
+	const struct metadata_dst *a, *b;
+
+	if (!(skb_a->_skb_refdst | skb_b->_skb_refdst))
+		return 0;
+
+	a = (const struct metadata_dst *) skb_dst(skb_a);
+	b = (const struct metadata_dst *) skb_dst(skb_b);
+
+	if (!a != !b || a->type != b->type)
+		return 1;
+
+	switch (a->type) {
+	case METADATA_HW_PORT_MUX:
+		return memcmp(&a->u.port_info, &b->u.port_info,
+			      sizeof(a->u.port_info));
+	case METADATA_IP_TUNNEL:
+		return memcmp(&a->u.tun_info, &b->u.tun_info,
+			      sizeof(a->u.tun_info) +
+					 a->u.tun_info.options_len);
+	case METADATA_MACSEC:
+		return memcmp(&a->u.macsec_info, &b->u.macsec_info,
+			      sizeof(a->u.macsec_info));
+	case METADATA_XFRM:
+		return memcmp(&a->u.xfrm_info, &b->u.xfrm_info,
+			      sizeof(a->u.xfrm_info));
+	default:
+		return 1;
+	}
+}
+
+void metadata_dst_free(struct metadata_dst *);
+struct metadata_dst *metadata_dst_alloc(u8 optslen, enum metadata_type type,
+					gfp_t flags);
+void metadata_dst_free_percpu(struct metadata_dst __percpu *md_dst);
+struct metadata_dst __percpu *
+metadata_dst_alloc_percpu(u8 optslen, enum metadata_type type, gfp_t flags);
+
+static inline struct metadata_dst *tun_rx_dst(int md_size)
+{
+	struct metadata_dst *tun_dst;
+
+	tun_dst = metadata_dst_alloc(md_size, METADATA_IP_TUNNEL, GFP_ATOMIC);
+	if (!tun_dst)
+		return NULL;
+
+	tun_dst->u.tun_info.options_len = 0;
+	tun_dst->u.tun_info.mode = 0;
+	return tun_dst;
+}
+
+static inline struct metadata_dst *tun_dst_unclone(struct sk_buff *skb)
+{
+	struct metadata_dst *md_dst = skb_metadata_dst(skb);
+	int md_size;
+	struct metadata_dst *new_md;
+
+	if (!md_dst || md_dst->type != METADATA_IP_TUNNEL)
+		return ERR_PTR(-EINVAL);
+
+	md_size = md_dst->u.tun_info.options_len;
+	new_md = metadata_dst_alloc(md_size, METADATA_IP_TUNNEL, GFP_ATOMIC);
+	if (!new_md)
+		return ERR_PTR(-ENOMEM);
+
+	/* Copy in two stages to keep the __counted_by happy. */
+	new_md->u.tun_info = md_dst->u.tun_info;
+	memcpy(ip_tunnel_info_opts(&new_md->u.tun_info),
+	       ip_tunnel_info_opts(&md_dst->u.tun_info), md_size);
+
+#ifdef CONFIG_DST_CACHE
+	/* Unclone the dst cache if there is one */
+	if (new_md->u.tun_info.dst_cache.cache) {
+		int ret;
+
+		ret = dst_cache_init(&new_md->u.tun_info.dst_cache, GFP_ATOMIC);
+		if (ret) {
+			metadata_dst_free(new_md);
+			return ERR_PTR(ret);
+		}
+	}
+#endif
+
+	skb_dst_drop(skb);
+	skb_dst_set(skb, &new_md->dst);
+	return new_md;
+}
+
+static inline struct ip_tunnel_info *skb_tunnel_info_unclone(struct sk_buff *skb)
+{
+	struct metadata_dst *dst;
+
+	dst = tun_dst_unclone(skb);
+	if (IS_ERR(dst))
+		return NULL;
+
+	return &dst->u.tun_info;
+}
+
+static inline struct metadata_dst *__ip_tun_set_dst(__be32 saddr,
+						    __be32 daddr,
+						    __u8 tos, __u8 ttl,
+						    __be16 tp_dst,
+						    const unsigned long *flags,
+						    __be64 tunnel_id,
+						    int md_size)
+{
+	struct metadata_dst *tun_dst;
+
+	tun_dst = tun_rx_dst(md_size);
+	if (!tun_dst)
+		return NULL;
+
+	ip_tunnel_key_init(&tun_dst->u.tun_info.key,
+			   saddr, daddr, tos, ttl,
+			   0, 0, tp_dst, tunnel_id, flags);
+	return tun_dst;
+}
+
+static inline struct metadata_dst *ip_tun_rx_dst(struct sk_buff *skb,
+						 const unsigned long *flags,
+						 __be64 tunnel_id,
+						 int md_size)
+{
+	const struct iphdr *iph = ip_hdr(skb);
+	struct metadata_dst *tun_dst;
+
+	tun_dst = __ip_tun_set_dst(iph->saddr, iph->daddr, iph->tos, iph->ttl,
+				   0, flags, tunnel_id, md_size);
+
+	if (tun_dst && (iph->frag_off & htons(IP_DF)))
+		__set_bit(IP_TUNNEL_DONT_FRAGMENT_BIT,
+			  tun_dst->u.tun_info.key.tun_flags);
+	return tun_dst;
+}
+
+static inline struct metadata_dst *__ipv6_tun_set_dst(const struct in6_addr *saddr,
+						      const struct in6_addr *daddr,
+						      __u8 tos, __u8 ttl,
+						      __be16 tp_dst,
+						      __be32 label,
+						      const unsigned long *flags,
+						      __be64 tunnel_id,
+						      int md_size)
+{
+	struct metadata_dst *tun_dst;
+	struct ip_tunnel_info *info;
+
+	tun_dst = tun_rx_dst(md_size);
+	if (!tun_dst)
+		return NULL;
+
+	info = &tun_dst->u.tun_info;
+	info->mode = IP_TUNNEL_INFO_IPV6;
+	ip_tunnel_flags_copy(info->key.tun_flags, flags);
+	info->key.tun_id = tunnel_id;
+	info->key.tp_src = 0;
+	info->key.tp_dst = tp_dst;
+
+	info->key.u.ipv6.src = *saddr;
+	info->key.u.ipv6.dst = *daddr;
+
+	info->key.tos = tos;
+	info->key.ttl = ttl;
+	info->key.label = label;
+
+	return tun_dst;
+}
+
+static inline struct metadata_dst *ipv6_tun_rx_dst(struct sk_buff *skb,
+						   const unsigned long *flags,
+						   __be64 tunnel_id,
+						   int md_size)
+{
+	const struct ipv6hdr *ip6h = ipv6_hdr(skb);
+
+	return __ipv6_tun_set_dst(&ip6h->saddr, &ip6h->daddr,
+				  ipv6_get_dsfield(ip6h), ip6h->hop_limit,
+				  0, ip6_flowlabel(ip6h), flags, tunnel_id,
+				  md_size);
+}
+#endif /* __NET_DST_METADATA_H */