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, 85 insertions, 0 deletions

diff --git a/block/partitions/sgi.c b/block/partitions/sgi.c
new file mode 100644
index 000000000..2383ca63c
--- /dev/null
+++ b/block/partitions/sgi.c
@@ -0,0 +1,85 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  fs/partitions/sgi.c
+ *
+ *  Code extracted from drivers/block/genhd.c
+ */
+
+#include "check.h"
+
+#define SGI_LABEL_MAGIC 0x0be5a941
+
+enum {
+	LINUX_RAID_PARTITION = 0xfd,	/* autodetect RAID partition */
+};
+
+struct sgi_disklabel {
+	__be32 magic_mushroom;		/* Big fat spliff... */
+	__be16 root_part_num;		/* Root partition number */
+	__be16 swap_part_num;		/* Swap partition number */
+	s8 boot_file[16];		/* Name of boot file for ARCS */
+	u8 _unused0[48];		/* Device parameter useless crapola.. */
+	struct sgi_volume {
+		s8 name[8];		/* Name of volume */
+		__be32 block_num;		/* Logical block number */
+		__be32 num_bytes;		/* How big, in bytes */
+	} volume[15];
+	struct sgi_partition {
+		__be32 num_blocks;		/* Size in logical blocks */
+		__be32 first_block;	/* First logical block */
+		__be32 type;		/* Type of this partition */
+	} partitions[16];
+	__be32 csum;			/* Disk label checksum */
+	__be32 _unused1;			/* Padding */
+};
+
+int sgi_partition(struct parsed_partitions *state)
+{
+	int i, csum;
+	__be32 magic;
+	int slot = 1;
+	unsigned int start, blocks;
+	__be32 *ui, cs;
+	Sector sect;
+	struct sgi_disklabel *label;
+	struct sgi_partition *p;
+
+	label = read_part_sector(state, 0, &sect);
+	if (!label)
+		return -1;
+	p = &label->partitions[0];
+	magic = label->magic_mushroom;
+	if(be32_to_cpu(magic) != SGI_LABEL_MAGIC) {
+		put_dev_sector(sect);
+		return 0;
+	}
+	ui = ((__be32 *) (label + 1)) - 1;
+	for(csum = 0; ui >= ((__be32 *) label);) {
+		cs = *ui--;
+		csum += be32_to_cpu(cs);
+	}
+	if(csum) {
+		printk(KERN_WARNING "Dev %s SGI disklabel: csum bad, label corrupted\n",
+		       state->disk->disk_name);
+		put_dev_sector(sect);
+		return 0;
+	}
+	/* All SGI disk labels have 16 partitions, disks under Linux only
+	 * have 15 minor's.  Luckily there are always a few zero length
+	 * partitions which we don't care about so we never overflow the
+	 * current_minor.
+	 */
+	for(i = 0; i < 16; i++, p++) {
+		blocks = be32_to_cpu(p->num_blocks);
+		start  = be32_to_cpu(p->first_block);
+		if (blocks) {
+			put_partition(state, slot, start, blocks);
+			if (be32_to_cpu(p->type) == LINUX_RAID_PARTITION)
+				state->parts[slot].flags = ADDPART_FLAG_RAID;
+		}
+		slot++;
+	}
+	seq_buf_puts(&state->pp_buf, "\n");
+	put_dev_sector(sect);
+	return 1;
+}