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>

2 files changed, 131 insertions, 0 deletions

diff --git a/samples/tsm-mr/Makefile b/samples/tsm-mr/Makefile
new file mode 100644
index 000000000..587c3947b
--- /dev/null
+++ b/samples/tsm-mr/Makefile
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+obj-$(CONFIG_SAMPLE_TSM_MR) += tsm_mr_sample.o
diff --git a/samples/tsm-mr/tsm_mr_sample.c b/samples/tsm-mr/tsm_mr_sample.c
new file mode 100644
index 000000000..c79dbc1e0
--- /dev/null
+++ b/samples/tsm-mr/tsm_mr_sample.c
@@ -0,0 +1,129 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright(c) 2024-2005 Intel Corporation. All rights reserved. */
+
+#define pr_fmt(x) KBUILD_MODNAME ": " x
+
+#include <linux/module.h>
+#include <linux/tsm-mr.h>
+#include <linux/miscdevice.h>
+#include <crypto/sha2.h>
+
+static struct {
+	u8 static_mr[SHA384_DIGEST_SIZE];
+	u8 config_mr[SHA512_DIGEST_SIZE];
+	u8 rtmr0[SHA256_DIGEST_SIZE];
+	u8 rtmr1[SHA384_DIGEST_SIZE];
+	u8 report_digest[SHA512_DIGEST_SIZE];
+} sample_report = {
+	.static_mr = "static_mr",
+	.config_mr = "config_mr",
+	.rtmr0 = "rtmr0",
+	.rtmr1 = "rtmr1",
+};
+
+static int sample_report_refresh(const struct tsm_measurements *tm)
+{
+	sha512((const u8 *)&sample_report,
+	       offsetof(typeof(sample_report), report_digest),
+	       sample_report.report_digest);
+	return 0;
+}
+
+static int sample_report_extend_mr(const struct tsm_measurements *tm,
+				   const struct tsm_measurement_register *mr,
+				   const u8 *data)
+{
+	union {
+		struct sha256_ctx sha256;
+		struct sha384_ctx sha384;
+		struct sha512_ctx sha512;
+	} ctx;
+
+	switch (mr->mr_hash) {
+	case HASH_ALGO_SHA256:
+		sha256_init(&ctx.sha256);
+		sha256_update(&ctx.sha256, mr->mr_value, mr->mr_size);
+		sha256_update(&ctx.sha256, data, mr->mr_size);
+		sha256_final(&ctx.sha256, mr->mr_value);
+		return 0;
+	case HASH_ALGO_SHA384:
+		sha384_init(&ctx.sha384);
+		sha384_update(&ctx.sha384, mr->mr_value, mr->mr_size);
+		sha384_update(&ctx.sha384, data, mr->mr_size);
+		sha384_final(&ctx.sha384, mr->mr_value);
+		return 0;
+	case HASH_ALGO_SHA512:
+		sha512_init(&ctx.sha512);
+		sha512_update(&ctx.sha512, mr->mr_value, mr->mr_size);
+		sha512_update(&ctx.sha512, data, mr->mr_size);
+		sha512_final(&ctx.sha512, mr->mr_value);
+		return 0;
+	default:
+		pr_err("Unsupported hash algorithm: %d\n", mr->mr_hash);
+		return -EOPNOTSUPP;
+	}
+}
+
+#define MR_(mr, hash) .mr_value = &sample_report.mr, TSM_MR_(mr, hash)
+static const struct tsm_measurement_register sample_mrs[] = {
+	/* static MR, read-only */
+	{ MR_(static_mr, SHA384) },
+	/* config MR, read-only */
+	{ MR_(config_mr, SHA512) | TSM_MR_F_NOHASH },
+	/* RTMR, direct extension prohibited */
+	{ MR_(rtmr0, SHA256) | TSM_MR_F_LIVE },
+	/* RTMR, direct extension allowed */
+	{ MR_(rtmr1, SHA384) | TSM_MR_F_RTMR },
+	/* RTMR, crypto agile, alaised to rtmr0 and rtmr1, respectively */
+	{ .mr_value = &sample_report.rtmr0,
+	  TSM_MR_(rtmr_crypto_agile, SHA256) | TSM_MR_F_RTMR },
+	{ .mr_value = &sample_report.rtmr1,
+	  TSM_MR_(rtmr_crypto_agile, SHA384) | TSM_MR_F_RTMR },
+	/* sha512 digest of the whole structure */
+	{ MR_(report_digest, SHA512) | TSM_MR_F_LIVE },
+};
+#undef MR_
+
+static struct tsm_measurements sample_tm = {
+	.mrs = sample_mrs,
+	.nr_mrs = ARRAY_SIZE(sample_mrs),
+	.refresh = sample_report_refresh,
+	.write = sample_report_extend_mr,
+};
+
+static const struct attribute_group *sample_groups[] = {
+	NULL,
+	NULL,
+};
+
+static struct miscdevice sample_misc_dev = {
+	.name = KBUILD_MODNAME,
+	.minor = MISC_DYNAMIC_MINOR,
+	.groups = sample_groups,
+};
+
+static int __init tsm_mr_sample_init(void)
+{
+	int rc;
+
+	sample_groups[0] = tsm_mr_create_attribute_group(&sample_tm);
+	if (IS_ERR(sample_groups[0]))
+		return PTR_ERR(sample_groups[0]);
+
+	rc = misc_register(&sample_misc_dev);
+	if (rc)
+		tsm_mr_free_attribute_group(sample_groups[0]);
+	return rc;
+}
+
+static void __exit tsm_mr_sample_exit(void)
+{
+	misc_deregister(&sample_misc_dev);
+	tsm_mr_free_attribute_group(sample_groups[0]);
+}
+
+module_init(tsm_mr_sample_init);
+module_exit(tsm_mr_sample_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Sample module using tsm-mr to expose emulated MRs");