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, 94 insertions, 0 deletions

diff --git a/net/ethtool/phc_vclocks.c b/net/ethtool/phc_vclocks.c
new file mode 100644
index 000000000..15146e38a
--- /dev/null
+++ b/net/ethtool/phc_vclocks.c
@@ -0,0 +1,94 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright 2021 NXP
+ */
+#include "common.h"
+#include "netlink.h"
+
+struct phc_vclocks_req_info {
+	struct ethnl_req_info		base;
+};
+
+struct phc_vclocks_reply_data {
+	struct ethnl_reply_data		base;
+	int				num;
+	int				*index;
+};
+
+#define PHC_VCLOCKS_REPDATA(__reply_base) \
+	container_of(__reply_base, struct phc_vclocks_reply_data, base)
+
+const struct nla_policy ethnl_phc_vclocks_get_policy[] = {
+	[ETHTOOL_A_PHC_VCLOCKS_HEADER] = NLA_POLICY_NESTED(ethnl_header_policy),
+};
+
+static int phc_vclocks_prepare_data(const struct ethnl_req_info *req_base,
+				    struct ethnl_reply_data *reply_base,
+				    const struct genl_info *info)
+{
+	struct phc_vclocks_reply_data *data = PHC_VCLOCKS_REPDATA(reply_base);
+	struct net_device *dev = reply_base->dev;
+	int ret;
+
+	ret = ethnl_ops_begin(dev);
+	if (ret < 0)
+		return ret;
+	data->num = ethtool_get_phc_vclocks(dev, &data->index);
+	ethnl_ops_complete(dev);
+
+	return ret;
+}
+
+static int phc_vclocks_reply_size(const struct ethnl_req_info *req_base,
+				  const struct ethnl_reply_data *reply_base)
+{
+	const struct phc_vclocks_reply_data *data =
+		PHC_VCLOCKS_REPDATA(reply_base);
+	int len = 0;
+
+	if (data->num > 0) {
+		len += nla_total_size(sizeof(u32));
+		len += nla_total_size(sizeof(s32) * data->num);
+	}
+
+	return len;
+}
+
+static int phc_vclocks_fill_reply(struct sk_buff *skb,
+				  const struct ethnl_req_info *req_base,
+				  const struct ethnl_reply_data *reply_base)
+{
+	const struct phc_vclocks_reply_data *data =
+		PHC_VCLOCKS_REPDATA(reply_base);
+
+	if (data->num <= 0)
+		return 0;
+
+	if (nla_put_u32(skb, ETHTOOL_A_PHC_VCLOCKS_NUM, data->num) ||
+	    nla_put(skb, ETHTOOL_A_PHC_VCLOCKS_INDEX,
+		    sizeof(s32) * data->num, data->index))
+		return -EMSGSIZE;
+
+	return 0;
+}
+
+static void phc_vclocks_cleanup_data(struct ethnl_reply_data *reply_base)
+{
+	const struct phc_vclocks_reply_data *data =
+		PHC_VCLOCKS_REPDATA(reply_base);
+
+	kfree(data->index);
+}
+
+const struct ethnl_request_ops ethnl_phc_vclocks_request_ops = {
+	.request_cmd		= ETHTOOL_MSG_PHC_VCLOCKS_GET,
+	.reply_cmd		= ETHTOOL_MSG_PHC_VCLOCKS_GET_REPLY,
+	.hdr_attr		= ETHTOOL_A_PHC_VCLOCKS_HEADER,
+	.req_info_size		= sizeof(struct phc_vclocks_req_info),
+	.reply_data_size	= sizeof(struct phc_vclocks_reply_data),
+
+	.prepare_data		= phc_vclocks_prepare_data,
+	.reply_size		= phc_vclocks_reply_size,
+	.fill_reply		= phc_vclocks_fill_reply,
+	.cleanup_data		= phc_vclocks_cleanup_data,
+};