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, 29 insertions, 0 deletions

diff --git a/fs/btrfs/ioctl.h b/fs/btrfs/ioctl.h
new file mode 100644
index 000000000..ccf6bed9c
--- /dev/null
+++ b/fs/btrfs/ioctl.h
@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef BTRFS_IOCTL_H
+#define BTRFS_IOCTL_H
+
+#include <linux/types.h>
+
+struct file;
+struct dentry;
+struct mnt_idmap;
+struct file_kattr;
+struct io_uring_cmd;
+struct btrfs_inode;
+struct btrfs_fs_info;
+struct btrfs_ioctl_balance_args;
+
+long btrfs_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
+long btrfs_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
+int btrfs_fileattr_get(struct dentry *dentry, struct file_kattr *fa);
+int btrfs_fileattr_set(struct mnt_idmap *idmap,
+		       struct dentry *dentry, struct file_kattr *fa);
+int btrfs_ioctl_get_supported_features(void __user *arg);
+void btrfs_sync_inode_flags_to_i_flags(struct btrfs_inode *inode);
+void btrfs_update_ioctl_balance_args(struct btrfs_fs_info *fs_info,
+				     struct btrfs_ioctl_balance_args *bargs);
+int btrfs_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags);
+void btrfs_uring_read_extent_endio(void *ctx, int err);
+
+#endif