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, 366 insertions, 0 deletions

diff --git a/fs/backing-file.c b/fs/backing-file.c
new file mode 100644
index 000000000..cc101143f
--- /dev/null
+++ b/fs/backing-file.c
@@ -0,0 +1,366 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Common helpers for stackable filesystems and backing files.
+ *
+ * Forked from fs/overlayfs/file.c.
+ *
+ * Copyright (C) 2017 Red Hat, Inc.
+ * Copyright (C) 2023 CTERA Networks.
+ */
+
+#include <linux/fs.h>
+#include <linux/backing-file.h>
+#include <linux/splice.h>
+#include <linux/mm.h>
+#include <linux/security.h>
+
+#include "internal.h"
+
+/**
+ * backing_file_open - open a backing file for kernel internal use
+ * @user_file:  file the user requested to open
+ * @flags:	open flags
+ * @real_path:	path of the backing file
+ * @cred:	credentials for open
+ *
+ * Open a backing file for a stackable filesystem (e.g., overlayfs).
+ * @user_file->f_path may be on the stackable filesystem and @real_path
+ * on the underlying filesystem. In this case, we want to be able to
+ * return the path of the stackable filesystem. This is done by
+ * embedding the returned file into a container structure that also
+ * stores the stacked file's path, which can be retrieved using
+ * backing_file_user_path().
+ */
+struct file *backing_file_open(const struct file *user_file, int flags,
+			       const struct path *real_path,
+			       const struct cred *cred)
+{
+	const struct path *user_path = file_user_path(user_file);
+	struct file *f;
+	int error;
+
+	f = alloc_empty_backing_file(flags, cred, user_file);
+	if (IS_ERR(f))
+		return f;
+
+	path_get(user_path);
+	backing_file_set_user_path(f, user_path);
+	error = vfs_open(real_path, f);
+	if (error) {
+		fput(f);
+		f = ERR_PTR(error);
+	}
+
+	return f;
+}
+EXPORT_SYMBOL_GPL(backing_file_open);
+
+struct file *backing_tmpfile_open(const struct file *user_file, int flags,
+				  const struct path *real_parentpath,
+				  umode_t mode, const struct cred *cred)
+{
+	struct mnt_idmap *real_idmap = mnt_idmap(real_parentpath->mnt);
+	const struct path *user_path = &user_file->f_path;
+	struct file *f;
+	int error;
+
+	f = alloc_empty_backing_file(flags, cred, user_file);
+	if (IS_ERR(f))
+		return f;
+
+	path_get(user_path);
+	backing_file_set_user_path(f, user_path);
+	error = vfs_tmpfile(real_idmap, real_parentpath, f, mode);
+	if (error) {
+		fput(f);
+		f = ERR_PTR(error);
+	}
+	return f;
+}
+EXPORT_SYMBOL(backing_tmpfile_open);
+
+struct backing_aio {
+	struct kiocb iocb;
+	refcount_t ref;
+	struct kiocb *orig_iocb;
+	/* used for aio completion */
+	void (*end_write)(struct kiocb *iocb, ssize_t);
+	struct work_struct work;
+	long res;
+};
+
+static struct kmem_cache *backing_aio_cachep;
+
+#define BACKING_IOCB_MASK \
+	(IOCB_NOWAIT | IOCB_HIPRI | IOCB_DSYNC | IOCB_SYNC | IOCB_APPEND)
+
+static rwf_t iocb_to_rw_flags(int flags)
+{
+	return (__force rwf_t)(flags & BACKING_IOCB_MASK);
+}
+
+static void backing_aio_put(struct backing_aio *aio)
+{
+	if (refcount_dec_and_test(&aio->ref)) {
+		fput(aio->iocb.ki_filp);
+		kmem_cache_free(backing_aio_cachep, aio);
+	}
+}
+
+static void backing_aio_cleanup(struct backing_aio *aio, long res)
+{
+	struct kiocb *iocb = &aio->iocb;
+	struct kiocb *orig_iocb = aio->orig_iocb;
+
+	orig_iocb->ki_pos = iocb->ki_pos;
+	if (aio->end_write)
+		aio->end_write(orig_iocb, res);
+
+	backing_aio_put(aio);
+}
+
+static void backing_aio_rw_complete(struct kiocb *iocb, long res)
+{
+	struct backing_aio *aio = container_of(iocb, struct backing_aio, iocb);
+	struct kiocb *orig_iocb = aio->orig_iocb;
+
+	if (iocb->ki_flags & IOCB_WRITE)
+		kiocb_end_write(iocb);
+
+	backing_aio_cleanup(aio, res);
+	orig_iocb->ki_complete(orig_iocb, res);
+}
+
+static void backing_aio_complete_work(struct work_struct *work)
+{
+	struct backing_aio *aio = container_of(work, struct backing_aio, work);
+
+	backing_aio_rw_complete(&aio->iocb, aio->res);
+}
+
+static void backing_aio_queue_completion(struct kiocb *iocb, long res)
+{
+	struct backing_aio *aio = container_of(iocb, struct backing_aio, iocb);
+
+	/*
+	 * Punt to a work queue to serialize updates of mtime/size.
+	 */
+	aio->res = res;
+	INIT_WORK(&aio->work, backing_aio_complete_work);
+	queue_work(file_inode(aio->orig_iocb->ki_filp)->i_sb->s_dio_done_wq,
+		   &aio->work);
+}
+
+static int backing_aio_init_wq(struct kiocb *iocb)
+{
+	struct super_block *sb = file_inode(iocb->ki_filp)->i_sb;
+
+	if (sb->s_dio_done_wq)
+		return 0;
+
+	return sb_init_dio_done_wq(sb);
+}
+
+static int do_backing_file_read_iter(struct file *file, struct iov_iter *iter,
+				     struct kiocb *iocb, int flags)
+{
+	struct backing_aio *aio = NULL;
+	int ret;
+
+	if (is_sync_kiocb(iocb)) {
+		rwf_t rwf = iocb_to_rw_flags(flags);
+
+		return vfs_iter_read(file, iter, &iocb->ki_pos, rwf);
+	}
+
+	aio = kmem_cache_zalloc(backing_aio_cachep, GFP_KERNEL);
+	if (!aio)
+		return -ENOMEM;
+
+	aio->orig_iocb = iocb;
+	kiocb_clone(&aio->iocb, iocb, get_file(file));
+	aio->iocb.ki_complete = backing_aio_rw_complete;
+	refcount_set(&aio->ref, 2);
+	ret = vfs_iocb_iter_read(file, &aio->iocb, iter);
+	backing_aio_put(aio);
+	if (ret != -EIOCBQUEUED)
+		backing_aio_cleanup(aio, ret);
+	return ret;
+}
+
+ssize_t backing_file_read_iter(struct file *file, struct iov_iter *iter,
+			       struct kiocb *iocb, int flags,
+			       struct backing_file_ctx *ctx)
+{
+	ssize_t ret;
+
+	if (WARN_ON_ONCE(!(file->f_mode & FMODE_BACKING)))
+		return -EIO;
+
+	if (!iov_iter_count(iter))
+		return 0;
+
+	if (iocb->ki_flags & IOCB_DIRECT &&
+	    !(file->f_mode & FMODE_CAN_ODIRECT))
+		return -EINVAL;
+
+	scoped_with_creds(ctx->cred)
+		ret = do_backing_file_read_iter(file, iter, iocb, flags);
+
+	if (ctx->accessed)
+		ctx->accessed(iocb->ki_filp);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(backing_file_read_iter);
+
+static int do_backing_file_write_iter(struct file *file, struct iov_iter *iter,
+				      struct kiocb *iocb, int flags,
+				      void (*end_write)(struct kiocb *, ssize_t))
+{
+	struct backing_aio *aio;
+	int ret;
+
+	if (is_sync_kiocb(iocb)) {
+		rwf_t rwf = iocb_to_rw_flags(flags);
+
+		ret = vfs_iter_write(file, iter, &iocb->ki_pos, rwf);
+		if (end_write)
+			end_write(iocb, ret);
+		return ret;
+	}
+
+	ret = backing_aio_init_wq(iocb);
+	if (ret)
+		return ret;
+
+	aio = kmem_cache_zalloc(backing_aio_cachep, GFP_KERNEL);
+	if (!aio)
+		return -ENOMEM;
+
+	aio->orig_iocb = iocb;
+	aio->end_write = end_write;
+	kiocb_clone(&aio->iocb, iocb, get_file(file));
+	aio->iocb.ki_flags = flags;
+	aio->iocb.ki_complete = backing_aio_queue_completion;
+	refcount_set(&aio->ref, 2);
+	ret = vfs_iocb_iter_write(file, &aio->iocb, iter);
+	backing_aio_put(aio);
+	if (ret != -EIOCBQUEUED)
+		backing_aio_cleanup(aio, ret);
+	return ret;
+}
+
+ssize_t backing_file_write_iter(struct file *file, struct iov_iter *iter,
+				struct kiocb *iocb, int flags,
+				struct backing_file_ctx *ctx)
+{
+	ssize_t ret;
+
+	if (WARN_ON_ONCE(!(file->f_mode & FMODE_BACKING)))
+		return -EIO;
+
+	if (!iov_iter_count(iter))
+		return 0;
+
+	ret = file_remove_privs(iocb->ki_filp);
+	if (ret)
+		return ret;
+
+	if (iocb->ki_flags & IOCB_DIRECT &&
+	    !(file->f_mode & FMODE_CAN_ODIRECT))
+		return -EINVAL;
+
+	scoped_with_creds(ctx->cred)
+		return do_backing_file_write_iter(file, iter, iocb, flags, ctx->end_write);
+}
+EXPORT_SYMBOL_GPL(backing_file_write_iter);
+
+ssize_t backing_file_splice_read(struct file *in, struct kiocb *iocb,
+				 struct pipe_inode_info *pipe, size_t len,
+				 unsigned int flags,
+				 struct backing_file_ctx *ctx)
+{
+	ssize_t ret;
+
+	if (WARN_ON_ONCE(!(in->f_mode & FMODE_BACKING)))
+		return -EIO;
+
+	scoped_with_creds(ctx->cred)
+		ret = vfs_splice_read(in, &iocb->ki_pos, pipe, len, flags);
+
+	if (ctx->accessed)
+		ctx->accessed(iocb->ki_filp);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(backing_file_splice_read);
+
+ssize_t backing_file_splice_write(struct pipe_inode_info *pipe,
+				  struct file *out, struct kiocb *iocb,
+				  size_t len, unsigned int flags,
+				  struct backing_file_ctx *ctx)
+{
+	ssize_t ret;
+
+	if (WARN_ON_ONCE(!(out->f_mode & FMODE_BACKING)))
+		return -EIO;
+
+	if (!out->f_op->splice_write)
+		return -EINVAL;
+
+	ret = file_remove_privs(iocb->ki_filp);
+	if (ret)
+		return ret;
+
+	scoped_with_creds(ctx->cred) {
+		file_start_write(out);
+		ret = out->f_op->splice_write(pipe, out, &iocb->ki_pos, len, flags);
+		file_end_write(out);
+	}
+
+	if (ctx->end_write)
+		ctx->end_write(iocb, ret);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(backing_file_splice_write);
+
+int backing_file_mmap(struct file *file, struct vm_area_struct *vma,
+		      struct backing_file_ctx *ctx)
+{
+	struct file *user_file = vma->vm_file;
+	int ret;
+
+	if (WARN_ON_ONCE(!(file->f_mode & FMODE_BACKING)))
+		return -EIO;
+
+	if (!can_mmap_file(file))
+		return -ENODEV;
+
+	vma_set_file(vma, file);
+
+	scoped_with_creds(ctx->cred) {
+		ret = security_mmap_backing_file(vma, file, user_file);
+		if (ret)
+			return ret;
+
+		ret = vfs_mmap(vma->vm_file, vma);
+	}
+
+	if (ctx->accessed)
+		ctx->accessed(user_file);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(backing_file_mmap);
+
+static int __init backing_aio_init(void)
+{
+	backing_aio_cachep = KMEM_CACHE(backing_aio, SLAB_HWCACHE_ALIGN);
+	if (!backing_aio_cachep)
+		return -ENOMEM;
+
+	return 0;
+}
+fs_initcall(backing_aio_init);