[PATCH v2 07/21] fuse: prepare to setup backing inode passthrough on lookup

[Joanne Koong <joannelkoong@gmail.com>]

From: Amir Goldstein <amir73il@gmail.com>

Add a helper for requesting to associate a backing inode for inode
passthrough operations.

This helper is expected to be used to setup backing inode on lookup
response.

The minimal requirement for the backing inode is to support the
GETATTR passthrough op.

Unlike setting of passthrough from open, this mapping is created for
the entire lifetime of the inode and cannot be changed later.

Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/fuse/backing.c     | 32 ++++++++++++++++++++------------
 fs/fuse/fuse_i.h      |  6 ++++--
 fs/fuse/iomode.c      | 43 +++++++++++++++++++++++++++++++++++++++++++
 fs/fuse/passthrough.c | 12 +++++-------
 4 files changed, 72 insertions(+), 21 deletions(-)

diff --git a/fs/fuse/backing.c b/fs/fuse/backing.c
index 52fccb9ff283..2234ab47406a 100644
--- a/fs/fuse/backing.c
+++ b/fs/fuse/backing.c
@@ -17,6 +17,26 @@ struct fuse_backing *fuse_backing_get(struct fuse_backing *fb)
 	return NULL;
 }
 
+/*
+ * Get fuse backing object by backing id.
+ *
+ * Returns an fb object with elevated refcount to be stored in fuse inode.
+ */
+struct fuse_backing *fuse_backing_id_get(struct fuse_conn *fc, int backing_id)
+{
+	struct fuse_backing *fb;
+
+	if (backing_id <= 0)
+		return ERR_PTR(-EINVAL);
+
+	rcu_read_lock();
+	fb = idr_find(&fc->backing_files_map, backing_id);
+	fb = fuse_backing_get(fb);
+	rcu_read_unlock();
+
+	return fb;
+}
+
 static void fuse_backing_free(struct fuse_backing *fb)
 {
 	pr_debug("%s: fb=0x%p\n", __func__, fb);
@@ -178,15 +198,3 @@ int fuse_backing_close(struct fuse_conn *fc, int backing_id)
 
 	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/fuse_i.h b/fs/fuse/fuse_i.h
index 389e28497dc7..9e5142a94a09 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1289,7 +1289,7 @@ void fuse_file_release(struct inode *inode, struct fuse_file *ff,
 #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);
+struct fuse_backing *fuse_backing_id_get(struct fuse_conn *fc, int backing_id);
 #else
 
 static inline struct fuse_backing *fuse_backing_get(struct fuse_backing *fb)
@@ -1300,7 +1300,7 @@ 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,
+static inline struct fuse_backing *fuse_backing_id_get(struct fuse_conn *fc,
 						       int backing_id)
 {
 	return NULL;
@@ -1353,6 +1353,8 @@ ssize_t fuse_passthrough_splice_write(struct pipe_inode_info *pipe,
 ssize_t fuse_passthrough_mmap(struct file *file, struct vm_area_struct *vma);
 int fuse_passthrough_readdir(struct file *file, struct dir_context *ctx);
 
+int fuse_inode_set_passthrough(struct inode *inode, int backing_id);
+
 static inline bool fuse_passthrough_op(struct inode *inode, enum fuse_opcode op)
 {
 	struct fuse_inode *fi = get_fuse_inode(inode);
diff --git a/fs/fuse/iomode.c b/fs/fuse/iomode.c
index 6e2ddfe384b3..3483aaec698e 100644
--- a/fs/fuse/iomode.c
+++ b/fs/fuse/iomode.c
@@ -181,6 +181,49 @@ static void fuse_file_uncached_io_release(struct fuse_file *ff,
 	fuse_inode_uncached_io_end(inode);
 }
 
+/* Setup passthrough for inode operations without an open file */
+int fuse_inode_set_passthrough(struct inode *inode, int backing_id)
+{
+	struct fuse_conn *fc = get_fuse_conn(inode);
+	struct fuse_inode *fi = get_fuse_inode(inode);
+	struct fuse_backing *current_fb, *fb;
+	int err;
+
+	if (!IS_ENABLED(CONFIG_FUSE_PASSTHROUGH) || !fc->passthrough_ino)
+		return 0;
+
+	fb = fuse_backing_id_get(fc, backing_id);
+	if (IS_ERR_OR_NULL(fb)) {
+		err = fb ? PTR_ERR(fb) : -ENOENT;
+		fb = NULL;
+		goto fail;
+	}
+
+	current_fb = fuse_inode_backing(fi);
+	if (current_fb) {
+		/* Different backing file on same inode is not allowed */
+		err = current_fb == fb ? 0 : -EBUSY;
+		fuse_backing_put(fb);
+		return err;
+	}
+
+	/* Backing inode requires at least GETATTR op passthrough */
+	err = -EOPNOTSUPP;
+	if (!(fb->ops_mask & FUSE_PASSTHROUGH_OP_GETATTR))
+		goto fail;
+
+	err = fuse_inode_uncached_io_start(inode, NULL, fb);
+	if (err)
+		goto fail;
+
+	return 0;
+fail:
+	fuse_backing_put(fb);
+	pr_debug("failed to setup backing inode (ino=%lu, backing_id=%d, err=%i).\n",
+		 inode->i_ino, backing_id, err);
+	return err;
+}
+
 /*
  * Open flags that are allowed in combination with FOPEN_PASSTHROUGH.
  * A combination of FOPEN_PASSTHROUGH and FOPEN_DIRECT_IO means that read/write
diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
index 7de038960b2f..f2025772c9c1 100644
--- a/fs/fuse/passthrough.c
+++ b/fs/fuse/passthrough.c
@@ -181,14 +181,12 @@ struct fuse_backing *fuse_passthrough_open(struct file *file, int backing_id)
 	struct file *backing_file;
 	int err;
 
-	err = -EINVAL;
-	if (backing_id <= 0)
-		goto out;
-
-	err = -ENOENT;
-	fb = fuse_backing_lookup(fc, backing_id);
-	if (!fb)
+	fb = fuse_backing_id_get(fc, backing_id);
+	if (IS_ERR_OR_NULL(fb)) {
+		err = fb ? PTR_ERR(fb) : -ENOENT;
+		fb = NULL;
 		goto out;
+	}
 
 	/* Allocate backing file per fuse file to store fuse path */
 	backing_file = backing_file_open(&file->f_path, file->f_flags,
-- 
2.52.0