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, 386 insertions, 0 deletions

diff --git a/fs/ext4/readpage.c b/fs/ext4/readpage.c
new file mode 100644
index 000000000..c7b6cdb2e
--- /dev/null
+++ b/fs/ext4/readpage.c
@@ -0,0 +1,386 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * linux/fs/ext4/readpage.c
+ *
+ * Copyright (C) 2002, Linus Torvalds.
+ * Copyright (C) 2015, Google, Inc.
+ *
+ * This was originally taken from fs/mpage.c
+ *
+ * The ext4_mpage_readpages() function here is intended to
+ * replace mpage_readahead() in the general case, not just for
+ * encrypted files.  It has some limitations (see below), where it
+ * will fall back to read_block_full_page(), but these limitations
+ * should only be hit when page_size != block_size.
+ *
+ * This will allow us to attach a callback function to support ext4
+ * encryption.
+ *
+ * If anything unusual happens, such as:
+ *
+ * - encountering a page which has buffers
+ * - encountering a page which has a non-hole after a hole
+ * - encountering a page with non-contiguous blocks
+ *
+ * then this code just gives up and calls the buffer_head-based read function.
+ * It does handle a page which has holes at the end - that is a common case:
+ * the end-of-file on blocksize < PAGE_SIZE setups.
+ *
+ */
+
+#include <linux/kernel.h>
+#include <linux/export.h>
+#include <linux/mm.h>
+#include <linux/kdev_t.h>
+#include <linux/gfp.h>
+#include <linux/bio.h>
+#include <linux/fs.h>
+#include <linux/buffer_head.h>
+#include <linux/blk-crypto.h>
+#include <linux/blkdev.h>
+#include <linux/highmem.h>
+#include <linux/prefetch.h>
+#include <linux/mpage.h>
+#include <linux/writeback.h>
+#include <linux/backing-dev.h>
+
+#include "ext4.h"
+#include <trace/events/ext4.h>
+
+#define NUM_VERITY_WORKS 128
+
+static struct kmem_cache *ext4_verity_work_cache;
+static mempool_t *ext4_verity_work_pool;
+
+struct ext4_verity_work {
+	struct bio *bio;
+	struct fsverity_info *vi;
+	struct work_struct work;
+};
+
+static void __read_end_io(struct bio *bio)
+{
+	struct folio_iter fi;
+
+	bio_for_each_folio_all(fi, bio)
+		folio_end_read(fi.folio, bio->bi_status == 0);
+	if (bio->bi_private)
+		mempool_free(bio->bi_private, ext4_verity_work_pool);
+	bio_put(bio);
+}
+
+static void verity_work(struct work_struct *work)
+{
+	struct ext4_verity_work *ctx =
+		container_of(work, struct ext4_verity_work, work);
+	struct bio *bio = ctx->bio;
+	struct fsverity_info *vi = ctx->vi;
+
+	/*
+	 * Free the ext4_verity_work right away, since it's no longer needed.
+	 * This relieves the pressure on the mempool as much as possible.
+	 */
+	mempool_free(ctx, ext4_verity_work_pool);
+	bio->bi_private = NULL;
+
+	fsverity_verify_bio(vi, bio);
+
+	__read_end_io(bio);
+}
+
+/*
+ * I/O completion handler for multipage BIOs.
+ *
+ * The mpage code never puts partial pages into a BIO (except for end-of-file).
+ * If a page does not map to a contiguous run of blocks then it simply falls
+ * back to block_read_full_folio().
+ *
+ * Why is this?  If a page's completion depends on a number of different BIOs
+ * which can complete in any order (or at the same time) then determining the
+ * status of that page is hard.  See end_buffer_async_read() for the details.
+ * There is no point in duplicating all that complexity.
+ */
+static void mpage_end_io(struct bio *bio)
+{
+	if (IS_ENABLED(CONFIG_FS_VERITY) && bio->bi_private &&
+	    !bio->bi_status) {
+		struct ext4_verity_work *ctx = bio->bi_private;
+
+		INIT_WORK(&ctx->work, verity_work);
+		fsverity_enqueue_verify_work(&ctx->work);
+		return;
+	}
+	__read_end_io(bio);
+}
+
+static void ext4_set_verity_work(struct bio *bio, struct fsverity_info *vi)
+{
+	if (vi) {
+		/* Due to the mempool, this never fails. */
+		struct ext4_verity_work *ctx =
+			mempool_alloc(ext4_verity_work_pool, GFP_NOFS);
+
+		ctx->bio = bio;
+		ctx->vi = vi;
+		bio->bi_private = ctx;
+	}
+}
+
+static inline loff_t ext4_readpage_limit(struct inode *inode)
+{
+	if (IS_ENABLED(CONFIG_FS_VERITY) && IS_VERITY(inode))
+		return inode->i_sb->s_maxbytes;
+
+	return i_size_read(inode);
+}
+
+static int ext4_mpage_readpages(struct inode *inode, struct fsverity_info *vi,
+		struct readahead_control *rac, struct folio *folio)
+{
+	struct bio *bio = NULL;
+	sector_t last_block_in_bio = 0;
+	const unsigned blkbits = inode->i_blkbits;
+	const unsigned blocksize = 1 << blkbits;
+	sector_t block_in_file;
+	sector_t last_block;
+	sector_t last_block_in_file;
+	sector_t first_block;
+	loff_t pos;
+	unsigned page_block;
+	struct block_device *bdev = inode->i_sb->s_bdev;
+	int length;
+	unsigned relative_block = 0;
+	struct ext4_map_blocks map;
+	unsigned int nr_pages, folio_pages;
+
+	map.m_pblk = 0;
+	map.m_lblk = 0;
+	map.m_len = 0;
+	map.m_flags = 0;
+
+	nr_pages = rac ? readahead_count(rac) : folio_nr_pages(folio);
+	for (; nr_pages; nr_pages -= folio_pages) {
+		int fully_mapped = 1;
+		unsigned int first_hole;
+		unsigned int blocks_per_folio;
+
+		if (rac)
+			folio = readahead_folio(rac);
+
+		folio_pages = folio_nr_pages(folio);
+		prefetchw(&folio->flags);
+
+		if (folio_buffers(folio))
+			goto confused;
+
+		blocks_per_folio = folio_size(folio) >> blkbits;
+		first_hole = blocks_per_folio;
+		pos = folio_pos(folio);
+		block_in_file = pos >> blkbits;
+		last_block = EXT4_PG_TO_LBLK(inode, folio->index + nr_pages);
+		last_block_in_file = (ext4_readpage_limit(inode) +
+				      blocksize - 1) >> blkbits;
+		if (last_block > last_block_in_file)
+			last_block = last_block_in_file;
+		page_block = 0;
+
+		/*
+		 * Map blocks using the previous result first.
+		 */
+		if ((map.m_flags & EXT4_MAP_MAPPED) &&
+		    block_in_file > map.m_lblk &&
+		    block_in_file < (map.m_lblk + map.m_len)) {
+			unsigned map_offset = block_in_file - map.m_lblk;
+			unsigned last = map.m_len - map_offset;
+
+			first_block = map.m_pblk + map_offset;
+			for (relative_block = 0; ; relative_block++) {
+				if (relative_block == last) {
+					/* needed? */
+					map.m_flags &= ~EXT4_MAP_MAPPED;
+					break;
+				}
+				if (page_block == blocks_per_folio)
+					break;
+				page_block++;
+				block_in_file++;
+			}
+		}
+
+		/*
+		 * Then do more ext4_map_blocks() calls until we are
+		 * done with this folio.
+		 */
+		while (page_block < blocks_per_folio) {
+			if (block_in_file < last_block) {
+				map.m_lblk = block_in_file;
+				map.m_len = last_block - block_in_file;
+
+				if (ext4_map_blocks(NULL, inode, &map, 0) < 0) {
+				set_error_page:
+					folio_zero_segment(folio, 0,
+							  folio_size(folio));
+					folio_unlock(folio);
+					goto next_page;
+				}
+			}
+			if ((map.m_flags & EXT4_MAP_MAPPED) == 0) {
+				fully_mapped = 0;
+				if (first_hole == blocks_per_folio)
+					first_hole = page_block;
+				page_block++;
+				block_in_file++;
+				continue;
+			}
+			if (first_hole != blocks_per_folio)
+				goto confused;		/* hole -> non-hole */
+
+			/* Contiguous blocks? */
+			if (!page_block)
+				first_block = map.m_pblk;
+			else if (first_block + page_block != map.m_pblk)
+				goto confused;
+			for (relative_block = 0; ; relative_block++) {
+				if (relative_block == map.m_len) {
+					/* needed? */
+					map.m_flags &= ~EXT4_MAP_MAPPED;
+					break;
+				} else if (page_block == blocks_per_folio)
+					break;
+				page_block++;
+				block_in_file++;
+			}
+		}
+		if (first_hole != blocks_per_folio) {
+			folio_zero_segment(folio, first_hole << blkbits,
+					  folio_size(folio));
+			if (first_hole == 0) {
+				if (vi && !fsverity_verify_folio(vi, folio))
+					goto set_error_page;
+				folio_end_read(folio, true);
+				continue;
+			}
+		} else if (fully_mapped) {
+			folio_set_mappedtodisk(folio);
+		}
+
+		/*
+		 * This folio will go to BIO.  Do we need to send this
+		 * BIO off first?
+		 */
+		if (bio && (last_block_in_bio != first_block - 1 ||
+			    !fscrypt_mergeable_bio(bio, inode, pos))) {
+		submit_and_realloc:
+			blk_crypto_submit_bio(bio);
+			bio = NULL;
+		}
+		if (bio == NULL) {
+			/*
+			 * bio_alloc will _always_ be able to allocate a bio if
+			 * __GFP_DIRECT_RECLAIM is set, see bio_alloc_bioset().
+			 */
+			bio = bio_alloc(bdev, bio_max_segs(nr_pages),
+					REQ_OP_READ, GFP_KERNEL);
+			fscrypt_set_bio_crypt_ctx(bio, inode, pos, GFP_KERNEL);
+			ext4_set_verity_work(bio, vi);
+			bio->bi_iter.bi_sector = first_block << (blkbits - 9);
+			bio->bi_end_io = mpage_end_io;
+			if (rac)
+				bio->bi_opf |= REQ_RAHEAD;
+		}
+
+		length = first_hole << blkbits;
+		if (!bio_add_folio(bio, folio, length, 0))
+			goto submit_and_realloc;
+
+		if (((map.m_flags & EXT4_MAP_BOUNDARY) &&
+		     (relative_block == map.m_len)) ||
+		    (first_hole != blocks_per_folio)) {
+			blk_crypto_submit_bio(bio);
+			bio = NULL;
+		} else
+			last_block_in_bio = first_block + blocks_per_folio - 1;
+		continue;
+	confused:
+		if (bio) {
+			blk_crypto_submit_bio(bio);
+			bio = NULL;
+		}
+		if (!folio_test_uptodate(folio))
+			block_read_full_folio(folio, ext4_get_block);
+		else
+			folio_unlock(folio);
+next_page:
+		; /* A label shall be followed by a statement until C23 */
+	}
+	if (bio)
+		blk_crypto_submit_bio(bio);
+	return 0;
+}
+
+int ext4_read_folio(struct file *file, struct folio *folio)
+{
+	struct inode *inode = folio->mapping->host;
+	struct fsverity_info *vi = NULL;
+	int ret;
+
+	trace_ext4_read_folio(inode, folio);
+
+	if (ext4_has_inline_data(inode)) {
+		ret = ext4_readpage_inline(inode, folio);
+		if (ret != -EAGAIN)
+			return ret;
+	}
+
+	if (folio->index < DIV_ROUND_UP(inode->i_size, PAGE_SIZE))
+		vi = fsverity_get_info(inode);
+	if (vi)
+		fsverity_readahead(vi, folio->index, folio_nr_pages(folio));
+	return ext4_mpage_readpages(inode, vi, NULL, folio);
+}
+
+void ext4_readahead(struct readahead_control *rac)
+{
+	struct inode *inode = rac->mapping->host;
+	struct fsverity_info *vi = NULL;
+
+	/* If the file has inline data, no need to do readahead. */
+	if (ext4_has_inline_data(inode))
+		return;
+
+	if (readahead_index(rac) < DIV_ROUND_UP(inode->i_size, PAGE_SIZE))
+		vi = fsverity_get_info(inode);
+	if (vi)
+		fsverity_readahead(vi, readahead_index(rac),
+				   readahead_count(rac));
+	ext4_mpage_readpages(inode, vi, rac, NULL);
+}
+
+int __init ext4_init_verity_caches(void)
+{
+	if (!IS_ENABLED(CONFIG_FS_VERITY))
+		return 0;
+	ext4_verity_work_cache =
+		KMEM_CACHE(ext4_verity_work, SLAB_RECLAIM_ACCOUNT);
+
+	if (!ext4_verity_work_cache)
+		goto fail;
+	ext4_verity_work_pool = mempool_create_slab_pool(
+		NUM_VERITY_WORKS, ext4_verity_work_cache);
+	if (!ext4_verity_work_pool)
+		goto fail_free_cache;
+	return 0;
+
+fail_free_cache:
+	kmem_cache_destroy(ext4_verity_work_cache);
+fail:
+	return -ENOMEM;
+}
+
+void ext4_exit_verity_caches(void)
+{
+	if (!IS_ENABLED(CONFIG_FS_VERITY))
+		return;
+	mempool_destroy(ext4_verity_work_pool);
+	kmem_cache_destroy(ext4_verity_work_cache);
+}