Btrfs: fix how we mount subvol=<whatever>

We've only been able to mount with subvol=<whatever> where whatever was a subvol
within whatever root we had as the default.  This allows us to mount -o
subvol=path/to/subvol/you/want relative from the normal fs_tree root.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>

1 files changed, 135 insertions, 64 deletions

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 244fa46c50b8..934789f7fd33 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -40,6 +40,7 @@
 #include <linux/magic.h>
 #include <linux/slab.h>
 #include <linux/cleancache.h>
+#include <linux/mnt_namespace.h>
 #include "compat.h"
 #include "delayed-inode.h"
 #include "ctree.h"
@@ -58,6 +59,7 @@
 #include <trace/events/btrfs.h>
 
 static const struct super_operations btrfs_super_ops;
+static struct file_system_type btrfs_fs_type;
 
 static const char *btrfs_decode_error(struct btrfs_fs_info *fs_info, int errno,
                                       char nbuf[16])
@@ -411,7 +413,7 @@ static int btrfs_parse_early_options(const char *options, fmode_t flags,
         int intarg;
 
         if (!options)
-                goto out;
+                return 0;
 
         /*
          * strsep changes the string, duplicate it because parse_options
@@ -460,26 +462,15 @@ static int btrfs_parse_early_options(const char *options, fmode_t flags,
                         error = btrfs_scan_one_device(match_strdup(&args[0]),
                                         flags, holder, fs_devices);
                         if (error)
-                                goto out_free_opts;
+                                goto out;
                         break;
                 default:
                         break;
                 }
         }
 
- out_free_opts:
+out:
         kfree(orig);
- out:
-        /*
-         * If no subvolume name is specified we use the default one.  Allocate
-         * a copy of the string "." here so that code later in the
-         * mount path doesn't care if it's the default volume or another one.
-         */
-        if (!*subvol_name) {
-                *subvol_name = kstrdup(".", GFP_KERNEL);
-                if (!*subvol_name)
-                        return -ENOMEM;
-        }
         return error;
 }
 
@@ -730,6 +721,118 @@ static int btrfs_set_super(struct super_block *s, void *data)
         return set_anon_super(s, data);
 }
 
+/*
+ * This will strip out the subvol=%s argument for an argument string and add
+ * subvolid=0 to make sure we get the actual tree root for path walking to the
+ * subvol we want.
+ */
+static char *setup_root_args(char *args)
+{
+        unsigned copied = 0;
+        unsigned len = strlen(args) + 2;
+        char *pos;
+        char *ret;
+
+        /*
+         * We need the same args as before, but minus
+         *
+         * subvol=a
+         *
+         * and add
+         *
+         * subvolid=0
+         *
+         * which is a difference of 2 characters, so we allocate strlen(args) +
+         * 2 characters.
+         */
+        ret = kzalloc(len * sizeof(char), GFP_NOFS);
+        if (!ret)
+                return NULL;
+        pos = strstr(args, "subvol=");
+
+        /* This shouldn't happen, but just in case.. */
+        if (!pos) {
+                kfree(ret);
+                return NULL;
+        }
+
+        /*
+         * The subvol=<> arg is not at the front of the string, copy everybody
+         * up to that into ret.
+         */
+        if (pos != args) {
+                *pos = '\0';
+                strcpy(ret, args);
+                copied += strlen(args);
+                pos++;
+        }
+
+        strncpy(ret + copied, "subvolid=0", len - copied);
+
+        /* Length of subvolid=0 */
+        copied += 10;
+
+        /*
+         * If there is no , after the subvol= option then we know there's no
+         * other options and we can just return.
+         */
+        pos = strchr(pos, ',');
+        if (!pos)
+                return ret;
+
+        /* Copy the rest of the arguments into our buffer */
+        strncpy(ret + copied, pos, len - copied);
+        copied += strlen(pos);
+
+        return ret;
+}
+
+static struct dentry *mount_subvol(const char *subvol_name, int flags,
+                                   const char *device_name, char *data)
+{
+        struct super_block *s;
+        struct dentry *root;
+        struct vfsmount *mnt;
+        struct mnt_namespace *ns_private;
+        char *newargs;
+        struct path path;
+        int error;
+
+        newargs = setup_root_args(data);
+        if (!newargs)
+                return ERR_PTR(-ENOMEM);
+        mnt = vfs_kern_mount(&btrfs_fs_type, flags, device_name,
+                             newargs);
+        kfree(newargs);
+        if (IS_ERR(mnt))
+                return ERR_CAST(mnt);
+
+        ns_private = create_mnt_ns(mnt);
+        if (IS_ERR(ns_private)) {
+                mntput(mnt);
+                return ERR_CAST(ns_private);
+        }
+
+        /*
+         * This will trigger the automount of the subvol so we can just
+         * drop the mnt we have here and return the dentry that we
+         * found.
+         */
+        error = vfs_path_lookup(mnt->mnt_root, mnt, subvol_name,
+                                LOOKUP_FOLLOW, &path);
+        put_mnt_ns(ns_private);
+        if (error)
+                return ERR_PTR(error);
+
+        /* Get a ref to the sb and the dentry we found and return it */
+        s = path.mnt->mnt_sb;
+        atomic_inc(&s->s_active);
+        root = dget(path.dentry);
+        path_put(&path);
+        down_write(&s->s_umount);
+
+        return root;
+}
 
 /*
  * Find a superblock for the given device / mount point.
@@ -761,13 +864,19 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
         if (error)
                 return ERR_PTR(error);
 
+        if (subvol_name) {
+                root = mount_subvol(subvol_name, flags, device_name, data);
+                kfree(subvol_name);
+                return root;
+        }
+
         error = btrfs_scan_one_device(device_name, mode, fs_type, &fs_devices);
         if (error)
-                goto error_free_subvol_name;
+                return ERR_PTR(error);
 
         error = btrfs_open_devices(fs_devices, mode, fs_type);
         if (error)
-                goto error_free_subvol_name;
+                return ERR_PTR(error);
 
         if (!(flags & MS_RDONLY) && fs_devices->rw_devices == 0) {
                 error = -EACCES;
@@ -792,14 +901,15 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 
         bdev = fs_devices->latest_bdev;
         s = sget(fs_type, btrfs_test_super, btrfs_set_super, tree_root);
-        if (IS_ERR(s))
-                goto error_s;
+        if (IS_ERR(s)) {
+                error = PTR_ERR(s);
+                goto error_close_devices;
+        }
 
         if (s->s_root) {
                 if ((flags ^ s->s_flags) & MS_RDONLY) {
                         deactivate_locked_super(s);
-                        error = -EBUSY;
-                        goto error_close_devices;
+                        return ERR_PTR(-EBUSY);
                 }
 
                 btrfs_close_devices(fs_devices);
@@ -814,64 +924,25 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
                                          flags & MS_SILENT ? 1 : 0);
                 if (error) {
                         deactivate_locked_super(s);
-                        goto error_free_subvol_name;
+                        return ERR_PTR(error);
                 }
 
                 btrfs_sb(s)->fs_info->bdev_holder = fs_type;
                 s->s_flags |= MS_ACTIVE;
         }
 
-        /* if they gave us a subvolume name bind mount into that */
-        if (strcmp(subvol_name, ".")) {
-                struct dentry *new_root;
-
-                root = get_default_root(s, subvol_rootid);
-                if (IS_ERR(root)) {
-                        error = PTR_ERR(root);
-                        deactivate_locked_super(s);
-                        goto error_free_subvol_name;
-                }
-
-                mutex_lock(&root->d_inode->i_mutex);
-                new_root = lookup_one_len(subvol_name, root,
-                                      strlen(subvol_name));
-                mutex_unlock(&root->d_inode->i_mutex);
-
-                if (IS_ERR(new_root)) {
-                        dput(root);
-                        deactivate_locked_super(s);
-                        error = PTR_ERR(new_root);
-                        goto error_free_subvol_name;
-                }
-                if (!new_root->d_inode) {
-                        dput(root);
-                        dput(new_root);
-                        deactivate_locked_super(s);
-                        error = -ENXIO;
-                        goto error_free_subvol_name;
-                }
-                dput(root);
-                root = new_root;
-        } else {
-                root = get_default_root(s, subvol_objectid);
-                if (IS_ERR(root)) {
-                        error = PTR_ERR(root);
-                        deactivate_locked_super(s);
-                        goto error_free_subvol_name;
-                }
+        root = get_default_root(s, subvol_objectid);
+        if (IS_ERR(root)) {
+                deactivate_locked_super(s);
+                return root;
         }
 
-        kfree(subvol_name);
         return root;
 
-error_s:
-        error = PTR_ERR(s);
 error_close_devices:
         btrfs_close_devices(fs_devices);
         kfree(fs_info);
         kfree(tree_root);
-error_free_subvol_name:
-        kfree(subvol_name);
         return ERR_PTR(error);
 }