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, 102 insertions, 0 deletions

diff --git a/net/atm/atm_misc.c b/net/atm/atm_misc.c
new file mode 100644
index 000000000..a30b83c1c
--- /dev/null
+++ b/net/atm/atm_misc.c
@@ -0,0 +1,102 @@
+// SPDX-License-Identifier: GPL-2.0
+/* net/atm/atm_misc.c - Various functions for use by ATM drivers */
+
+/* Written 1995-2000 by Werner Almesberger, EPFL ICA */
+
+#include <linux/module.h>
+#include <linux/atm.h>
+#include <linux/atmdev.h>
+#include <linux/skbuff.h>
+#include <linux/sonet.h>
+#include <linux/bitops.h>
+#include <linux/errno.h>
+#include <linux/atomic.h>
+
+int atm_charge(struct atm_vcc *vcc, int truesize)
+{
+	atm_force_charge(vcc, truesize);
+	if (atomic_read(&sk_atm(vcc)->sk_rmem_alloc) <= sk_atm(vcc)->sk_rcvbuf)
+		return 1;
+	atm_return(vcc, truesize);
+	atomic_inc(&vcc->stats->rx_drop);
+	return 0;
+}
+EXPORT_SYMBOL(atm_charge);
+
+struct sk_buff *atm_alloc_charge(struct atm_vcc *vcc, int pdu_size,
+				 gfp_t gfp_flags)
+{
+	struct sock *sk = sk_atm(vcc);
+	int guess = SKB_TRUESIZE(pdu_size);
+
+	atm_force_charge(vcc, guess);
+	if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf) {
+		struct sk_buff *skb = alloc_skb(pdu_size, gfp_flags);
+
+		if (skb) {
+			atomic_add(skb->truesize-guess,
+				   &sk->sk_rmem_alloc);
+			return skb;
+		}
+	}
+	atm_return(vcc, guess);
+	atomic_inc(&vcc->stats->rx_drop);
+	return NULL;
+}
+EXPORT_SYMBOL(atm_alloc_charge);
+
+
+/*
+ * atm_pcr_goal returns the positive PCR if it should be rounded up, the
+ * negative PCR if it should be rounded down, and zero if the maximum available
+ * bandwidth should be used.
+ *
+ * The rules are as follows (* = maximum, - = absent (0), x = value "x",
+ * (x+ = x or next value above x, x- = x or next value below):
+ *
+ *	min max pcr	result		min max pcr	result
+ *	-   -   -	* (UBR only)	x   -   -	x+
+ *	-   -   *	*		x   -   *	*
+ *	-   -   z	z-		x   -   z	z-
+ *	-   *   -	*		x   *   -	x+
+ *	-   *   *	*		x   *   *	*
+ *	-   *   z	z-		x   *   z	z-
+ *	-   y   -	y-		x   y   -	x+
+ *	-   y   *	y-		x   y   *	y-
+ *	-   y   z	z-		x   y   z	z-
+ *
+ * All non-error cases can be converted with the following simple set of rules:
+ *
+ *   if pcr == z then z-
+ *   else if min == x && pcr == - then x+
+ *     else if max == y then y-
+ *	 else *
+ */
+
+int atm_pcr_goal(const struct atm_trafprm *tp)
+{
+	if (tp->pcr && tp->pcr != ATM_MAX_PCR)
+		return -tp->pcr;
+	if (tp->min_pcr && !tp->pcr)
+		return tp->min_pcr;
+	if (tp->max_pcr != ATM_MAX_PCR)
+		return -tp->max_pcr;
+	return 0;
+}
+EXPORT_SYMBOL(atm_pcr_goal);
+
+void sonet_copy_stats(struct k_sonet_stats *from, struct sonet_stats *to)
+{
+#define __HANDLE_ITEM(i) to->i = atomic_read(&from->i)
+	__SONET_ITEMS
+#undef __HANDLE_ITEM
+}
+EXPORT_SYMBOL(sonet_copy_stats);
+
+void sonet_subtract_stats(struct k_sonet_stats *from, struct sonet_stats *to)
+{
+#define __HANDLE_ITEM(i) atomic_sub(to->i, &from->i)
+	__SONET_ITEMS
+#undef __HANDLE_ITEM
+}
+EXPORT_SYMBOL(sonet_subtract_stats);