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, 136 insertions, 0 deletions

diff --git a/fs/jffs2/compr_rtime.c b/fs/jffs2/compr_rtime.c
new file mode 100644
index 000000000..3bd9d2f3b
--- /dev/null
+++ b/fs/jffs2/compr_rtime.c
@@ -0,0 +1,136 @@
+/*
+ * JFFS2 -- Journalling Flash File System, Version 2.
+ *
+ * Copyright © 2001-2007 Red Hat, Inc.
+ * Copyright © 2004-2010 David Woodhouse <dwmw2@infradead.org>
+ *
+ * Created by Arjan van de Ven <arjanv@redhat.com>
+ *
+ * For licensing information, see the file 'LICENCE' in this directory.
+ *
+ *
+ *
+ * Very simple lz77-ish encoder.
+ *
+ * Theory of operation: Both encoder and decoder have a list of "last
+ * occurrences" for every possible source-value; after sending the
+ * first source-byte, the second byte indicated the "run" length of
+ * matches
+ *
+ * The algorithm is intended to only send "whole bytes", no bit-messing.
+ *
+ */
+
+#include <linux/kernel.h>
+#include <linux/types.h>
+#include <linux/errno.h>
+#include <linux/string.h>
+#include <linux/jffs2.h>
+#include "compr.h"
+
+/* _compress returns the compressed size, -1 if bigger */
+static int jffs2_rtime_compress(unsigned char *data_in,
+				unsigned char *cpage_out,
+				uint32_t *sourcelen, uint32_t *dstlen)
+{
+	unsigned short positions[256];
+	int outpos = 0;
+	int pos=0;
+
+	if (*dstlen <= 3)
+		return -1;
+
+	memset(positions,0,sizeof(positions));
+
+	while (pos < (*sourcelen) && outpos <= (*dstlen)-2) {
+		int backpos, runlen=0;
+		unsigned char value;
+
+		value = data_in[pos];
+
+		cpage_out[outpos++] = data_in[pos++];
+
+		backpos = positions[value];
+		positions[value]=pos;
+
+		while ((backpos < pos) && (pos < (*sourcelen)) &&
+		       (data_in[pos]==data_in[backpos++]) && (runlen<255)) {
+			pos++;
+			runlen++;
+		}
+		cpage_out[outpos++] = runlen;
+	}
+
+	if (outpos >= pos) {
+		/* We failed */
+		return -1;
+	}
+
+	/* Tell the caller how much we managed to compress, and how much space it took */
+	*sourcelen = pos;
+	*dstlen = outpos;
+	return 0;
+}
+
+
+static int jffs2_rtime_decompress(unsigned char *data_in,
+				  unsigned char *cpage_out,
+				  uint32_t srclen, uint32_t destlen)
+{
+	unsigned short positions[256];
+	int outpos = 0;
+	int pos=0;
+
+	memset(positions,0,sizeof(positions));
+
+	while (outpos<destlen) {
+		unsigned char value;
+		int backoffs;
+		int repeat;
+
+		value = data_in[pos++];
+		cpage_out[outpos++] = value; /* first the verbatim copied byte */
+		repeat = data_in[pos++];
+		backoffs = positions[value];
+
+		positions[value]=outpos;
+		if (repeat) {
+			if ((outpos + repeat) > destlen) {
+				return 1;
+			}
+			if (backoffs + repeat >= outpos) {
+				while(repeat) {
+					cpage_out[outpos++] = cpage_out[backoffs++];
+					repeat--;
+				}
+			} else {
+				memcpy(&cpage_out[outpos],&cpage_out[backoffs],repeat);
+				outpos+=repeat;
+			}
+		}
+	}
+	return 0;
+}
+
+static struct jffs2_compressor jffs2_rtime_comp = {
+    .priority = JFFS2_RTIME_PRIORITY,
+    .name = "rtime",
+    .compr = JFFS2_COMPR_RTIME,
+    .compress = &jffs2_rtime_compress,
+    .decompress = &jffs2_rtime_decompress,
+#ifdef JFFS2_RTIME_DISABLED
+    .disabled = 1,
+#else
+    .disabled = 0,
+#endif
+};
+
+int jffs2_rtime_init(void)
+{
+    return jffs2_register_compressor(&jffs2_rtime_comp);
+}
+
+void jffs2_rtime_exit(void)
+{
+    jffs2_unregister_compressor(&jffs2_rtime_comp);
+}