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, 87 insertions, 0 deletions

diff --git a/block/partitions/osf.c b/block/partitions/osf.c
new file mode 100644
index 000000000..2a692584d
--- /dev/null
+++ b/block/partitions/osf.c
@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  fs/partitions/osf.c
+ *
+ *  Code extracted from drivers/block/genhd.c
+ *
+ *  Copyright (C) 1991-1998  Linus Torvalds
+ *  Re-organised Feb 1998 Russell King
+ */
+
+#include "check.h"
+
+#define MAX_OSF_PARTITIONS 18
+#define DISKLABELMAGIC (0x82564557UL)
+
+int osf_partition(struct parsed_partitions *state)
+{
+	int i;
+	int slot = 1;
+	unsigned int npartitions;
+	Sector sect;
+	unsigned char *data;
+	struct disklabel {
+		__le32 d_magic;
+		__le16 d_type,d_subtype;
+		u8 d_typename[16];
+		u8 d_packname[16];
+		__le32 d_secsize;
+		__le32 d_nsectors;
+		__le32 d_ntracks;
+		__le32 d_ncylinders;
+		__le32 d_secpercyl;
+		__le32 d_secprtunit;
+		__le16 d_sparespertrack;
+		__le16 d_sparespercyl;
+		__le32 d_acylinders;
+		__le16 d_rpm, d_interleave, d_trackskew, d_cylskew;
+		__le32 d_headswitch, d_trkseek, d_flags;
+		__le32 d_drivedata[5];
+		__le32 d_spare[5];
+		__le32 d_magic2;
+		__le16 d_checksum;
+		__le16 d_npartitions;
+		__le32 d_bbsize, d_sbsize;
+		struct d_partition {
+			__le32 p_size;
+			__le32 p_offset;
+			__le32 p_fsize;
+			u8  p_fstype;
+			u8  p_frag;
+			__le16 p_cpg;
+		} d_partitions[MAX_OSF_PARTITIONS];
+	} * label;
+	struct d_partition * partition;
+
+	data = read_part_sector(state, 0, &sect);
+	if (!data)
+		return -1;
+
+	label = (struct disklabel *) (data+64);
+	partition = label->d_partitions;
+	if (le32_to_cpu(label->d_magic) != DISKLABELMAGIC) {
+		put_dev_sector(sect);
+		return 0;
+	}
+	if (le32_to_cpu(label->d_magic2) != DISKLABELMAGIC) {
+		put_dev_sector(sect);
+		return 0;
+	}
+	npartitions = le16_to_cpu(label->d_npartitions);
+	if (npartitions > MAX_OSF_PARTITIONS) {
+		put_dev_sector(sect);
+		return 0;
+	}
+	for (i = 0 ; i < npartitions; i++, partition++) {
+		if (slot == state->limit)
+		        break;
+		if (le32_to_cpu(partition->p_size))
+			put_partition(state, slot,
+				le32_to_cpu(partition->p_offset),
+				le32_to_cpu(partition->p_size));
+		slot++;
+	}
+	seq_buf_puts(&state->pp_buf, "\n");
+	put_dev_sector(sect);
+	return 1;
+}