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, 55 insertions, 0 deletions

diff --git a/fs/befs/inode.c b/fs/befs/inode.c
new file mode 100644
index 000000000..791b46a6f
--- /dev/null
+++ b/fs/befs/inode.c
@@ -0,0 +1,55 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * inode.c
+ *
+ * Copyright (C) 2001 Will Dyson <will_dyson@pobox.com>
+ */
+
+#include <linux/fs.h>
+
+#include "befs.h"
+#include "inode.h"
+
+/*
+ * Validates the correctness of the befs inode
+ * Returns BEFS_OK if the inode should be used, otherwise
+ * returns BEFS_BAD_INODE
+ */
+int
+befs_check_inode(struct super_block *sb, befs_inode *raw_inode,
+		 befs_blocknr_t inode)
+{
+	u32 magic1 = fs32_to_cpu(sb, raw_inode->magic1);
+	befs_inode_addr ino_num = fsrun_to_cpu(sb, raw_inode->inode_num);
+	u32 flags = fs32_to_cpu(sb, raw_inode->flags);
+
+	/* check magic header. */
+	if (magic1 != BEFS_INODE_MAGIC1) {
+		befs_error(sb,
+			   "Inode has a bad magic header - inode = %lu",
+			   (unsigned long)inode);
+		return BEFS_BAD_INODE;
+	}
+
+	/*
+	 * Sanity check2: inodes store their own block address. Check it.
+	 */
+	if (inode != iaddr2blockno(sb, &ino_num)) {
+		befs_error(sb, "inode blocknr field disagrees with vfs "
+			   "VFS: %lu, Inode %lu", (unsigned long)
+			   inode, (unsigned long)iaddr2blockno(sb, &ino_num));
+		return BEFS_BAD_INODE;
+	}
+
+	/*
+	 * check flag
+	 */
+
+	if (!(flags & BEFS_INODE_IN_USE)) {
+		befs_error(sb, "inode is not used - inode = %lu",
+			   (unsigned long)inode);
+		return BEFS_BAD_INODE;
+	}
+
+	return BEFS_OK;
+}