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, 232 insertions, 0 deletions

diff --git a/fs/hpfs/buffer.c b/fs/hpfs/buffer.c
new file mode 100644
index 000000000..d39246865
--- /dev/null
+++ b/fs/hpfs/buffer.c
@@ -0,0 +1,232 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  linux/fs/hpfs/buffer.c
+ *
+ *  Mikulas Patocka (mikulas@artax.karlin.mff.cuni.cz), 1998-1999
+ *
+ *  general buffer i/o
+ */
+#include <linux/sched.h>
+#include <linux/slab.h>
+#include <linux/blkdev.h>
+#include "hpfs_fn.h"
+
+secno hpfs_search_hotfix_map(struct super_block *s, secno sec)
+{
+	unsigned i;
+	struct hpfs_sb_info *sbi = hpfs_sb(s);
+	for (i = 0; unlikely(i < sbi->n_hotfixes); i++) {
+		if (sbi->hotfix_from[i] == sec) {
+			return sbi->hotfix_to[i];
+		}
+	}
+	return sec;
+}
+
+unsigned hpfs_search_hotfix_map_for_range(struct super_block *s, secno sec, unsigned n)
+{
+	unsigned i;
+	struct hpfs_sb_info *sbi = hpfs_sb(s);
+	for (i = 0; unlikely(i < sbi->n_hotfixes); i++) {
+		if (sbi->hotfix_from[i] >= sec && sbi->hotfix_from[i] < sec + n) {
+			n = sbi->hotfix_from[i] - sec;
+		}
+	}
+	return n;
+}
+
+void hpfs_prefetch_sectors(struct super_block *s, unsigned secno, int n)
+{
+	struct buffer_head *bh;
+	struct blk_plug plug;
+
+	if (n <= 0 || unlikely(secno >= hpfs_sb(s)->sb_fs_size))
+		return;
+
+	if (unlikely(hpfs_search_hotfix_map_for_range(s, secno, n) != n))
+		return;
+
+	bh = sb_find_get_block(s, secno);
+	if (bh) {
+		if (buffer_uptodate(bh)) {
+			brelse(bh);
+			return;
+		}
+		brelse(bh);
+	}
+
+	blk_start_plug(&plug);
+	while (n > 0) {
+		if (unlikely(secno >= hpfs_sb(s)->sb_fs_size))
+			break;
+		sb_breadahead(s, secno);
+		secno++;
+		n--;
+	}
+	blk_finish_plug(&plug);
+}
+
+/* Map a sector into a buffer and return pointers to it and to the buffer. */
+
+void *hpfs_map_sector(struct super_block *s, unsigned secno, struct buffer_head **bhp,
+		 int ahead)
+{
+	struct buffer_head *bh;
+
+	hpfs_lock_assert(s);
+
+	hpfs_prefetch_sectors(s, secno, ahead);
+
+	cond_resched();
+
+	*bhp = bh = sb_bread(s, hpfs_search_hotfix_map(s, secno));
+	if (bh != NULL)
+		return bh->b_data;
+	else {
+		pr_err("%s(): read error\n", __func__);
+		return NULL;
+	}
+}
+
+/* Like hpfs_map_sector but don't read anything */
+
+void *hpfs_get_sector(struct super_block *s, unsigned secno, struct buffer_head **bhp)
+{
+	struct buffer_head *bh;
+	/*return hpfs_map_sector(s, secno, bhp, 0);*/
+
+	hpfs_lock_assert(s);
+
+	cond_resched();
+
+	if ((*bhp = bh = sb_getblk(s, hpfs_search_hotfix_map(s, secno))) != NULL) {
+		if (!buffer_uptodate(bh)) wait_on_buffer(bh);
+		set_buffer_uptodate(bh);
+		return bh->b_data;
+	} else {
+		pr_err("%s(): getblk failed\n", __func__);
+		return NULL;
+	}
+}
+
+/* Map 4 sectors into a 4buffer and return pointers to it and to the buffer. */
+
+void *hpfs_map_4sectors(struct super_block *s, unsigned secno, struct quad_buffer_head *qbh,
+		   int ahead)
+{
+	char *data;
+
+	hpfs_lock_assert(s);
+
+	cond_resched();
+
+	if (secno & 3) {
+		pr_err("%s(): unaligned read\n", __func__);
+		return NULL;
+	}
+
+	hpfs_prefetch_sectors(s, secno, 4 + ahead);
+
+	if (!hpfs_map_sector(s, secno + 0, &qbh->bh[0], 0)) goto bail0;
+	if (!hpfs_map_sector(s, secno + 1, &qbh->bh[1], 0)) goto bail1;
+	if (!hpfs_map_sector(s, secno + 2, &qbh->bh[2], 0)) goto bail2;
+	if (!hpfs_map_sector(s, secno + 3, &qbh->bh[3], 0)) goto bail3;
+
+	if (likely(qbh->bh[1]->b_data == qbh->bh[0]->b_data + 1 * 512) &&
+	    likely(qbh->bh[2]->b_data == qbh->bh[0]->b_data + 2 * 512) &&
+	    likely(qbh->bh[3]->b_data == qbh->bh[0]->b_data + 3 * 512)) {
+		return qbh->data = qbh->bh[0]->b_data;
+	}
+
+	qbh->data = data = kmalloc(2048, GFP_NOFS);
+	if (!data) {
+		pr_err("%s(): out of memory\n", __func__);
+		goto bail4;
+	}
+
+	memcpy(data + 0 * 512, qbh->bh[0]->b_data, 512);
+	memcpy(data + 1 * 512, qbh->bh[1]->b_data, 512);
+	memcpy(data + 2 * 512, qbh->bh[2]->b_data, 512);
+	memcpy(data + 3 * 512, qbh->bh[3]->b_data, 512);
+
+	return data;
+
+ bail4:
+	brelse(qbh->bh[3]);
+ bail3:
+	brelse(qbh->bh[2]);
+ bail2:
+	brelse(qbh->bh[1]);
+ bail1:
+	brelse(qbh->bh[0]);
+ bail0:
+	return NULL;
+}
+
+/* Don't read sectors */
+
+void *hpfs_get_4sectors(struct super_block *s, unsigned secno,
+                          struct quad_buffer_head *qbh)
+{
+	cond_resched();
+
+	hpfs_lock_assert(s);
+
+	if (secno & 3) {
+		pr_err("%s(): unaligned read\n", __func__);
+		return NULL;
+	}
+
+	if (!hpfs_get_sector(s, secno + 0, &qbh->bh[0])) goto bail0;
+	if (!hpfs_get_sector(s, secno + 1, &qbh->bh[1])) goto bail1;
+	if (!hpfs_get_sector(s, secno + 2, &qbh->bh[2])) goto bail2;
+	if (!hpfs_get_sector(s, secno + 3, &qbh->bh[3])) goto bail3;
+
+	if (likely(qbh->bh[1]->b_data == qbh->bh[0]->b_data + 1 * 512) &&
+	    likely(qbh->bh[2]->b_data == qbh->bh[0]->b_data + 2 * 512) &&
+	    likely(qbh->bh[3]->b_data == qbh->bh[0]->b_data + 3 * 512)) {
+		return qbh->data = qbh->bh[0]->b_data;
+	}
+
+	if (!(qbh->data = kmalloc(2048, GFP_NOFS))) {
+		pr_err("%s(): out of memory\n", __func__);
+		goto bail4;
+	}
+	return qbh->data;
+
+bail4:
+	brelse(qbh->bh[3]);
+bail3:
+	brelse(qbh->bh[2]);
+bail2:
+	brelse(qbh->bh[1]);
+bail1:
+	brelse(qbh->bh[0]);
+bail0:
+	return NULL;
+}
+	
+
+void hpfs_brelse4(struct quad_buffer_head *qbh)
+{
+	if (unlikely(qbh->data != qbh->bh[0]->b_data))
+		kfree(qbh->data);
+	brelse(qbh->bh[0]);
+	brelse(qbh->bh[1]);
+	brelse(qbh->bh[2]);
+	brelse(qbh->bh[3]);
+}	
+
+void hpfs_mark_4buffers_dirty(struct quad_buffer_head *qbh)
+{
+	if (unlikely(qbh->data != qbh->bh[0]->b_data)) {
+		memcpy(qbh->bh[0]->b_data, qbh->data + 0 * 512, 512);
+		memcpy(qbh->bh[1]->b_data, qbh->data + 1 * 512, 512);
+		memcpy(qbh->bh[2]->b_data, qbh->data + 2 * 512, 512);
+		memcpy(qbh->bh[3]->b_data, qbh->data + 3 * 512, 512);
+	}
+	mark_buffer_dirty(qbh->bh[0]);
+	mark_buffer_dirty(qbh->bh[1]);
+	mark_buffer_dirty(qbh->bh[2]);
+	mark_buffer_dirty(qbh->bh[3]);
+}