On Thu, Aug 21, 2025 at 09:42:07AM +0200, Amir Goldstein wrote:
> On Thu, Aug 21, 2025 at 9:21 AM Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> >
> > On Thu, Aug 21, 2025 at 2:53 AM Darrick J. Wong <djwong@xxxxxxxxxx> wrote:
> > >
> > > From: Darrick J. Wong <djwong@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
> >
> > Are you going to make FUSE_IOMAP depend on FUSE_PASSTHROUGH later on?
> > I can't think of a reason why not.
>
> Ah I see. They will both depend on FUSE_BACKING
> cool
Yep. Thanks for your feedback! :)
--D
> >
> > Thanks,
> > Amir.
> >
> > > ---
> > > fs/fuse/fuse_i.h | 47 ++++++++-----
> > > fs/fuse/Makefile | 2 -
> > > fs/fuse/backing.c | 174 +++++++++++++++++++++++++++++++++++++++++++++++++
> > > fs/fuse/passthrough.c | 158 --------------------------------------------
> > > 4 files changed, 203 insertions(+), 178 deletions(-)
> > > create mode 100644 fs/fuse/backing.c
> > >
> > >
> > > diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> > > index 2cd9f4cdc6a7ef..2be2cbdf060536 100644
> > > --- a/fs/fuse/fuse_i.h
> > > +++ b/fs/fuse/fuse_i.h
> > > @@ -1535,29 +1535,11 @@ 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)
> > > @@ -1568,6 +1550,11 @@ 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);
> > > @@ -1575,6 +1562,26 @@ 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 70709a7a3f9523..c79f786d0c90c3 100644
> > > --- a/fs/fuse/Makefile
> > > +++ b/fs/fuse/Makefile
> > > @@ -14,7 +14,7 @@ fuse-y := trace.o # put trace.o first so we see ftrace errors sooner
> > > 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-$(CONFIG_FUSE_IO_URING) += dev_uring.o
> > > fuse-$(CONFIG_FUSE_IOMAP) += file_iomap.o
> > > diff --git a/fs/fuse/backing.c b/fs/fuse/backing.c
> > > new file mode 100644
> > > index 00000000000000..ddb23b7400fc72
> > > --- /dev/null
> > > +++ b/fs/fuse/backing.c
> > > @@ -0,0 +1,174 @@
> > > +// 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;
> > > +
> > > + 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 607ef735ad4ab3..e0b8d885bc81f3 100644
> > > --- a/fs/fuse/passthrough.c
> > > +++ b/fs/fuse/passthrough.c
> > > @@ -144,158 +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;
> > > -
> > > - 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.
> > > *
> > > @@ -315,12 +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;
> > >
> > >
> > >
>
