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, 128 insertions, 0 deletions

diff --git a/drivers/misc/altera-stapl/altera-comp.c b/drivers/misc/altera-stapl/altera-comp.c
new file mode 100644
index 000000000..4a63f51cc
--- /dev/null
+++ b/drivers/misc/altera-stapl/altera-comp.c
@@ -0,0 +1,128 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * altera-comp.c
+ *
+ * altera FPGA driver
+ *
+ * Copyright (C) Altera Corporation 1998-2001
+ * Copyright (C) 2010 NetUP Inc.
+ * Copyright (C) 2010 Igor M. Liplianin <liplianin@netup.ru>
+ */
+
+#include <linux/kernel.h>
+#include "altera-exprt.h"
+
+#define	SHORT_BITS		16
+#define	CHAR_BITS		8
+#define	DATA_BLOB_LENGTH	3
+#define	MATCH_DATA_LENGTH	8192
+#define ALTERA_REQUEST_SIZE	1024
+#define ALTERA_BUFFER_SIZE	(MATCH_DATA_LENGTH + ALTERA_REQUEST_SIZE)
+
+static u32 altera_bits_req(u32 n)
+{
+	u32 result = SHORT_BITS;
+
+	if (n == 0)
+		result = 1;
+	else {
+		/* Look for the highest non-zero bit position */
+		while ((n & (1 << (SHORT_BITS - 1))) == 0) {
+			n <<= 1;
+			--result;
+		}
+	}
+
+	return result;
+}
+
+static u32 altera_read_packed(u8 *buffer, u32 bits, u32 *bits_avail,
+							u32 *in_index)
+{
+	u32 result = 0;
+	u32 shift = 0;
+	u32 databyte = 0;
+
+	while (bits > 0) {
+		databyte = buffer[*in_index];
+		result |= (((databyte >> (CHAR_BITS - *bits_avail))
+			& (0xff >> (CHAR_BITS - *bits_avail))) << shift);
+
+		if (bits <= *bits_avail) {
+			result &= (0xffff >> (SHORT_BITS - (bits + shift)));
+			*bits_avail -= bits;
+			bits = 0;
+		} else {
+			++(*in_index);
+			shift += *bits_avail;
+			bits -= *bits_avail;
+			*bits_avail = CHAR_BITS;
+		}
+	}
+
+	return result;
+}
+
+u32 altera_shrink(u8 *in, u32 in_length, u8 *out, u32 out_length, s32 version)
+{
+	u32 i, j, data_length = 0L;
+	u32 offset, length;
+	u32 match_data_length = MATCH_DATA_LENGTH;
+	u32 bits_avail = CHAR_BITS;
+	u32 in_index = 0L;
+
+	if (version > 0)
+		--match_data_length;
+
+	for (i = 0; i < out_length; ++i)
+		out[i] = 0;
+
+	/* Read number of bytes in data. */
+	for (i = 0; i < sizeof(in_length); ++i) {
+		data_length = data_length | (
+			altera_read_packed(in,
+					CHAR_BITS,
+					&bits_avail,
+					&in_index) << (i * CHAR_BITS));
+	}
+
+	if (data_length > out_length) {
+		data_length = 0L;
+		return data_length;
+	}
+
+	i = 0;
+	while (i < data_length) {
+		/* A 0 bit indicates literal data. */
+		if (altera_read_packed(in, 1, &bits_avail,
+						&in_index) == 0) {
+			for (j = 0; j < DATA_BLOB_LENGTH; ++j) {
+				if (i < data_length) {
+					out[i] = (u8)altera_read_packed(in,
+							CHAR_BITS,
+							&bits_avail,
+							&in_index);
+					i++;
+				}
+			}
+		} else {
+			/* A 1 bit indicates offset/length to follow. */
+			offset = altera_read_packed(in, altera_bits_req((s16)
+					(i > match_data_length ?
+						match_data_length : i)),
+					&bits_avail,
+					&in_index);
+			length = altera_read_packed(in, CHAR_BITS,
+					&bits_avail,
+					&in_index);
+			for (j = 0; j < length; ++j) {
+				if (i < data_length) {
+					out[i] = out[i - offset];
+					i++;
+				}
+			}
+		}
+	}
+
+	return data_length;
+}