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, 105 insertions, 0 deletions

diff --git a/net/sctp/objcnt.c b/net/sctp/objcnt.c
new file mode 100644
index 000000000..0400c964e
--- /dev/null
+++ b/net/sctp/objcnt.c
@@ -0,0 +1,105 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* SCTP kernel implementation
+ * (C) Copyright IBM Corp. 2001, 2004
+ *
+ * This file is part of the SCTP kernel implementation
+ *
+ * Support for memory object debugging.  This allows one to monitor the
+ * object allocations/deallocations for types instrumented for this
+ * via the proc fs.
+ *
+ * Please send any bug reports or fixes you make to the
+ * email address(es):
+ *    lksctp developers <linux-sctp@vger.kernel.org>
+ *
+ * Written or modified by:
+ *    Jon Grimm             <jgrimm@us.ibm.com>
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/kernel.h>
+#include <net/sctp/sctp.h>
+
+/*
+ * Global counters to count raw object allocation counts.
+ * To add new counters, choose a unique suffix for the variable
+ * name as the helper macros key off this suffix to make
+ * life easier for the programmer.
+ */
+
+SCTP_DBG_OBJCNT(sock);
+SCTP_DBG_OBJCNT(ep);
+SCTP_DBG_OBJCNT(transport);
+SCTP_DBG_OBJCNT(assoc);
+SCTP_DBG_OBJCNT(bind_addr);
+SCTP_DBG_OBJCNT(bind_bucket);
+SCTP_DBG_OBJCNT(chunk);
+SCTP_DBG_OBJCNT(addr);
+SCTP_DBG_OBJCNT(datamsg);
+SCTP_DBG_OBJCNT(keys);
+
+/* An array to make it easy to pretty print the debug information
+ * to the proc fs.
+ */
+static struct sctp_dbg_objcnt_entry sctp_dbg_objcnt[] = {
+	SCTP_DBG_OBJCNT_ENTRY(sock),
+	SCTP_DBG_OBJCNT_ENTRY(ep),
+	SCTP_DBG_OBJCNT_ENTRY(assoc),
+	SCTP_DBG_OBJCNT_ENTRY(transport),
+	SCTP_DBG_OBJCNT_ENTRY(chunk),
+	SCTP_DBG_OBJCNT_ENTRY(bind_addr),
+	SCTP_DBG_OBJCNT_ENTRY(bind_bucket),
+	SCTP_DBG_OBJCNT_ENTRY(addr),
+	SCTP_DBG_OBJCNT_ENTRY(datamsg),
+	SCTP_DBG_OBJCNT_ENTRY(keys),
+};
+
+/* Callback from procfs to read out objcount information.
+ * Walk through the entries in the sctp_dbg_objcnt array, dumping
+ * the raw object counts for each monitored type.
+ */
+static int sctp_objcnt_seq_show(struct seq_file *seq, void *v)
+{
+	int i;
+
+	i = (int)*(loff_t *)v;
+	seq_setwidth(seq, 127);
+	seq_printf(seq, "%s: %d", sctp_dbg_objcnt[i].label,
+				atomic_read(sctp_dbg_objcnt[i].counter));
+	seq_pad(seq, '\n');
+	return 0;
+}
+
+static void *sctp_objcnt_seq_start(struct seq_file *seq, loff_t *pos)
+{
+	return (*pos >= ARRAY_SIZE(sctp_dbg_objcnt)) ? NULL : (void *)pos;
+}
+
+static void sctp_objcnt_seq_stop(struct seq_file *seq, void *v)
+{
+}
+
+static void *sctp_objcnt_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+{
+	++*pos;
+	return (*pos >= ARRAY_SIZE(sctp_dbg_objcnt)) ? NULL : (void *)pos;
+}
+
+static const struct seq_operations sctp_objcnt_seq_ops = {
+	.start = sctp_objcnt_seq_start,
+	.next  = sctp_objcnt_seq_next,
+	.stop  = sctp_objcnt_seq_stop,
+	.show  = sctp_objcnt_seq_show,
+};
+
+/* Initialize the objcount in the proc filesystem.  */
+void sctp_dbg_objcnt_init(struct net *net)
+{
+	struct proc_dir_entry *ent;
+
+	ent = proc_create_seq("sctp_dbg_objcnt", 0,
+			  net->sctp.proc_net_sctp, &sctp_objcnt_seq_ops);
+	if (!ent)
+		pr_warn("sctp_dbg_objcnt: Unable to create /proc entry.\n");
+}