fuse: support opening 64 bit backing ID

Add backing_id_64 field to fuse_open_out to allow opening files with
64-bit server-allocated backing IDs.

If the connection was initialized with FUSE_PASSTHROUGH_V2, the kernel
reads the backing ID from open_out.backing_id_64 instead of
open_out.backing_id.

The open reply is now variable-length for backward compatibility with
servers that don't send the new backing_id_64 field.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dir.c

@@ -876,6 +876,7 @@ static int fuse_create_open(struct mnt_idmap *idmap, struct inode *dir,
 	args.out_args[0].value = &outentry;
 	/* Store outarg for fuse_finish_open() */
 	outopenp = &ff->args->open_outarg;
+	args.out_argvar = true; /* compat */
 	args.out_args[1].size = sizeof(*outopenp);
 	args.out_args[1].value = outopenp;
 
@@ -885,7 +886,7 @@ static int fuse_create_open(struct mnt_idmap *idmap, struct inode *dir,
 
 	err = fuse_simple_idmap_request(idmap, fm, &args);
 	free_ext_value(&args);
-	if (err)
+	if (err < 0)
 		goto out_free_ff;
 
 	err = -EIO;

fs/fuse/file.c

@@ -28,6 +28,7 @@ static int fuse_send_open(struct fuse_mount *fm, u64 nodeid,
 {
 	struct fuse_open_in inarg;
 	FUSE_ARGS(args);
+	int res;
 
 	memset(&inarg, 0, sizeof(inarg));
 	inarg.flags = open_flags & ~(O_CREAT | O_EXCL | O_NOCTTY);
@@ -45,10 +46,13 @@ static int fuse_send_open(struct fuse_mount *fm, u64 nodeid,
 	args.in_args[0].size = sizeof(inarg);
 	args.in_args[0].value = &inarg;
 	args.out_numargs = 1;
+	args.out_argvar = true; /* compat */
 	args.out_args[0].size = sizeof(*outargp);
 	args.out_args[0].value = outargp;
 
-	return fuse_simple_request(fm, &args);
+	res = fuse_simple_request(fm, &args);
+
+	return res < 0 ? res : 0;
 }
 
 struct fuse_file *fuse_file_alloc(struct fuse_mount *fm, bool release)

fs/fuse/fuse_i.h

@@ -1314,7 +1314,7 @@ static inline struct fuse_backing *fuse_inode_backing_set(struct fuse_inode *fi,
 #endif
 }
 
-struct fuse_backing *fuse_passthrough_open(struct file *file, int backing_id);
+int fuse_passthrough_open(struct file *file, struct fuse_backing *fb);
 void fuse_passthrough_release(struct fuse_file *ff, struct fuse_backing *fb);
 
 static inline bool fuse_is_passthrough(struct fuse_file *ff)

fs/fuse/iomode.c

@@ -165,7 +165,9 @@ static int fuse_file_passthrough_open(struct inode *inode, struct file *file)
 {
 	struct fuse_file *ff = file->private_data;
 	struct fuse_conn *fc = get_fuse_conn(inode);
+	struct fuse_open_out *outarg = &ff->args->open_outarg;
 	struct fuse_backing *fb;
+	u64 backing_id;
 	int err;
 
 	/* Check allowed conditions for file open in passthrough mode */
@@ -175,18 +177,39 @@ static int fuse_file_passthrough_open(struct inode *inode, struct file *file)
 	if (ff->open_flags & ~FOPEN_PASSTHROUGH_MASK)
 		return fuse_EIO("conflicting open flags");
 
-	fb = fuse_passthrough_open(file, ff->args->open_outarg.backing_id);
-	if (IS_ERR(fb))
-		return PTR_ERR(fb);
+	if (!fc->backing_id_64) {
+		if (outarg->backing_id_64 != 0)
+			return fuse_EIO("64 bit backing ID set");
+
+		if (outarg->backing_id <= 0)
+			return fuse_EIO("invalid backing ID");
+
+		backing_id = outarg->backing_id;
+	} else {
+		if (outarg->backing_id != 0)
+			return fuse_EIO("32 bit backing ID set");
+
+		backing_id = outarg->backing_id_64;
+	}
+	fb = fuse_backing_lookup(fc, backing_id);
+	if (!fb)
+		return fuse_EIO("backing not found");
+
+	err = fuse_passthrough_open(file, fb);
+	if (err)
+		goto backing_put;
 
 	/* First passthrough file open denies caching inode io mode */
 	err = fuse_file_uncached_io_open(inode, ff, fb);
-	if (!err)
-		return 0;
+	if (err)
+		goto passthrough_release;
 
+	return 0;
+
+passthrough_release:
 	fuse_passthrough_release(ff, fb);
+backing_put:
 	fuse_backing_put(fb);
-
 	return err;
 }
 

fs/fuse/passthrough.c

@@ -163,41 +163,25 @@ int fuse_passthrough_fadvise(struct fuse_file *ff, loff_t offset, loff_t len, in
 
 /*
  * Setup passthrough to a backing file.
- *
- * Returns an fb object with elevated refcount to be stored in fuse inode.
  */
-struct fuse_backing *fuse_passthrough_open(struct file *file, int backing_id)
+int fuse_passthrough_open(struct file *file, struct fuse_backing *fb)
 {
 	struct fuse_file *ff = file->private_data;
-	struct fuse_conn *fc = ff->fm->fc;
-	struct fuse_backing *fb = NULL;
 	struct file *backing_file;
 
-	if (fc->backing_id_64)
-		return ERR_PTR(fuse_EIO("incompatible backing version"));
-
-	if (backing_id <= 0)
-		return ERR_PTR(fuse_EIO("invalid backing_id"));
-
-	fb = fuse_backing_lookup(fc, backing_id);
-	if (!fb)
-		return ERR_PTR(fuse_EIO("backing not found"));
-
 	/* Allocate backing file per fuse file to store fuse path */
 	backing_file = backing_file_open(file, file->f_flags,
 					 &fb->file->f_path, fb->cred);
-	if (IS_ERR(backing_file)) {
-		fuse_backing_put(fb);
-		return ERR_PTR(fuse_EIO("failed to open backing file (%ld)", PTR_ERR(backing_file)));
-	}
+	if (IS_ERR(backing_file))
+		return fuse_EIO("failed to open backing file (%ld)", PTR_ERR(backing_file));
 
 	ff->passthrough = backing_file;
 	ff->cred = get_cred(fb->cred);
 
-	pr_debug("%s: backing_id=%d, fb=0x%p, backing_file=0x%p\n", __func__,
-		 backing_id, fb, ff->passthrough);
+	pr_debug("%s: backing_id=%llu, fb=0x%p, backing_file=0x%p\n", __func__,
+		 fb->backing_id, fb, ff->passthrough);
 
-	return fb;
+	return 0;
 }
 
 void fuse_passthrough_release(struct fuse_file *ff, struct fuse_backing *fb)

include/uapi/linux/fuse.h

@@ -254,6 +254,7 @@
  *  - add FUSE_PASSTHROUGH_V2
  *  - add FUSE_DEV_IOC_BACKING_CREATE, struct fuse_backing_create_in
  *  - add FUSE_NOTIFY_BACKING_REMOVE, struct fuse_notify_backing_remove_out
+ *  - add backing_id_64 to fuse_open_out
  */
 
 #ifndef _LINUX_FUSE_H
@@ -841,6 +842,7 @@ struct fuse_open_out {
 	uint64_t	fh;
 	uint32_t	open_flags;
 	int32_t		backing_id;
+	uint64_t	backing_id_64;
 };
 
 struct fuse_release_in {