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, 203 insertions, 0 deletions

diff --git a/fs/bfs/file.c b/fs/bfs/file.c
new file mode 100644
index 000000000..d33d6bde9
--- /dev/null
+++ b/fs/bfs/file.c
@@ -0,0 +1,203 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *	fs/bfs/file.c
+ *	BFS file operations.
+ *	Copyright (C) 1999-2018 Tigran Aivazian <aivazian.tigran@gmail.com>
+ *
+ *	Make the file block allocation algorithm understand the size
+ *	of the underlying block device.
+ *	Copyright (C) 2007 Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
+ *
+ */
+
+#include <linux/fs.h>
+#include <linux/mpage.h>
+#include <linux/buffer_head.h>
+#include "bfs.h"
+
+#undef DEBUG
+
+#ifdef DEBUG
+#define dprintf(x...)	printf(x)
+#else
+#define dprintf(x...)
+#endif
+
+const struct file_operations bfs_file_operations = {
+	.llseek 	= generic_file_llseek,
+	.read_iter	= generic_file_read_iter,
+	.write_iter	= generic_file_write_iter,
+	.mmap_prepare	= generic_file_mmap_prepare,
+	.splice_read	= filemap_splice_read,
+};
+
+static int bfs_move_block(unsigned long from, unsigned long to,
+					struct super_block *sb)
+{
+	struct buffer_head *bh, *new;
+
+	bh = sb_bread(sb, from);
+	if (!bh)
+		return -EIO;
+	new = sb_getblk(sb, to);
+	memcpy(new->b_data, bh->b_data, bh->b_size);
+	mark_buffer_dirty(new);
+	bforget(bh);
+	brelse(new);
+	return 0;
+}
+
+static int bfs_move_blocks(struct super_block *sb, unsigned long start,
+				unsigned long end, unsigned long where)
+{
+	unsigned long i;
+
+	dprintf("%08lx-%08lx->%08lx\n", start, end, where);
+	for (i = start; i <= end; i++)
+		if(bfs_move_block(i, where + i, sb)) {
+			dprintf("failed to move block %08lx -> %08lx\n", i,
+								where + i);
+			return -EIO;
+		}
+	return 0;
+}
+
+static int bfs_get_block(struct inode *inode, sector_t block,
+			struct buffer_head *bh_result, int create)
+{
+	unsigned long phys;
+	int err;
+	struct super_block *sb = inode->i_sb;
+	struct bfs_sb_info *info = BFS_SB(sb);
+	struct bfs_inode_info *bi = BFS_I(inode);
+
+	phys = bi->i_sblock + block;
+	if (!create) {
+		if (phys <= bi->i_eblock) {
+			dprintf("c=%d, b=%08lx, phys=%09lx (granted)\n",
+                                create, (unsigned long)block, phys);
+			map_bh(bh_result, sb, phys);
+		}
+		return 0;
+	}
+
+	/*
+	 * If the file is not empty and the requested block is within the
+	 * range of blocks allocated for this file, we can grant it.
+	 */
+	if (bi->i_sblock && (phys <= bi->i_eblock)) {
+		dprintf("c=%d, b=%08lx, phys=%08lx (interim block granted)\n", 
+				create, (unsigned long)block, phys);
+		map_bh(bh_result, sb, phys);
+		return 0;
+	}
+
+	/* The file will be extended, so let's see if there is enough space. */
+	if (phys >= info->si_blocks)
+		return -ENOSPC;
+
+	/* The rest has to be protected against itself. */
+	mutex_lock(&info->bfs_lock);
+
+	/*
+	 * If the last data block for this file is the last allocated
+	 * block, we can extend the file trivially, without moving it
+	 * anywhere.
+	 */
+	if (bi->i_eblock == info->si_lf_eblk) {
+		dprintf("c=%d, b=%08lx, phys=%08lx (simple extension)\n", 
+				create, (unsigned long)block, phys);
+		map_bh(bh_result, sb, phys);
+		info->si_freeb -= phys - bi->i_eblock;
+		info->si_lf_eblk = bi->i_eblock = phys;
+		mark_inode_dirty(inode);
+		err = 0;
+		goto out;
+	}
+
+	/* Ok, we have to move this entire file to the next free block. */
+	phys = info->si_lf_eblk + 1;
+	if (phys + block >= info->si_blocks) {
+		err = -ENOSPC;
+		goto out;
+	}
+
+	if (bi->i_sblock) {
+		err = bfs_move_blocks(inode->i_sb, bi->i_sblock, 
+						bi->i_eblock, phys);
+		if (err) {
+			dprintf("failed to move ino=%08lx -> fs corruption\n",
+								inode->i_ino);
+			goto out;
+		}
+	} else
+		err = 0;
+
+	dprintf("c=%d, b=%08lx, phys=%08lx (moved)\n",
+                create, (unsigned long)block, phys);
+	bi->i_sblock = phys;
+	phys += block;
+	info->si_lf_eblk = bi->i_eblock = phys;
+
+	/*
+	 * This assumes nothing can write the inode back while we are here
+	 * and thus update inode->i_blocks! (XXX)
+	 */
+	info->si_freeb -= bi->i_eblock - bi->i_sblock + 1 - inode->i_blocks;
+	mark_inode_dirty(inode);
+	map_bh(bh_result, sb, phys);
+out:
+	mutex_unlock(&info->bfs_lock);
+	return err;
+}
+
+static int bfs_writepages(struct address_space *mapping,
+		struct writeback_control *wbc)
+{
+	return mpage_writepages(mapping, wbc, bfs_get_block);
+}
+
+static int bfs_read_folio(struct file *file, struct folio *folio)
+{
+	return block_read_full_folio(folio, bfs_get_block);
+}
+
+static void bfs_write_failed(struct address_space *mapping, loff_t to)
+{
+	struct inode *inode = mapping->host;
+
+	if (to > inode->i_size)
+		truncate_pagecache(inode, inode->i_size);
+}
+
+static int bfs_write_begin(const struct kiocb *iocb,
+			   struct address_space *mapping,
+			   loff_t pos, unsigned len,
+			   struct folio **foliop, void **fsdata)
+{
+	int ret;
+
+	ret = block_write_begin(mapping, pos, len, foliop, bfs_get_block);
+	if (unlikely(ret))
+		bfs_write_failed(mapping, pos + len);
+
+	return ret;
+}
+
+static sector_t bfs_bmap(struct address_space *mapping, sector_t block)
+{
+	return generic_block_bmap(mapping, block, bfs_get_block);
+}
+
+const struct address_space_operations bfs_aops = {
+	.dirty_folio	= block_dirty_folio,
+	.invalidate_folio = block_invalidate_folio,
+	.read_folio	= bfs_read_folio,
+	.writepages	= bfs_writepages,
+	.write_begin	= bfs_write_begin,
+	.write_end	= generic_write_end,
+	.migrate_folio	= buffer_migrate_folio,
+	.bmap		= bfs_bmap,
+};
+
+const struct inode_operations bfs_file_inops;