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, 175 insertions, 0 deletions

diff --git a/lib/oid_registry.c b/lib/oid_registry.c
new file mode 100644
index 000000000..9b757a117
--- /dev/null
+++ b/lib/oid_registry.c
@@ -0,0 +1,175 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* ASN.1 Object identifier (OID) registry
+ *
+ * Copyright (C) 2012 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#include <linux/module.h>
+#include <linux/export.h>
+#include <linux/oid_registry.h>
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/bug.h>
+#include <linux/asn1.h>
+#include "oid_registry_data.c"
+
+MODULE_DESCRIPTION("OID Registry");
+MODULE_AUTHOR("Red Hat, Inc.");
+MODULE_LICENSE("GPL");
+
+/**
+ * look_up_OID - Find an OID registration for the specified data
+ * @data: Binary representation of the OID
+ * @datasize: Size of the binary representation
+ */
+enum OID look_up_OID(const void *data, size_t datasize)
+{
+	const unsigned char *octets = data;
+	enum OID oid;
+	unsigned char xhash;
+	unsigned i, j, k, hash;
+	size_t len;
+
+	/* Hash the OID data */
+	hash = datasize - 1;
+
+	for (i = 0; i < datasize; i++)
+		hash += octets[i] * 33;
+	hash = (hash >> 24) ^ (hash >> 16) ^ (hash >> 8) ^ hash;
+	hash &= 0xff;
+
+	/* Binary search the OID registry.  OIDs are stored in ascending order
+	 * of hash value then ascending order of size and then in ascending
+	 * order of reverse value.
+	 */
+	i = 0;
+	k = OID__NR;
+	while (i < k) {
+		j = (i + k) / 2;
+
+		xhash = oid_search_table[j].hash;
+		if (xhash > hash) {
+			k = j;
+			continue;
+		}
+		if (xhash < hash) {
+			i = j + 1;
+			continue;
+		}
+
+		oid = oid_search_table[j].oid;
+		len = oid_index[oid + 1] - oid_index[oid];
+		if (len > datasize) {
+			k = j;
+			continue;
+		}
+		if (len < datasize) {
+			i = j + 1;
+			continue;
+		}
+
+		/* Variation is most likely to be at the tail end of the
+		 * OID, so do the comparison in reverse.
+		 */
+		while (len > 0) {
+			unsigned char a = oid_data[oid_index[oid] + --len];
+			unsigned char b = octets[len];
+			if (a > b) {
+				k = j;
+				goto next;
+			}
+			if (a < b) {
+				i = j + 1;
+				goto next;
+			}
+		}
+		return oid;
+	next:
+		;
+	}
+
+	return OID__NR;
+}
+EXPORT_SYMBOL_GPL(look_up_OID);
+
+/**
+ * parse_OID - Parse an OID from a bytestream
+ * @data: Binary representation of the header + OID
+ * @datasize: Size of the binary representation
+ * @oid: Pointer to oid to return result
+ *
+ * Parse an OID from a bytestream that holds the OID in the format
+ * ASN1_OID | length | oid. The length indicator must equal to datasize - 2.
+ * -EBADMSG is returned if the bytestream is too short.
+ */
+int parse_OID(const void *data, size_t datasize, enum OID *oid)
+{
+	const unsigned char *v = data;
+
+	/* we need 2 bytes of header and at least 1 byte for oid */
+	if (datasize < 3 || v[0] != ASN1_OID || v[1] != datasize - 2)
+		return -EBADMSG;
+
+	*oid = look_up_OID(data + 2, datasize - 2);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(parse_OID);
+
+/*
+ * sprint_oid - Print an Object Identifier into a buffer
+ * @data: The encoded OID to print
+ * @datasize: The size of the encoded OID
+ * @buffer: The buffer to render into
+ * @bufsize: The size of the buffer
+ *
+ * The OID is rendered into the buffer in "a.b.c.d" format and the number of
+ * bytes is returned.  -EBADMSG is returned if the data could not be interpreted
+ * and -ENOBUFS if the buffer was too small.
+ */
+int sprint_oid(const void *data, size_t datasize, char *buffer, size_t bufsize)
+{
+	const unsigned char *v = data, *end = v + datasize;
+	unsigned long num;
+	unsigned char n;
+	size_t ret;
+	int count;
+
+	if (v >= end)
+		goto bad;
+
+	n = *v++;
+	ret = count = snprintf(buffer, bufsize, "%u.%u", n / 40, n % 40);
+	if (count >= bufsize)
+		return -ENOBUFS;
+	buffer += count;
+	bufsize -= count;
+
+	while (v < end) {
+		n = *v++;
+		if (!(n & 0x80)) {
+			num = n;
+		} else {
+			num = n & 0x7f;
+			do {
+				if (v >= end)
+					goto bad;
+				n = *v++;
+				num <<= 7;
+				num |= n & 0x7f;
+			} while (n & 0x80);
+		}
+		ret += count = snprintf(buffer, bufsize, ".%lu", num);
+		if (count >= bufsize)
+			return -ENOBUFS;
+		buffer += count;
+		bufsize -= count;
+	}
+
+	return ret;
+
+bad:
+	snprintf(buffer, bufsize, "(bad)");
+	return -EBADMSG;
+}
+EXPORT_SYMBOL_GPL(sprint_oid);