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, 110 insertions, 0 deletions

diff --git a/net/ipv4/tcp_plb.c b/net/ipv4/tcp_plb.c
new file mode 100644
index 000000000..bcc2f0add
--- /dev/null
+++ b/net/ipv4/tcp_plb.c
@@ -0,0 +1,110 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Protective Load Balancing (PLB)
+ *
+ * PLB was designed to reduce link load imbalance across datacenter
+ * switches. PLB is a host-based optimization; it leverages congestion
+ * signals from the transport layer to randomly change the path of the
+ * connection experiencing sustained congestion. PLB prefers to repath
+ * after idle periods to minimize packet reordering. It repaths by
+ * changing the IPv6 Flow Label on the packets of a connection, which
+ * datacenter switches include as part of ECMP/WCMP hashing.
+ *
+ * PLB is described in detail in:
+ *
+ *	Mubashir Adnan Qureshi, Yuchung Cheng, Qianwen Yin, Qiaobin Fu,
+ *	Gautam Kumar, Masoud Moshref, Junhua Yan, Van Jacobson,
+ *	David Wetherall,Abdul Kabbani:
+ *	"PLB: Congestion Signals are Simple and Effective for
+ *	 Network Load Balancing"
+ *	In ACM SIGCOMM 2022, Amsterdam Netherlands.
+ *
+ */
+
+#include <net/tcp.h>
+
+/* Called once per round-trip to update PLB state for a connection. */
+void tcp_plb_update_state(const struct sock *sk, struct tcp_plb_state *plb,
+			  const int cong_ratio)
+{
+	struct net *net = sock_net(sk);
+
+	if (!READ_ONCE(net->ipv4.sysctl_tcp_plb_enabled))
+		return;
+
+	if (cong_ratio >= 0) {
+		if (cong_ratio < READ_ONCE(net->ipv4.sysctl_tcp_plb_cong_thresh))
+			plb->consec_cong_rounds = 0;
+		else if (plb->consec_cong_rounds <
+			 READ_ONCE(net->ipv4.sysctl_tcp_plb_rehash_rounds))
+			plb->consec_cong_rounds++;
+	}
+}
+EXPORT_SYMBOL_GPL(tcp_plb_update_state);
+
+/* Check whether recent congestion has been persistent enough to warrant
+ * a load balancing decision that switches the connection to another path.
+ */
+void tcp_plb_check_rehash(struct sock *sk, struct tcp_plb_state *plb)
+{
+	struct net *net = sock_net(sk);
+	u32 max_suspend;
+	bool forced_rehash = false, idle_rehash = false;
+
+	if (!READ_ONCE(net->ipv4.sysctl_tcp_plb_enabled))
+		return;
+
+	forced_rehash = plb->consec_cong_rounds >=
+			READ_ONCE(net->ipv4.sysctl_tcp_plb_rehash_rounds);
+	/* If sender goes idle then we check whether to rehash. */
+	idle_rehash = READ_ONCE(net->ipv4.sysctl_tcp_plb_idle_rehash_rounds) &&
+		      !tcp_sk(sk)->packets_out &&
+		      plb->consec_cong_rounds >=
+		      READ_ONCE(net->ipv4.sysctl_tcp_plb_idle_rehash_rounds);
+
+	if (!forced_rehash && !idle_rehash)
+		return;
+
+	/* Note that tcp_jiffies32 can wrap; we detect wraps by checking for
+	 * cases where the max suspension end is before the actual suspension
+	 * end. We clear pause_until to 0 to indicate there is no recent
+	 * RTO event that constrains PLB rehashing.
+	 */
+	max_suspend = 2 * READ_ONCE(net->ipv4.sysctl_tcp_plb_suspend_rto_sec) * HZ;
+	if (plb->pause_until &&
+	    (!before(tcp_jiffies32, plb->pause_until) ||
+	     before(tcp_jiffies32 + max_suspend, plb->pause_until)))
+		plb->pause_until = 0;
+
+	if (plb->pause_until)
+		return;
+
+	__sk_rethink_txhash_reset_dst(sk);
+	plb->consec_cong_rounds = 0;
+	WRITE_ONCE(tcp_sk(sk)->plb_rehash, tcp_sk(sk)->plb_rehash + 1);
+	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPLBREHASH);
+}
+EXPORT_SYMBOL_GPL(tcp_plb_check_rehash);
+
+/* Upon RTO, disallow load balancing for a while, to avoid having load
+ * balancing decisions switch traffic to a black-holed path that was
+ * previously avoided with a sk_rethink_txhash() call at RTO time.
+ */
+void tcp_plb_update_state_upon_rto(struct sock *sk, struct tcp_plb_state *plb)
+{
+	struct net *net = sock_net(sk);
+	u32 pause;
+
+	if (!READ_ONCE(net->ipv4.sysctl_tcp_plb_enabled))
+		return;
+
+	pause = READ_ONCE(net->ipv4.sysctl_tcp_plb_suspend_rto_sec) * HZ;
+	pause += get_random_u32_below(pause);
+	plb->pause_until = tcp_jiffies32 + pause;
+
+	/* Reset PLB state upon RTO, since an RTO causes a sk_rethink_txhash() call
+	 * that may switch this connection to a path with completely different
+	 * congestion characteristics.
+	 */
+	plb->consec_cong_rounds = 0;
+}
+EXPORT_SYMBOL_GPL(tcp_plb_update_state_upon_rto);