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, 179 insertions, 0 deletions

diff --git a/net/ipv4/udp_bpf.c b/net/ipv4/udp_bpf.c
new file mode 100644
index 000000000..ad57c4c9e
--- /dev/null
+++ b/net/ipv4/udp_bpf.c
@@ -0,0 +1,179 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Cloudflare Ltd https://cloudflare.com */
+
+#include <linux/skmsg.h>
+#include <net/sock.h>
+#include <net/udp.h>
+#include <net/inet_common.h>
+#include <asm/ioctls.h>
+
+static struct proto *udpv6_prot_saved __read_mostly;
+
+static int sk_udp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
+			  int flags)
+{
+#if IS_ENABLED(CONFIG_IPV6)
+	if (sk->sk_family == AF_INET6)
+		return udpv6_prot_saved->recvmsg(sk, msg, len, flags);
+#endif
+	return udp_prot.recvmsg(sk, msg, len, flags);
+}
+
+static bool udp_sk_has_data(struct sock *sk)
+{
+	return !skb_queue_empty(&udp_sk(sk)->reader_queue) ||
+	       !skb_queue_empty(&sk->sk_receive_queue);
+}
+
+static bool psock_has_data(struct sk_psock *psock)
+{
+	return !skb_queue_empty(&psock->ingress_skb) ||
+	       !sk_psock_queue_empty(psock);
+}
+
+#define udp_msg_has_data(__sk, __psock)	\
+		({ udp_sk_has_data(__sk) || psock_has_data(__psock); })
+
+static int udp_msg_wait_data(struct sock *sk, struct sk_psock *psock,
+			     long timeo)
+{
+	DEFINE_WAIT_FUNC(wait, woken_wake_function);
+	int ret = 0;
+
+	if (sk->sk_shutdown & RCV_SHUTDOWN)
+		return 1;
+
+	if (!timeo)
+		return ret;
+
+	add_wait_queue(sk_sleep(sk), &wait);
+	sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk);
+	ret = udp_msg_has_data(sk, psock);
+	if (!ret) {
+		release_sock(sk);
+		wait_woken(&wait, TASK_INTERRUPTIBLE, timeo);
+		lock_sock(sk);
+		ret = udp_msg_has_data(sk, psock);
+	}
+	sk_clear_bit(SOCKWQ_ASYNC_WAITDATA, sk);
+	remove_wait_queue(sk_sleep(sk), &wait);
+	return ret;
+}
+
+static int udp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
+			   int flags)
+{
+	struct sk_psock *psock;
+	int copied, ret;
+
+	if (unlikely(flags & MSG_ERRQUEUE))
+		return inet_recv_error(sk, msg, len);
+
+	if (!len)
+		return 0;
+
+	psock = sk_psock_get(sk);
+	if (unlikely(!psock))
+		return sk_udp_recvmsg(sk, msg, len, flags);
+
+	if (!psock_has_data(psock)) {
+		ret = sk_udp_recvmsg(sk, msg, len, flags);
+		goto out;
+	}
+
+	lock_sock(sk);
+msg_bytes_ready:
+	copied = sk_msg_recvmsg(sk, psock, msg, len, flags);
+	if (!copied) {
+		long timeo;
+		int data;
+
+		timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT);
+		data = udp_msg_wait_data(sk, psock, timeo);
+		if (data) {
+			if (psock_has_data(psock))
+				goto msg_bytes_ready;
+
+			release_sock(sk);
+
+			ret = sk_udp_recvmsg(sk, msg, len, flags);
+			goto out;
+		}
+		copied = -EAGAIN;
+	}
+
+	release_sock(sk);
+
+	ret = copied;
+out:
+	sk_psock_put(sk, psock);
+	return ret;
+}
+
+enum {
+	UDP_BPF_IPV4,
+	UDP_BPF_IPV6,
+	UDP_BPF_NUM_PROTS,
+};
+
+static DEFINE_SPINLOCK(udpv6_prot_lock);
+static struct proto udp_bpf_prots[UDP_BPF_NUM_PROTS];
+
+static int udp_bpf_ioctl(struct sock *sk, int cmd, int *karg)
+{
+	if (cmd != SIOCINQ)
+		return udp_ioctl(sk, cmd, karg);
+
+	/* Since we don't hold a lock, sk_receive_queue may contain data.
+	 * BPF might only be processing this data at the moment. We only
+	 * care about the data in the ingress_msg here.
+	 */
+	*karg = sk_msg_first_len(sk);
+	return 0;
+}
+
+static void udp_bpf_rebuild_protos(struct proto *prot, const struct proto *base)
+{
+	*prot			= *base;
+	prot->close		= sock_map_close;
+	prot->recvmsg		= udp_bpf_recvmsg;
+	prot->sock_is_readable	= sk_msg_is_readable;
+	prot->ioctl		= udp_bpf_ioctl;
+}
+
+static void udp_bpf_check_v6_needs_rebuild(struct proto *ops)
+{
+	if (unlikely(ops != smp_load_acquire(&udpv6_prot_saved))) {
+		spin_lock_bh(&udpv6_prot_lock);
+		if (likely(ops != udpv6_prot_saved)) {
+			udp_bpf_rebuild_protos(&udp_bpf_prots[UDP_BPF_IPV6], ops);
+			smp_store_release(&udpv6_prot_saved, ops);
+		}
+		spin_unlock_bh(&udpv6_prot_lock);
+	}
+}
+
+static int __init udp_bpf_v4_build_proto(void)
+{
+	udp_bpf_rebuild_protos(&udp_bpf_prots[UDP_BPF_IPV4], &udp_prot);
+	return 0;
+}
+late_initcall(udp_bpf_v4_build_proto);
+
+int udp_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool restore)
+{
+	int family = sk->sk_family == AF_INET ? UDP_BPF_IPV4 : UDP_BPF_IPV6;
+
+	if (restore) {
+		WRITE_ONCE(sk->sk_write_space, psock->saved_write_space);
+		sock_replace_proto(sk, psock->sk_proto);
+		return 0;
+	}
+
+	if (sk->sk_family == AF_INET6)
+		udp_bpf_check_v6_needs_rebuild(psock->sk_proto);
+
+	sock_replace_proto(sk, &udp_bpf_prots[family]);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(udp_bpf_update_proto);