[PATCH 3/4] fuse: Call vfs_get_tree() for submounts

Greg Kurz
2021-05-25T15:03:12Z

We recently fixed an infinite loop by setting the SB_BORN flag on
submounts along with the write barrier needed by super_cache_count().
This is the job of vfs_get_tree() and FUSE shouldn't have to care
about the barrier at all.

Split out some code from fuse_dentry_automount() to a new dedicated
fuse_get_tree_submount() handler for submounts and call vfs_get_tree().

The fs_private field of the filesystem context isn't used with
submounts : hijack it to pass the FUSE inode of the mount point
down to fuse_get_tree_submount().

Finally, adapt virtiofs to use this.

Signed-off-by: Greg Kurz <groug@kaod.org>
---
fs/fuse/dir.c | 58 +++++++--------------------------------------
fs/fuse/fuse_i.h | 6 +++++
fs/fuse/inode.c | 44 ++++++++++++++++++++++++++++++++++
fs/fuse/virtio_fs.c | 3 +++
4 files changed, 62 insertions(+), 49 deletions(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 3b0482738741..97649dcfeccd 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -309,12 +309,8 @@ static int fuse_dentry_delete(const struct dentry *dentry)
static struct vfsmount *fuse_dentry_automount(struct path *path)
{
struct fs_context *fsc;
- struct fuse_mount *parent_fm = get_fuse_mount_super(path->mnt->mnt_sb);
- struct fuse_conn *fc = parent_fm->fc;
- struct fuse_mount *fm;
struct vfsmount *mnt;
struct fuse_inode *mp_fi = get_fuse_inode(d_inode(path->dentry));
- struct super_block *sb;
int err;

fsc = fs_context_for_submount(path->mnt->mnt_sb->s_type, path->dentry);
@@ -323,47 +319,17 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
goto out;
}

- err = -ENOMEM;
- fm = kzalloc(sizeof(struct fuse_mount), GFP_KERNEL);
- if (!fm)
- goto out_put_fsc;
-
- fsc->s_fs_info = fm;
- sb = sget_fc(fsc, NULL, set_anon_super_fc);
- if (IS_ERR(sb)) {
- err = PTR_ERR(sb);
- kfree(fm);
- goto out_put_fsc;
- }
- fm->fc = fuse_conn_get(fc);
-
- /* Initialize superblock, making @mp_fi its root */
- err = fuse_fill_super_submount(sb, mp_fi);
- if (err) {
- fuse_conn_put(fc);
- kfree(fm);
- sb->s_fs_info = NULL;
- goto out_put_sb;
- }
-
/*
- * FIXME: setting SB_BORN requires a write barrier for
- * super_cache_count(). We should actually come
- * up with a proper ->get_tree() implementation
- * for submounts and call vfs_get_tree() to take
- * care of the write barrier.
+ * Hijack fsc->fs_private to pass the mount point inode to
+ * fuse_get_tree_submount(). It *must* be NULLified afterwards
+ * to avoid the inode pointer to be passed to kfree() when
+ * the context gets freed.
*/
- smp_wmb();
- sb->s_flags |= SB_BORN;
-
- sb->s_flags |= SB_ACTIVE;
- fsc->root = dget(sb->s_root);
- /* We are done configuring the superblock, so unlock it */
- up_write(&sb->s_umount);
-
- down_write(&fc->killsb);
- list_add_tail(&fm->fc_entry, &fc->mounts);
- up_write(&fc->killsb);
+ fsc->fs_private = mp_fi;
+ err = vfs_get_tree(fsc);
+ fsc->fs_private = NULL;
+ if (err)
+ goto out_put_fsc;

/* Create the submount */
mnt = vfs_create_mount(fsc);
@@ -375,12 +341,6 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
put_fs_context(fsc);
return mnt;

-out_put_sb:
- /*
- * Only jump here when fsc->root is NULL and sb is still locked
- * (otherwise put_fs_context() will put the superblock)
- */
- deactivate_locked_super(sb);
out_put_fsc:
put_fs_context(fsc);
out:
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 7e463e220053..d7fcf59a6a0e 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1090,6 +1090,12 @@ int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx);
int fuse_fill_super_submount(struct super_block *sb,
struct fuse_inode *parent_fi);

+/*
+ * Get the mountable root for the submount
+ * @fsc: superblock configuration context
+ */
+int fuse_get_tree_submount(struct fs_context *fsc);
+
/*
* Remove the mount from the connection
*
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 393e36b74dc4..433ca2b13046 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1313,6 +1313,50 @@ int fuse_fill_super_submount(struct super_block *sb,
return 0;
}

+/* Filesystem context private data holds the FUSE inode of the mount point */
+int fuse_get_tree_submount(struct fs_context *fsc)
+{
+ struct fuse_mount *fm;
+ struct fuse_inode *mp_fi = fsc->fs_private;
+ struct fuse_conn *fc = get_fuse_conn(&mp_fi->inode);
+ struct super_block *sb;
+ int err;
+
+ fm = kzalloc(sizeof(struct fuse_mount), GFP_KERNEL);
+ if (!fm)
+ return -ENOMEM;
+
+ fsc->s_fs_info = fm;
+ sb = sget_fc(fsc, NULL, set_anon_super_fc);
+ if (IS_ERR(sb)) {
+ kfree(fm);
+ return PTR_ERR(sb);
+ }
+ fm->fc = fuse_conn_get(fc);
+
+ /* Initialize superblock, making @mp_fi its root */
+ err = fuse_fill_super_submount(sb, mp_fi);
+ if (err) {
+ fuse_conn_put(fc);
+ kfree(fm);
+ sb->s_fs_info = NULL;
+ deactivate_locked_super(sb);
+ return err;
+ }
+
+ sb->s_flags |= SB_ACTIVE;
+ fsc->root = dget(sb->s_root);
+ /* We are done configuring the superblock, so unlock it */
+ up_write(&sb->s_umount);
+
+ down_write(&fc->killsb);
+ list_add_tail(&fm->fc_entry, &fc->mounts);
+ up_write(&fc->killsb);
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(fuse_get_tree_submount);
+
int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx)
{
struct fuse_dev *fud = NULL;
diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index bcb8a02e2d8b..e12e5190352c 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -1420,6 +1420,9 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
unsigned int virtqueue_size;
int err = -EIO;

+ if (fsc->purpose == FS_CONTEXT_FOR_SUBMOUNT)
+ return fuse_get_tree_submount(fsc);
+
/* This gets a reference on virtio_fs object. This ptr gets installed
* in fc->iq->priv. Once fuse_conn is going away, it calls ->put()
* to drop the reference to this object.
--
2.31.1