[PATCHv3] btrfs: Introduce new BTRFS_IOC_SNAP_DESTROY_V2 ioctl

[Marcos Paulo de Souza <marcos.souza.org@gmail.com>]

From: Marcos Paulo de Souza <mpdesouza@suse.com>

This ioctl will be responsible for deleting a subvolume using it's id.
This can be used when a system has a file system mounted from a
subvolume, rather than the root file system, like below:

/
|- @subvol1
|- @subvol2
\- @subvol_default

If only @subvol_default is mounted, we have no path to reach
@subvol1 and @subvol2, thus no way to delete them. Current subvolume
delete ioctl takes a file handler point as argument, and if
@subvol_default is mounted, we can't reach @subvol1 and @subvol2 from
the same mount point.

This patch introduces a new flag to allow BTRFS_IOC_SNAP_DESTROY_V2
to delete subvolume using subvolid.

Now, we can use this new ioctl specifying the subvolume id and refer to
the same mount point. It doesn't matter which subvolume was mounted,
since we can reach to the desired one using the subvolume id, and then
delete it.

Also in this patch:
* export get_subvol_name_from_objectid, adding btrfs suffix
* add BTRFS_SUBVOL_SPEC_BY_ID flag
* add subvolid as a union member in struct btrfs_ioctl_vol_args_v2.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
---

 Changes from v2:
 * Commit message improved, explaining how to use the new ioctl (David)
 * Moved subvolid member to the union which already contained devid and name
   (David)
 * Changed name_ptr to subvol_name_ptr, since it'll point to the "full"
   subvolume name, but we need the basename of this char, which was also renamed
   to subvol_name (David).
 * Change the check for a valid subvolid to be >= BTRFS_FIRST_FREE_OBJECTID
   (David)
 * Now BTRFS_IOC_SNAP_DESTROY_V2 can handle both cases where the user uses the
   subvolid and just the subvolume name (David)
 * Changed BTRFS_SUBVOL_DELETE_BY_ID to BTRFS_SUBVOL_SPEC_BY_ID, since this flag
   can be used for other actions rather than deleting a subvolume (David, Christoph)
 * Rewritten comment about the getting/releasing the dentry before doing the
   lookup, explaining why this dentry can be released in order to get a new one
   from lookup (David)
 * Moved mnt_want_write_file call sites right after the flag validation (David)

 Changes from v1:
 * make btrfs_ioctl_snap_destroy handle both SNAP_DESTROY and SNAP_DESTROY_V2
   (suggested by Josef)
 * Change BTRFS_SUBVOL_DELETE_BY_ID to BTRFS_SUBVOL_BY_ID (David)
 * Send patches for btrfs-progs and xfstests along this change

 fs/btrfs/ctree.h           |   2 +
 fs/btrfs/export.c          |   4 +-
 fs/btrfs/export.h          |   5 ++
 fs/btrfs/ioctl.c           | 128 +++++++++++++++++++++++++++++++------
 fs/btrfs/super.c           |   4 +-
 include/uapi/linux/btrfs.h |   8 ++-
 6 files changed, 127 insertions(+), 24 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 54efb21c2727..2d56517c4bca 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -2963,6 +2963,8 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
 int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
 			unsigned long new_flags);
 int btrfs_sync_fs(struct super_block *sb, int wait);
+char *btrfs_get_subvol_name_from_objectid(struct btrfs_fs_info *fs_info,
+					   u64 subvol_objectid);
 
 static inline __printf(2, 3) __cold
 void btrfs_no_printk(const struct btrfs_fs_info *fs_info, const char *fmt, ...)
diff --git a/fs/btrfs/export.c b/fs/btrfs/export.c
index 72e312cae69d..027411cdbae7 100644
--- a/fs/btrfs/export.c
+++ b/fs/btrfs/export.c
@@ -57,7 +57,7 @@ static int btrfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
 	return type;
 }
 
-static struct dentry *btrfs_get_dentry(struct super_block *sb, u64 objectid,
+struct dentry *btrfs_get_dentry(struct super_block *sb, u64 objectid,
 				       u64 root_objectid, u32 generation,
 				       int check_generation)
 {
@@ -152,7 +152,7 @@ static struct dentry *btrfs_fh_to_dentry(struct super_block *sb, struct fid *fh,
 	return btrfs_get_dentry(sb, objectid, root_objectid, generation, 1);
 }
 
-static struct dentry *btrfs_get_parent(struct dentry *child)
+struct dentry *btrfs_get_parent(struct dentry *child)
 {
 	struct inode *dir = d_inode(child);
 	struct btrfs_fs_info *fs_info = btrfs_sb(dir->i_sb);
diff --git a/fs/btrfs/export.h b/fs/btrfs/export.h
index 57488ecd7d4e..f981e8103d64 100644
--- a/fs/btrfs/export.h
+++ b/fs/btrfs/export.h
@@ -18,4 +18,9 @@ struct btrfs_fid {
 	u64 parent_root_objectid;
 } __attribute__ ((packed));
 
+struct dentry *btrfs_get_dentry(struct super_block *sb, u64 objectid,
+				       u64 root_objectid, u32 generation,
+				       int check_generation);
+struct dentry *btrfs_get_parent(struct dentry *child);
+
 #endif
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 12ae31e1813e..be5350582955 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -28,6 +28,7 @@
 #include <linux/iversion.h>
 #include "ctree.h"
 #include "disk-io.h"
+#include "export.h"
 #include "transaction.h"
 #include "btrfs_inode.h"
 #include "print-tree.h"
@@ -2836,7 +2837,8 @@ static int btrfs_ioctl_get_subvol_rootref(struct file *file, void __user *argp)
 }
 
 static noinline int btrfs_ioctl_snap_destroy(struct file *file,
-					     void __user *arg)
+					     void __user *arg,
+					     bool destroy_v2)
 {
 	struct dentry *parent = file->f_path.dentry;
 	struct btrfs_fs_info *fs_info = btrfs_sb(parent->d_sb);
@@ -2845,34 +2847,114 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 	struct inode *inode;
 	struct btrfs_root *root = BTRFS_I(dir)->root;
 	struct btrfs_root *dest = NULL;
-	struct btrfs_ioctl_vol_args *vol_args;
-	int namelen;
+	struct btrfs_ioctl_vol_args *vol_args = NULL;
+	struct btrfs_ioctl_vol_args_v2 *vol_args2 = NULL;
+	char *subvol_name, *subvol_name_ptr = NULL;
+	int subvol_namelen;
 	int err = 0;
+	bool destroy_parent = false;
 
-	if (!S_ISDIR(dir->i_mode))
-		return -ENOTDIR;
+	if (destroy_v2) {
+		vol_args2 = memdup_user(arg, sizeof(*vol_args2));
+		if (IS_ERR(vol_args2))
+			return PTR_ERR(vol_args2);
 
-	vol_args = memdup_user(arg, sizeof(*vol_args));
-	if (IS_ERR(vol_args))
-		return PTR_ERR(vol_args);
+		/*
+		 * If SPEC_BY_ID is not set, we are looking for the subvolume by
+		 * name, same as v1 currently does.
+		 */
+		if (!(vol_args2->flags & BTRFS_SUBVOL_SPEC_BY_ID)) {
+			vol_args2->name[BTRFS_PATH_NAME_MAX] = '\0';
+			subvol_name = vol_args2->name;
 
-	vol_args->name[BTRFS_PATH_NAME_MAX] = '\0';
-	namelen = strlen(vol_args->name);
-	if (strchr(vol_args->name, '/') ||
-	    strncmp(vol_args->name, "..", namelen) == 0) {
-		err = -EINVAL;
-		goto out;
+			err = mnt_want_write_file(file);
+			if (err)
+				goto out;
+		} else {
+			if (vol_args2->subvolid < BTRFS_FIRST_FREE_OBJECTID) {
+				err = -EINVAL;
+				goto out;
+			}
+
+			err = mnt_want_write_file(file);
+			if (err)
+				goto out;
+
+			dentry = btrfs_get_dentry(fs_info->sb,
+					BTRFS_FIRST_FREE_OBJECTID,
+					vol_args2->subvolid, 0, 0);
+			if (IS_ERR(dentry)) {
+				err = PTR_ERR(dentry);
+				goto out_drop_write;
+			}
+
+			/*
+			 * Change the default parent since the subvolume being
+			 * deleted can be outside of the current mount point.
+			 */
+			parent = btrfs_get_parent(dentry);
+
+			/*
+			 * At this point dentry->d_name can point to '/' if the
+			 * subvolume we want to destroy is outsite of the
+			 * current mount point, so we need to released the
+			 * current dentry and execute the lookup to return a new
+			 * one with ->d_name pointing to the
+			 * <mount point>/subvol_name.
+			 */
+			dput(dentry);
+			if (IS_ERR(parent)) {
+				err = PTR_ERR(parent);
+				goto out_drop_write;
+			}
+			dir = d_inode(parent);
+
+			/* If v2 was used with SPEC_BY_ID, a new parent was
+			 * allocated since the subvolume can be outside of the
+			 * current moutn point. Later on we need to release this
+			 * new parent dentry.
+			 */
+			destroy_parent = true;
+
+			subvol_name_ptr = btrfs_get_subvol_name_from_objectid(fs_info,
+					vol_args2->subvolid);
+			if (IS_ERR(subvol_name_ptr)) {
+				err = PTR_ERR(subvol_name_ptr);
+				goto free_parent;
+			}
+			/* subvol_name_ptr is already NULL termined */
+			subvol_name = (char *)kbasename(subvol_name_ptr);
+		}
+	} else {
+		vol_args = memdup_user(arg, sizeof(*vol_args));
+		if (IS_ERR(vol_args))
+			return PTR_ERR(vol_args);
+
+		vol_args->name[BTRFS_PATH_NAME_MAX] = '\0';
+		subvol_name = vol_args->name;
+
+		err = mnt_want_write_file(file);
+		if (err)
+			goto out;
 	}
 
-	err = mnt_want_write_file(file);
-	if (err)
-		goto out;
+	subvol_namelen = strlen(subvol_name);
 
+	if (strchr(subvol_name, '/') ||
+	    strncmp(subvol_name, "..", subvol_namelen) == 0) {
+		err = -EINVAL;
+		goto free_subvol_name;
+	}
+
+	if (!S_ISDIR(dir->i_mode)) {
+		err = -ENOTDIR;
+		goto free_subvol_name;
+	}
 
 	err = down_write_killable_nested(&dir->i_rwsem, I_MUTEX_PARENT);
 	if (err == -EINTR)
 		goto out_drop_write;
-	dentry = lookup_one_len(vol_args->name, parent, namelen);
+	dentry = lookup_one_len(subvol_name, parent, subvol_namelen);
 	if (IS_ERR(dentry)) {
 		err = PTR_ERR(dentry);
 		goto out_unlock_dir;
@@ -2941,9 +3023,15 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 	dput(dentry);
 out_unlock_dir:
 	inode_unlock(dir);
+free_subvol_name:
+	kfree(subvol_name_ptr);
+free_parent:
+	if (destroy_parent)
+		dput(parent);
 out_drop_write:
 	mnt_drop_write_file(file);
 out:
+	kfree(vol_args2);
 	kfree(vol_args);
 	return err;
 }
@@ -5464,7 +5552,9 @@ long btrfs_ioctl(struct file *file, unsigned int
 	case BTRFS_IOC_SUBVOL_CREATE_V2:
 		return btrfs_ioctl_snap_create_v2(file, argp, 1);
 	case BTRFS_IOC_SNAP_DESTROY:
-		return btrfs_ioctl_snap_destroy(file, argp);
+		return btrfs_ioctl_snap_destroy(file, argp, false);
+	case BTRFS_IOC_SNAP_DESTROY_V2:
+		return btrfs_ioctl_snap_destroy(file, argp, true);
 	case BTRFS_IOC_SUBVOL_GETFLAGS:
 		return btrfs_ioctl_subvol_getflags(file, argp);
 	case BTRFS_IOC_SUBVOL_SETFLAGS:
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index f452a94abdc3..649531e92a1d 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1005,7 +1005,7 @@ static int btrfs_parse_subvol_options(const char *options, char **subvol_name,
 	return error;
 }
 
-static char *get_subvol_name_from_objectid(struct btrfs_fs_info *fs_info,
+char *btrfs_get_subvol_name_from_objectid(struct btrfs_fs_info *fs_info,
 					   u64 subvol_objectid)
 {
 	struct btrfs_root *root = fs_info->tree_root;
@@ -1417,7 +1417,7 @@ static struct dentry *mount_subvol(const char *subvol_name, u64 subvol_objectid,
 				goto out;
 			}
 		}
-		subvol_name = get_subvol_name_from_objectid(btrfs_sb(mnt->mnt_sb),
+		subvol_name = btrfs_get_subvol_name_from_objectid(btrfs_sb(mnt->mnt_sb),
 							    subvol_objectid);
 		if (IS_ERR(subvol_name)) {
 			root = ERR_CAST(subvol_name);
diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h
index 7a8bc8b920f5..280f6ded2104 100644
--- a/include/uapi/linux/btrfs.h
+++ b/include/uapi/linux/btrfs.h
@@ -42,11 +42,14 @@ struct btrfs_ioctl_vol_args {
 
 #define BTRFS_DEVICE_SPEC_BY_ID		(1ULL << 3)
 
+#define BTRFS_SUBVOL_SPEC_BY_ID	(1ULL << 4)
+
 #define BTRFS_VOL_ARG_V2_FLAGS_SUPPORTED		\
 			(BTRFS_SUBVOL_CREATE_ASYNC |	\
 			BTRFS_SUBVOL_RDONLY |		\
 			BTRFS_SUBVOL_QGROUP_INHERIT |	\
-			BTRFS_DEVICE_SPEC_BY_ID)
+			BTRFS_DEVICE_SPEC_BY_ID |	\
+			BTRFS_SUBVOL_SPEC_BY_ID)
 
 #define BTRFS_FSID_SIZE 16
 #define BTRFS_UUID_SIZE 16
@@ -120,6 +123,7 @@ struct btrfs_ioctl_vol_args_v2 {
 	};
 	union {
 		char name[BTRFS_SUBVOL_NAME_MAX + 1];
+		__u64 subvolid;
 		__u64 devid;
 	};
 };
@@ -949,5 +953,7 @@ enum btrfs_err_code {
 				struct btrfs_ioctl_get_subvol_rootref_args)
 #define BTRFS_IOC_INO_LOOKUP_USER _IOWR(BTRFS_IOCTL_MAGIC, 62, \
 				struct btrfs_ioctl_ino_lookup_user_args)
+#define BTRFS_IOC_SNAP_DESTROY_V2 _IOW(BTRFS_IOCTL_MAGIC, 63, \
+				struct btrfs_ioctl_vol_args_v2)
 
 #endif /* _UAPI_LINUX_BTRFS_H */
-- 
2.24.0