[PATCH 2/5] fuse: move the backing file idr and code into a new source file

Darrick J. Wong
2025-09-16T00:27:12Z

iomap support for fuse is also going to want the ability to attach
backing files to a fuse filesystem. Move the fuse_backing code into a
separate file so that both can use it.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
---
fs/fuse/fuse_i.h | 47 +++++++------
fs/fuse/Makefile | 2 -
fs/fuse/backing.c | 179 +++++++++++++++++++++++++++++++++++++++++++++++++
fs/fuse/passthrough.c | 163 ---------------------------------------------
4 files changed, 208 insertions(+), 183 deletions(-)
create mode 100644 fs/fuse/backing.c

diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 02f0138e2fe443..52db609e63eb54 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1570,29 +1570,11 @@ struct fuse_file *fuse_file_open(struct fuse_mount *fm, u64 nodeid,
void fuse_file_release(struct inode *inode, struct fuse_file *ff,
unsigned int open_flags, fl_owner_t id, bool isdir);

-/* passthrough.c */
-static inline struct fuse_backing *fuse_inode_backing(struct fuse_inode *fi)
-{
-#ifdef CONFIG_FUSE_PASSTHROUGH
- return READ_ONCE(fi->fb);
-#else
- return NULL;
-#endif
-}
-
-static inline struct fuse_backing *fuse_inode_backing_set(struct fuse_inode *fi,
- struct fuse_backing *fb)
-{
-#ifdef CONFIG_FUSE_PASSTHROUGH
- return xchg(&fi->fb, fb);
-#else
- return NULL;
-#endif
-}
-
+/* backing.c */
#ifdef CONFIG_FUSE_PASSTHROUGH
struct fuse_backing *fuse_backing_get(struct fuse_backing *fb);
void fuse_backing_put(struct fuse_backing *fb);
+struct fuse_backing *fuse_backing_lookup(struct fuse_conn *fc, int backing_id);
#else

static inline struct fuse_backing *fuse_backing_get(struct fuse_backing *fb)
@@ -1603,6 +1585,11 @@ static inline struct fuse_backing *fuse_backing_get(struct fuse_backing *fb)
static inline void fuse_backing_put(struct fuse_backing *fb)
{
}
+static inline struct fuse_backing *fuse_backing_lookup(struct fuse_conn *fc,
+ int backing_id)
+{
+ return NULL;
+}
#endif

void fuse_backing_files_init(struct fuse_conn *fc);
@@ -1610,6 +1597,26 @@ void fuse_backing_files_free(struct fuse_conn *fc);
int fuse_backing_open(struct fuse_conn *fc, struct fuse_backing_map *map);
int fuse_backing_close(struct fuse_conn *fc, int backing_id);

+/* passthrough.c */
+static inline struct fuse_backing *fuse_inode_backing(struct fuse_inode *fi)
+{
+#ifdef CONFIG_FUSE_PASSTHROUGH
+ return READ_ONCE(fi->fb);
+#else
+ return NULL;
+#endif
+}
+
+static inline struct fuse_backing *fuse_inode_backing_set(struct fuse_inode *fi,
+ struct fuse_backing *fb)
+{
+#ifdef CONFIG_FUSE_PASSTHROUGH
+ return xchg(&fi->fb, fb);
+#else
+ return NULL;
+#endif
+}
+
struct fuse_backing *fuse_passthrough_open(struct file *file,
struct inode *inode,
int backing_id);
diff --git a/fs/fuse/Makefile b/fs/fuse/Makefile
index 3f0f312a31c1cc..8ddd8f0b204ee5 100644
--- a/fs/fuse/Makefile
+++ b/fs/fuse/Makefile
@@ -13,7 +13,7 @@ obj-$(CONFIG_VIRTIO_FS) += virtiofs.o
fuse-y := dev.o dir.o file.o inode.o control.o xattr.o acl.o readdir.o ioctl.o
fuse-y += iomode.o
fuse-$(CONFIG_FUSE_DAX) += dax.o
-fuse-$(CONFIG_FUSE_PASSTHROUGH) += passthrough.o
+fuse-$(CONFIG_FUSE_PASSTHROUGH) += passthrough.o backing.o
fuse-$(CONFIG_SYSCTL) += sysctl.o
fuse-$(CONFIG_FUSE_IO_URING) += dev_uring.o

diff --git a/fs/fuse/backing.c b/fs/fuse/backing.c
new file mode 100644
index 00000000000000..4afda419dd1416
--- /dev/null
+++ b/fs/fuse/backing.c
@@ -0,0 +1,179 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * FUSE passthrough to backing file.
+ *
+ * Copyright (c) 2023 CTERA Networks.
+ */
+
+#include "fuse_i.h"
+
+#include <linux/file.h>
+
+struct fuse_backing *fuse_backing_get(struct fuse_backing *fb)
+{
+ if (fb && refcount_inc_not_zero(&fb->count))
+ return fb;
+ return NULL;
+}
+
+static void fuse_backing_free(struct fuse_backing *fb)
+{
+ pr_debug("%s: fb=0x%p\n", __func__, fb);
+
+ if (fb->file)
+ fput(fb->file);
+ put_cred(fb->cred);
+ kfree_rcu(fb, rcu);
+}
+
+void fuse_backing_put(struct fuse_backing *fb)
+{
+ if (fb && refcount_dec_and_test(&fb->count))
+ fuse_backing_free(fb);
+}
+
+void fuse_backing_files_init(struct fuse_conn *fc)
+{
+ idr_init(&fc->backing_files_map);
+}
+
+static int fuse_backing_id_alloc(struct fuse_conn *fc, struct fuse_backing *fb)
+{
+ int id;
+
+ idr_preload(GFP_KERNEL);
+ spin_lock(&fc->lock);
+ /* FIXME: xarray might be space inefficient */
+ id = idr_alloc_cyclic(&fc->backing_files_map, fb, 1, 0, GFP_ATOMIC);
+ spin_unlock(&fc->lock);
+ idr_preload_end();
+
+ WARN_ON_ONCE(id == 0);
+ return id;
+}
+
+static struct fuse_backing *fuse_backing_id_remove(struct fuse_conn *fc,
+ int id)
+{
+ struct fuse_backing *fb;
+
+ spin_lock(&fc->lock);
+ fb = idr_remove(&fc->backing_files_map, id);
+ spin_unlock(&fc->lock);
+
+ return fb;
+}
+
+static int fuse_backing_id_free(int id, void *p, void *data)
+{
+ struct fuse_backing *fb = p;
+
+ WARN_ON_ONCE(refcount_read(&fb->count) != 1);
+ fuse_backing_free(fb);
+ return 0;
+}
+
+void fuse_backing_files_free(struct fuse_conn *fc)
+{
+ idr_for_each(&fc->backing_files_map, fuse_backing_id_free, NULL);
+ idr_destroy(&fc->backing_files_map);
+}
+
+int fuse_backing_open(struct fuse_conn *fc, struct fuse_backing_map *map)
+{
+ struct file *file;
+ struct super_block *backing_sb;
+ struct fuse_backing *fb = NULL;
+ int res;
+
+ pr_debug("%s: fd=%d flags=0x%x\n", __func__, map->fd, map->flags);
+
+ /* TODO: relax CAP_SYS_ADMIN once backing files are visible to lsof */
+ res = -EPERM;
+ if (!fc->passthrough || !capable(CAP_SYS_ADMIN))
+ goto out;
+
+ res = -EINVAL;
+ if (map->flags || map->padding)
+ goto out;
+
+ file = fget_raw(map->fd);
+ res = -EBADF;
+ if (!file)
+ goto out;
+
+ /* read/write/splice/mmap passthrough only relevant for regular files */
+ res = d_is_dir(file->f_path.dentry) ? -EISDIR : -EINVAL;
+ if (!d_is_reg(file->f_path.dentry))
+ goto out_fput;
+
+ backing_sb = file_inode(file)->i_sb;
+ res = -ELOOP;
+ if (backing_sb->s_stack_depth >= fc->max_stack_depth)
+ goto out_fput;
+
+ fb = kmalloc(sizeof(struct fuse_backing), GFP_KERNEL);
+ res = -ENOMEM;
+ if (!fb)
+ goto out_fput;
+
+ fb->file = file;
+ fb->cred = prepare_creds();
+ refcount_set(&fb->count, 1);
+
+ res = fuse_backing_id_alloc(fc, fb);
+ if (res < 0) {
+ fuse_backing_free(fb);
+ fb = NULL;
+ }
+
+out:
+ pr_debug("%s: fb=0x%p, ret=%i\n", __func__, fb, res);
+
+ return res;
+
+out_fput:
+ fput(file);
+ goto out;
+}
+
+int fuse_backing_close(struct fuse_conn *fc, int backing_id)
+{
+ struct fuse_backing *fb = NULL;
+ int err;
+
+ pr_debug("%s: backing_id=%d\n", __func__, backing_id);
+
+ /* TODO: relax CAP_SYS_ADMIN once backing files are visible to lsof */
+ err = -EPERM;
+ if (!fc->passthrough || !capable(CAP_SYS_ADMIN))
+ goto out;
+
+ err = -EINVAL;
+ if (backing_id <= 0)
+ goto out;
+
+ err = -ENOENT;
+ fb = fuse_backing_id_remove(fc, backing_id);
+ if (!fb)
+ goto out;
+
+ fuse_backing_put(fb);
+ err = 0;
+out:
+ pr_debug("%s: fb=0x%p, err=%i\n", __func__, fb, err);
+
+ return err;
+}
+
+struct fuse_backing *fuse_backing_lookup(struct fuse_conn *fc, int backing_id)
+{
+ struct fuse_backing *fb;
+
+ rcu_read_lock();
+ fb = idr_find(&fc->backing_files_map, backing_id);
+ fb = fuse_backing_get(fb);
+ rcu_read_unlock();
+
+ return fb;
+}
diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
index eb97ac009e75d9..e0b8d885bc81f3 100644
--- a/fs/fuse/passthrough.c
+++ b/fs/fuse/passthrough.c
@@ -144,163 +144,6 @@ ssize_t fuse_passthrough_mmap(struct file *file, struct vm_area_struct *vma)
return backing_file_mmap(backing_file, vma, &ctx);
}

-struct fuse_backing *fuse_backing_get(struct fuse_backing *fb)
-{
- if (fb && refcount_inc_not_zero(&fb->count))
- return fb;
- return NULL;
-}
-
-static void fuse_backing_free(struct fuse_backing *fb)
-{
- pr_debug("%s: fb=0x%p\n", __func__, fb);
-
- if (fb->file)
- fput(fb->file);
- put_cred(fb->cred);
- kfree_rcu(fb, rcu);
-}
-
-void fuse_backing_put(struct fuse_backing *fb)
-{
- if (fb && refcount_dec_and_test(&fb->count))
- fuse_backing_free(fb);
-}
-
-void fuse_backing_files_init(struct fuse_conn *fc)
-{
- idr_init(&fc->backing_files_map);
-}
-
-static int fuse_backing_id_alloc(struct fuse_conn *fc, struct fuse_backing *fb)
-{
- int id;
-
- idr_preload(GFP_KERNEL);
- spin_lock(&fc->lock);
- /* FIXME: xarray might be space inefficient */
- id = idr_alloc_cyclic(&fc->backing_files_map, fb, 1, 0, GFP_ATOMIC);
- spin_unlock(&fc->lock);
- idr_preload_end();
-
- WARN_ON_ONCE(id == 0);
- return id;
-}
-
-static struct fuse_backing *fuse_backing_id_remove(struct fuse_conn *fc,
- int id)
-{
- struct fuse_backing *fb;
-
- spin_lock(&fc->lock);
- fb = idr_remove(&fc->backing_files_map, id);
- spin_unlock(&fc->lock);
-
- return fb;
-}
-
-static int fuse_backing_id_free(int id, void *p, void *data)
-{
- struct fuse_backing *fb = p;
-
- WARN_ON_ONCE(refcount_read(&fb->count) != 1);
- fuse_backing_free(fb);
- return 0;
-}
-
-void fuse_backing_files_free(struct fuse_conn *fc)
-{
- idr_for_each(&fc->backing_files_map, fuse_backing_id_free, NULL);
- idr_destroy(&fc->backing_files_map);
-}
-
-int fuse_backing_open(struct fuse_conn *fc, struct fuse_backing_map *map)
-{
- struct file *file;
- struct super_block *backing_sb;
- struct fuse_backing *fb = NULL;
- int res;
-
- pr_debug("%s: fd=%d flags=0x%x\n", __func__, map->fd, map->flags);
-
- /* TODO: relax CAP_SYS_ADMIN once backing files are visible to lsof */
- res = -EPERM;
- if (!fc->passthrough || !capable(CAP_SYS_ADMIN))
- goto out;
-
- res = -EINVAL;
- if (map->flags || map->padding)
- goto out;
-
- file = fget_raw(map->fd);
- res = -EBADF;
- if (!file)
- goto out;
-
- /* read/write/splice/mmap passthrough only relevant for regular files */
- res = d_is_dir(file->f_path.dentry) ? -EISDIR : -EINVAL;
- if (!d_is_reg(file->f_path.dentry))
- goto out_fput;
-
- backing_sb = file_inode(file)->i_sb;
- res = -ELOOP;
- if (backing_sb->s_stack_depth >= fc->max_stack_depth)
- goto out_fput;
-
- fb = kmalloc(sizeof(struct fuse_backing), GFP_KERNEL);
- res = -ENOMEM;
- if (!fb)
- goto out_fput;
-
- fb->file = file;
- fb->cred = prepare_creds();
- refcount_set(&fb->count, 1);
-
- res = fuse_backing_id_alloc(fc, fb);
- if (res < 0) {
- fuse_backing_free(fb);
- fb = NULL;
- }
-
-out:
- pr_debug("%s: fb=0x%p, ret=%i\n", __func__, fb, res);
-
- return res;
-
-out_fput:
- fput(file);
- goto out;
-}
-
-int fuse_backing_close(struct fuse_conn *fc, int backing_id)
-{
- struct fuse_backing *fb = NULL;
- int err;
-
- pr_debug("%s: backing_id=%d\n", __func__, backing_id);
-
- /* TODO: relax CAP_SYS_ADMIN once backing files are visible to lsof */
- err = -EPERM;
- if (!fc->passthrough || !capable(CAP_SYS_ADMIN))
- goto out;
-
- err = -EINVAL;
- if (backing_id <= 0)
- goto out;
-
- err = -ENOENT;
- fb = fuse_backing_id_remove(fc, backing_id);
- if (!fb)
- goto out;
-
- fuse_backing_put(fb);
- err = 0;
-out:
- pr_debug("%s: fb=0x%p, err=%i\n", __func__, fb, err);
-
- return err;
-}
-
/*
* Setup passthrough to a backing file.
*
@@ -320,12 +163,8 @@ struct fuse_backing *fuse_passthrough_open(struct file *file,
if (backing_id <= 0)
goto out;

- rcu_read_lock();
- fb = idr_find(&fc->backing_files_map, backing_id);
- fb = fuse_backing_get(fb);
- rcu_read_unlock();
-
err = -ENOENT;
+ fb = fuse_backing_lookup(fc, backing_id);
if (!fb)
goto out;