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, 80 insertions, 0 deletions

diff --git a/fs/verity/pagecache.c b/fs/verity/pagecache.c
new file mode 100644
index 000000000..99f5f53ee
--- /dev/null
+++ b/fs/verity/pagecache.c
@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright 2019 Google LLC
+ */
+#include "fsverity_private.h"
+
+#include <linux/export.h>
+#include <linux/fsverity.h>
+#include <linux/pagemap.h>
+
+/**
+ * generic_read_merkle_tree_page - generic ->read_merkle_tree_page helper
+ * @inode:	inode containing the Merkle tree
+ * @index:	0-based index of the Merkle tree page in the inode
+ *
+ * The caller needs to adjust @index from the Merkle-tree relative index passed
+ * to ->read_merkle_tree_page to the actual index where the Merkle tree is
+ * stored in the page cache for @inode.
+ */
+struct page *generic_read_merkle_tree_page(struct inode *inode, pgoff_t index)
+{
+	struct folio *folio;
+
+	folio = read_mapping_folio(inode->i_mapping, index, NULL);
+	if (IS_ERR(folio))
+		return ERR_CAST(folio);
+	return folio_file_page(folio, index);
+}
+EXPORT_SYMBOL_GPL(generic_read_merkle_tree_page);
+
+/**
+ * generic_readahead_merkle_tree() - generic ->readahead_merkle_tree helper
+ * @inode:	inode containing the Merkle tree
+ * @index:	0-based index of the first Merkle tree page to read ahead in the
+ *		inode
+ * @nr_pages:	the number of Merkle tree pages that should be read ahead
+ *
+ * The caller needs to adjust @index from the Merkle-tree relative index passed
+ * to ->read_merkle_tree_page to the actual index where the Merkle tree is
+ * stored in the page cache for @inode.
+ */
+void generic_readahead_merkle_tree(struct inode *inode, pgoff_t index,
+				   unsigned long nr_pages)
+{
+	struct folio *folio;
+
+	lockdep_assert_held(&inode->i_mapping->invalidate_lock);
+
+	folio = __filemap_get_folio(inode->i_mapping, index, FGP_ACCESSED, 0);
+	if (folio == ERR_PTR(-ENOENT) ||
+	    (!IS_ERR(folio) && !folio_test_uptodate(folio))) {
+		DEFINE_READAHEAD(ractl, NULL, NULL, inode->i_mapping, index);
+
+		page_cache_ra_unbounded(&ractl, nr_pages, 0);
+	}
+	if (!IS_ERR(folio))
+		folio_put(folio);
+}
+EXPORT_SYMBOL_GPL(generic_readahead_merkle_tree);
+
+/**
+ * fsverity_fill_zerohash() - fill folio with hashes of zero data block
+ * @folio:	folio to fill
+ * @offset:	offset in the folio to start
+ * @len:	length of the range to fill with hashes
+ * @vi:		fsverity info
+ */
+void fsverity_fill_zerohash(struct folio *folio, size_t offset, size_t len,
+			      struct fsverity_info *vi)
+{
+	size_t off = offset;
+
+	WARN_ON_ONCE(!IS_ALIGNED(offset, vi->tree_params.digest_size));
+	WARN_ON_ONCE(!IS_ALIGNED(len, vi->tree_params.digest_size));
+
+	for (; off < (offset + len); off += vi->tree_params.digest_size)
+		memcpy_to_folio(folio, off, vi->tree_params.zero_digest,
+				vi->tree_params.digest_size);
+}
+EXPORT_SYMBOL_GPL(fsverity_fill_zerohash);