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, 143 insertions, 0 deletions

diff --git a/arch/powerpc/boot/decompress.c b/arch/powerpc/boot/decompress.c
new file mode 100644
index 000000000..6835cb53f
--- /dev/null
+++ b/arch/powerpc/boot/decompress.c
@@ -0,0 +1,143 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Wrapper around the kernel's pre-boot decompression library.
+ *
+ * Copyright (C) IBM Corporation 2016.
+ */
+
+#include "elf.h"
+#include "page.h"
+#include "string.h"
+#include "stdio.h"
+#include "ops.h"
+#include "reg.h"
+#include "types.h"
+
+/*
+ * The decompressor_*.c files play #ifdef games so they can be used in both
+ * pre-boot and regular kernel code. We need these definitions to make the
+ * includes work.
+ */
+
+#define STATIC static
+#define INIT
+
+/*
+ * The build process will copy the required zlib source files and headers
+ * out of lib/ and "fix" the includes so they do not pull in other kernel
+ * headers.
+ */
+
+#ifdef CONFIG_KERNEL_GZIP
+#	include "decompress_inflate.c"
+#endif
+
+#ifdef CONFIG_KERNEL_XZ
+#	include "xz_config.h"
+#	include "../../../lib/decompress_unxz.c"
+#endif
+
+/* globals for tracking the state of the decompression */
+static unsigned long decompressed_bytes;
+static unsigned long limit;
+static unsigned long skip;
+static char *output_buffer;
+
+/*
+ * flush() is called by __decompress() when the decompressor's scratch buffer is
+ * full.
+ */
+static long flush(void *v, unsigned long buffer_size)
+{
+	unsigned long end = decompressed_bytes + buffer_size;
+	unsigned long size = buffer_size;
+	unsigned long offset = 0;
+	char *in = v;
+	char *out;
+
+	/*
+	 * if we hit our decompression limit, we need to fake an error to abort
+	 * the in-progress decompression.
+	 */
+	if (decompressed_bytes >= limit)
+		return -1;
+
+	/* skip this entire block */
+	if (end <= skip) {
+		decompressed_bytes += buffer_size;
+		return buffer_size;
+	}
+
+	/* skip some data at the start, but keep the rest of the block */
+	if (decompressed_bytes < skip && end > skip) {
+		offset = skip - decompressed_bytes;
+
+		in += offset;
+		size -= offset;
+		decompressed_bytes += offset;
+	}
+
+	out = &output_buffer[decompressed_bytes - skip];
+	size = min(decompressed_bytes + size, limit) - decompressed_bytes;
+
+	memcpy(out, in, size);
+	decompressed_bytes += size;
+
+	return buffer_size;
+}
+
+static void print_err(char *s)
+{
+	/* suppress the "error" when we terminate the decompressor */
+	if (decompressed_bytes >= limit)
+		return;
+
+	printf("Decompression error: '%s'\n\r", s);
+}
+
+/**
+ * partial_decompress - decompresses part or all of a compressed buffer
+ * @inbuf:       input buffer
+ * @input_size:  length of the input buffer
+ * @outbuf:      output buffer
+ * @output_size: length of the output buffer
+ * @_skip:       number of output bytes to ignore
+ *
+ * This function takes compressed data from inbuf, decompresses and write it to
+ * outbuf. Once output_size bytes are written to the output buffer, or the
+ * stream is exhausted the function will return the number of bytes that were
+ * decompressed. Otherwise it will return whatever error code the decompressor
+ * reported (NB: This is specific to each decompressor type).
+ *
+ * The skip functionality is mainly there so the program and discover
+ * the size of the compressed image so that it can ask firmware (if present)
+ * for an appropriately sized buffer.
+ */
+long partial_decompress(void *inbuf, unsigned long input_size,
+	void *outbuf, unsigned long output_size, unsigned long _skip)
+{
+	int ret;
+
+	/*
+	 * The skipped bytes needs to be included in the size of data we want
+	 * to decompress.
+	 */
+	output_size += _skip;
+
+	decompressed_bytes = 0;
+	output_buffer = outbuf;
+	limit = output_size;
+	skip = _skip;
+
+	ret = __decompress(inbuf, input_size, NULL, flush, outbuf,
+		output_size, NULL, print_err);
+
+	/*
+	 * If decompression was aborted due to an actual error rather than
+	 * a fake error that we used to abort, then we should report it.
+	 */
+	if (decompressed_bytes < limit)
+		return ret;
+
+	return decompressed_bytes - skip;
+}