Blame SOURCES/0041-grub2-btrfs-Add-ability-to-boot-from-subvolumes.patch

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From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Jeff Mahoney <jeffm@suse.com>
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Date: Tue, 9 Jul 2019 13:39:45 +0200
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Subject: [PATCH] grub2/btrfs: Add ability to boot from subvolumes
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This patch adds the ability to specify a different root on a btrfs
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filesystem too boot from other than the default one.
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btrfs-list-snapshots <dev> will list the subvolumes available on the
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filesystem.
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set btrfs_subvol=<path> and set btrfs_subvolid=<subvolid> will specify
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which subvolume to use and any pathnames provided with either of those
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variables set will start using that root. If the subvolume or subvolume id
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doesn't exist, then an error case will result.
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It is possible to boot into a separate GRUB instance by exporting the
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variable and loading the config file from the subvolume.
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Signed-off-by: Jeff Mahoney <jeffm@suse.com>
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---
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 grub-core/fs/btrfs.c | 552 +++++++++++++++++++++++++++++++++++++++++++++++++--
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 include/grub/btrfs.h |   1 +
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 2 files changed, 533 insertions(+), 20 deletions(-)
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diff --git a/grub-core/fs/btrfs.c b/grub-core/fs/btrfs.c
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index 63203034df..f1fff7385b 100644
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--- a/grub-core/fs/btrfs.c
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+++ b/grub-core/fs/btrfs.c
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@@ -38,6 +38,9 @@
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 #include <zstd.h>
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 #include <grub/i18n.h>
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 #include <grub/btrfs.h>
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+#include <grub/command.h>
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+#include <grub/env.h>
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+#include <grub/extcmd.h>
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 #include <grub/crypto.h>
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 #include <grub/diskfilter.h>
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 #include <grub/safemath.h>
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@@ -79,9 +82,11 @@ struct grub_btrfs_superblock
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   grub_uint64_t generation;
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   grub_uint64_t root_tree;
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   grub_uint64_t chunk_tree;
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-  grub_uint8_t dummy2[0x20];
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+  grub_uint8_t dummy2[0x18];
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+  grub_uint64_t bytes_used;
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   grub_uint64_t root_dir_objectid;
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-  grub_uint8_t dummy3[0x41];
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+  grub_uint64_t num_devices;
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+  grub_uint8_t dummy3[0x39];
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   struct grub_btrfs_device this_device;
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   char label[0x100];
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   grub_uint8_t dummy4[0x100];
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@@ -121,6 +126,7 @@ struct grub_btrfs_data
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   grub_uint64_t exttree;
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   grub_size_t extsize;
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   struct grub_btrfs_extent_data *extent;
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+  grub_uint64_t fs_tree;
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 };
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 struct grub_btrfs_chunk_item
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@@ -191,6 +197,14 @@ struct grub_btrfs_leaf_descriptor
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   } *data;
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 };
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+struct grub_btrfs_root_ref
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+{
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+  grub_uint64_t dirid;
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+  grub_uint64_t sequence;
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+  grub_uint16_t name_len;
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+  const char name[0];
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+} __attribute__ ((packed));
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+
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 struct grub_btrfs_time
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 {
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   grub_int64_t sec;
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@@ -236,6 +250,14 @@ struct grub_btrfs_extent_data
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 #define GRUB_BTRFS_OBJECT_ID_CHUNK 0x100
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+#define GRUB_BTRFS_ROOT_TREE_OBJECTID 1ULL
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+#define GRUB_BTRFS_FS_TREE_OBJECTID 5ULL
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+#define GRUB_BTRFS_ROOT_REF_KEY     156
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+#define GRUB_BTRFS_ROOT_ITEM_KEY     132
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+
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+static grub_uint64_t btrfs_default_subvolid = 0;
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+static char *btrfs_default_subvol = NULL;
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+
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 static grub_disk_addr_t superblock_sectors[] = { 64 * 2, 64 * 1024 * 2,
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   256 * 1048576 * 2, 1048576ULL * 1048576ULL * 2
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 };
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@@ -1173,6 +1195,62 @@ grub_btrfs_read_logical (struct grub_btrfs_data *data, grub_disk_addr_t addr,
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   return GRUB_ERR_NONE;
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 }
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+static grub_err_t
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+get_fs_root(struct grub_btrfs_data *data, grub_uint64_t tree,
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+            grub_uint64_t objectid, grub_uint64_t offset,
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+            grub_uint64_t *fs_root);
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+
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+static grub_err_t
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+lookup_root_by_id(struct grub_btrfs_data *data, grub_uint64_t id)
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+{
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+  grub_err_t err;
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+  grub_uint64_t tree;
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+
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+  err = get_fs_root(data, data->sblock.root_tree, id, -1, &tree);
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+  if (!err)
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+    data->fs_tree = tree;
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+  return err;
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+}
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+
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+static grub_err_t
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+find_path (struct grub_btrfs_data *data,
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+	   const char *path, struct grub_btrfs_key *key,
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+	   grub_uint64_t *tree, grub_uint8_t *type);
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+
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+static grub_err_t
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+lookup_root_by_name(struct grub_btrfs_data *data, const char *path)
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+{
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+  grub_err_t err;
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+  grub_uint64_t tree = 0;
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+  grub_uint8_t type;
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+  struct grub_btrfs_key key;
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+
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+  err = find_path (data, path, &key, &tree, &type);
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+  if (err)
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+      return grub_error(GRUB_ERR_FILE_NOT_FOUND, "couldn't locate %s\n", path);
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+
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+  if (key.object_id != grub_cpu_to_le64_compile_time (GRUB_BTRFS_OBJECT_ID_CHUNK) || tree == 0)
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+    return grub_error(GRUB_ERR_BAD_FILE_TYPE, "%s: not a subvolume\n", path);
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+
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+  data->fs_tree = tree;
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+  return GRUB_ERR_NONE;
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+}
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+
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+static grub_err_t
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+btrfs_handle_subvol(struct grub_btrfs_data *data __attribute__ ((unused)))
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+{
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+  if (btrfs_default_subvol)
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+    return lookup_root_by_name(data, btrfs_default_subvol);
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+
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+  if (btrfs_default_subvolid)
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+    return lookup_root_by_id(data, btrfs_default_subvolid);
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+
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+  data->fs_tree = 0;
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+
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+  return GRUB_ERR_NONE;
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+}
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+
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+
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 static struct grub_btrfs_data *
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 grub_btrfs_mount (grub_device_t dev)
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 {
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@@ -1208,6 +1286,13 @@ grub_btrfs_mount (grub_device_t dev)
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   data->devices_attached[0].dev = dev;
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   data->devices_attached[0].id = data->sblock.this_device.device_id;
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+  err = btrfs_handle_subvol (data);
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+  if (err)
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+    {
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+      grub_free (data);
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+      return NULL;
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+    }
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+
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   return data;
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 }
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@@ -1673,6 +1758,91 @@ get_root (struct grub_btrfs_data *data, struct grub_btrfs_key *key,
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   return GRUB_ERR_NONE;
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 }
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+static grub_err_t
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+find_pathname(struct grub_btrfs_data *data, grub_uint64_t objectid,
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+              grub_uint64_t fs_root, const char *name, char **pathname)
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+{
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+  grub_err_t err;
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+  struct grub_btrfs_key key = {
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+    .object_id = objectid,
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+    .type = GRUB_BTRFS_ITEM_TYPE_INODE_REF,
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+    .offset = 0,
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+  };
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+  struct grub_btrfs_key key_out;
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+  struct grub_btrfs_leaf_descriptor desc;
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+  char *p = grub_strdup (name);
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+  grub_disk_addr_t elemaddr;
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+  grub_size_t elemsize;
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+  grub_size_t alloc = grub_strlen(name) + 1;
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+
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+  err = lower_bound(data, &key, &key_out, fs_root,
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+                    &elemaddr, &elemsize, &desc, 0);
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+  if (err)
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+    return grub_error(err, "lower_bound caught %d\n", err);
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+
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+  if (key_out.type != GRUB_BTRFS_ITEM_TYPE_INODE_REF)
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+    next(data, &desc, &elemaddr, &elemsize, &key_out);
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+
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+  if (key_out.type != GRUB_BTRFS_ITEM_TYPE_INODE_REF)
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+    {
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+      return grub_error(GRUB_ERR_FILE_NOT_FOUND,
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+                        "Can't find inode ref for {%"PRIuGRUB_UINT64_T
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+                        ", %u, %"PRIuGRUB_UINT64_T"} %"PRIuGRUB_UINT64_T
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+                        "/%"PRIuGRUB_SIZE"\n",
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+                        key_out.object_id, key_out.type,
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+                        key_out.offset, elemaddr, elemsize);
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+    }
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+
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+
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+  while (key_out.type == GRUB_BTRFS_ITEM_TYPE_INODE_REF &&
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+         key_out.object_id != key_out.offset) {
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+    struct grub_btrfs_inode_ref *inode_ref;
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+    char *new;
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+
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+    inode_ref = grub_malloc(elemsize + 1);
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+    if (!inode_ref)
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+      return grub_error(GRUB_ERR_OUT_OF_MEMORY,
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+                        "couldn't allocate memory for inode_ref (%"PRIuGRUB_SIZE")\n", elemsize);
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+
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+    err = grub_btrfs_read_logical(data, elemaddr, inode_ref, elemsize, 0);
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+    if (err)
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+      return grub_error(err, "read_logical caught %d\n", err);
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+
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+    alloc += grub_le_to_cpu16 (inode_ref->n) + 2;
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+    new = grub_malloc(alloc);
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+    if (!new)
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+      return grub_error(GRUB_ERR_OUT_OF_MEMORY,
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+                        "couldn't allocate memory for name (%"PRIuGRUB_SIZE")\n", alloc);
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+
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+    grub_memcpy(new, inode_ref->name, grub_le_to_cpu16 (inode_ref->n));
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+    if (p)
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+      {
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+        new[grub_le_to_cpu16 (inode_ref->n)] = '/';
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+        grub_strcpy (new + grub_le_to_cpu16 (inode_ref->n) + 1, p);
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+        grub_free(p);
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+      }
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+    else
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+      new[grub_le_to_cpu16 (inode_ref->n)] = 0;
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+    grub_free(inode_ref);
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+
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+    p = new;
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+
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+    key.object_id = key_out.offset;
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+
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+    err = lower_bound(data, &key, &key_out, fs_root, &elemaddr,
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+                      &elemsize, &desc, 0);
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+    if (err)
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+      return grub_error(err, "lower_bound caught %d\n", err);
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+
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+    if (key_out.type != GRUB_BTRFS_ITEM_TYPE_INODE_REF)
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+      next(data, &desc, &elemaddr, &elemsize, &key_out);
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+
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+  }
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+
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+  *pathname = p;
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+  return 0;
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+}
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+
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 static grub_err_t
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 find_path (struct grub_btrfs_data *data,
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 	   const char *path, struct grub_btrfs_key *key,
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@@ -1691,14 +1861,26 @@ find_path (struct grub_btrfs_data *data,
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   char *origpath = NULL;
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   unsigned symlinks_max = 32;
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-  err = get_root (data, key, tree, type);
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-  if (err)
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-    return err;
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-
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   origpath = grub_strdup (path);
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   if (!origpath)
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     return grub_errno;
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+  if (data->fs_tree)
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+    {
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+      *type = GRUB_BTRFS_DIR_ITEM_TYPE_DIRECTORY;
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+      *tree = data->fs_tree;
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+      /* This is a tree root, so everything starts at objectid 256 */
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+      key->object_id = grub_cpu_to_le64_compile_time (GRUB_BTRFS_OBJECT_ID_CHUNK);
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+      key->type = GRUB_BTRFS_ITEM_TYPE_DIR_ITEM;
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+      key->offset = 0;
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+    }
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+  else
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+    {
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+      err = get_root (data, key, tree, type);
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+      if (err)
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+	return err;
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+    }
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+
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   while (1)
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     {
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       while (path[0] == '/')
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@@ -1871,9 +2053,21 @@ find_path (struct grub_btrfs_data *data,
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 	  path = path_alloc = tmp;
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 	  if (path[0] == '/')
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 	    {
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-	      err = get_root (data, key, tree, type);
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-	      if (err)
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-		return err;
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+	      if (data->fs_tree)
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+		{
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+		  *type = GRUB_BTRFS_DIR_ITEM_TYPE_DIRECTORY;
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+		  *tree = data->fs_tree;
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+		  /* This is a tree root, so everything starts at objectid 256 */
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+		  key->object_id = grub_cpu_to_le64_compile_time (GRUB_BTRFS_OBJECT_ID_CHUNK);
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+		  key->type = GRUB_BTRFS_ITEM_TYPE_DIR_ITEM;
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+		  key->offset = 0;
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+		}
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+	      else
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+		{
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+		  err = get_root (data, key, tree, type);
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+		  if (err)
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+		    return err;
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+		}
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 	    }
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 	  continue;
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 	}
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@@ -2114,18 +2308,10 @@ grub_btrfs_read (grub_file_t file, char *buf, grub_size_t len)
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 				 data->tree, file->offset, buf, len);
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 }
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-static grub_err_t
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-grub_btrfs_uuid (grub_device_t device, char **uuid)
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+static char *
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+btrfs_unparse_uuid(struct grub_btrfs_data *data)
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 {
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-  struct grub_btrfs_data *data;
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-
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-  *uuid = NULL;
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-
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-  data = grub_btrfs_mount (device);
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-  if (!data)
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-    return grub_errno;
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-
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-  *uuid = grub_xasprintf ("%04x%04x-%04x-%04x-%04x-%04x%04x%04x",
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+  return  grub_xasprintf ("%04x%04x-%04x-%04x-%04x-%04x%04x%04x",
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 			  grub_be_to_cpu16 (data->sblock.uuid[0]),
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 			  grub_be_to_cpu16 (data->sblock.uuid[1]),
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 			  grub_be_to_cpu16 (data->sblock.uuid[2]),
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@@ -2134,6 +2320,20 @@ grub_btrfs_uuid (grub_device_t device, char **uuid)
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 			  grub_be_to_cpu16 (data->sblock.uuid[5]),
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 			  grub_be_to_cpu16 (data->sblock.uuid[6]),
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 			  grub_be_to_cpu16 (data->sblock.uuid[7]));
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+}
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+
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+static grub_err_t
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+grub_btrfs_uuid (grub_device_t device, char **uuid)
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+{
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+  struct grub_btrfs_data *data;
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+
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+  *uuid = NULL;
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+
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+  data = grub_btrfs_mount (device);
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+  if (!data)
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+    return grub_errno;
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+
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+  *uuid = btrfs_unparse_uuid(data);
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   grub_btrfs_unmount (data);
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@@ -2190,6 +2390,242 @@ grub_btrfs_embed (grub_device_t device __attribute__ ((unused)),
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 }
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 #endif
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+static grub_err_t
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+grub_cmd_btrfs_info (grub_command_t cmd __attribute__ ((unused)), int argc,
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+		     char **argv)
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+{
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+  grub_device_t dev;
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+  char *devname;
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+  struct grub_btrfs_data *data;
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+  char *uuid;
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+
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+  if (argc < 1)
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+    return grub_error (GRUB_ERR_BAD_ARGUMENT, "device name required");
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+
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+  devname = grub_file_get_device_name(argv[0]);
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+
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+  if (!devname)
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+    return grub_errno;
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+
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+  dev = grub_device_open (devname);
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+  grub_free (devname);
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+  if (!dev)
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+    return grub_errno;
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+
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+  data = grub_btrfs_mount (dev);
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+  if (!data)
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+    {
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+      grub_device_close(dev);
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+      return grub_error (GRUB_ERR_BAD_ARGUMENT, "failed to open fs");
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+    }
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+
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+  if (data->sblock.label)
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+    grub_printf("Label: '%s' ", data->sblock.label);
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+  else
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+    grub_printf("Label: none ");
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+
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+  uuid = btrfs_unparse_uuid(data);
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+
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+  grub_printf(" uuid: %s\n\tTotal devices %" PRIuGRUB_UINT64_T
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+              " FS bytes used %" PRIuGRUB_UINT64_T "\n",
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+	      uuid, grub_cpu_to_le64(data->sblock.num_devices),
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+	      grub_cpu_to_le64(data->sblock.bytes_used));
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+
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+  grub_btrfs_unmount (data);
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+
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+  return 0;
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+}
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+
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+static grub_err_t
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+get_fs_root(struct grub_btrfs_data *data, grub_uint64_t tree,
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+            grub_uint64_t objectid, grub_uint64_t offset,
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+            grub_uint64_t *fs_root)
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+{
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+  grub_err_t err;
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+  struct grub_btrfs_key key_in = {
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+    .object_id = objectid,
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+    .type = GRUB_BTRFS_ROOT_ITEM_KEY,
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+    .offset = offset,
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+  }, key_out;
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+  struct grub_btrfs_leaf_descriptor desc;
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+  grub_disk_addr_t elemaddr;
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+  grub_size_t elemsize;
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+  struct grub_btrfs_root_item ri;
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+
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+  err = lower_bound(data, &key_in, &key_out, tree,
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+                    &elemaddr, &elemsize, &desc, 0);
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+
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+  if (err)
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+    return err;
8e15ce
+
8e15ce
+  if (key_out.type != GRUB_BTRFS_ITEM_TYPE_ROOT_ITEM || elemaddr == 0)
8e15ce
+    return grub_error(GRUB_ERR_FILE_NOT_FOUND,
8e15ce
+                    N_("can't find fs root for subvol %"PRIuGRUB_UINT64_T"\n"),
8e15ce
+                    key_in.object_id);
8e15ce
+
8e15ce
+  err = grub_btrfs_read_logical (data, elemaddr, &ri, sizeof (ri), 0);
8e15ce
+  if (err)
8e15ce
+    return err;
8e15ce
+
8e15ce
+  *fs_root = ri.tree;
8e15ce
+
8e15ce
+  return GRUB_ERR_NONE;
8e15ce
+}
8e15ce
+
8e15ce
+static const struct grub_arg_option options[] = {
8e15ce
+  {"output", 'o', 0, N_("Output to a variable instead of the console."),
8e15ce
+   N_("VARNAME"), ARG_TYPE_STRING},
8e15ce
+  {"path-only", 'p', 0, N_("Show only the path of the subvolume."), 0, 0},
8e15ce
+  {"id-only", 'i', 0, N_("Show only the id of the subvolume."), 0, 0},
8e15ce
+  {0, 0, 0, 0, 0, 0}
8e15ce
+};
8e15ce
+
8e15ce
+static grub_err_t
8e15ce
+grub_cmd_btrfs_list_subvols (struct grub_extcmd_context *ctxt,
8e15ce
+			     int argc, char **argv)
8e15ce
+{
8e15ce
+  struct grub_btrfs_data *data;
8e15ce
+  grub_device_t dev;
8e15ce
+  char *devname;
8e15ce
+  grub_uint64_t tree;
8e15ce
+  struct grub_btrfs_key key_in = {
8e15ce
+    .object_id = grub_cpu_to_le64_compile_time (GRUB_BTRFS_FS_TREE_OBJECTID),
8e15ce
+    .type = GRUB_BTRFS_ROOT_REF_KEY,
8e15ce
+    .offset = 0,
8e15ce
+  }, key_out;
8e15ce
+  struct grub_btrfs_leaf_descriptor desc;
8e15ce
+  grub_disk_addr_t elemaddr;
8e15ce
+  grub_uint64_t fs_root = 0;
8e15ce
+  grub_size_t elemsize;
8e15ce
+  grub_size_t allocated = 0;
8e15ce
+  int r = 0;
8e15ce
+  grub_err_t err;
8e15ce
+  char *buf = NULL;
8e15ce
+  int print = 1;
8e15ce
+  int path_only = ctxt->state[1].set;
8e15ce
+  int num_only = ctxt->state[2].set;
8e15ce
+  char *varname = NULL;
8e15ce
+  char *output = NULL;
8e15ce
+
8e15ce
+  if (ctxt->state[0].set) {
8e15ce
+    varname = ctxt->state[0].arg;
8e15ce
+    print = 0;
8e15ce
+  }
8e15ce
+
8e15ce
+  if (argc < 1)
8e15ce
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, "device name required");
8e15ce
+
8e15ce
+  devname = grub_file_get_device_name(argv[0]);
8e15ce
+  if (!devname)
8e15ce
+    return grub_errno;
8e15ce
+
8e15ce
+  dev = grub_device_open (devname);
8e15ce
+  grub_free (devname);
8e15ce
+  if (!dev)
8e15ce
+    return grub_errno;
8e15ce
+
8e15ce
+  data = grub_btrfs_mount(dev);
8e15ce
+  if (!data)
8e15ce
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, "could not open device");
8e15ce
+
8e15ce
+  tree = data->sblock.root_tree;
8e15ce
+  err = get_fs_root(data, tree, grub_cpu_to_le64_compile_time (GRUB_BTRFS_FS_TREE_OBJECTID),
8e15ce
+                    0, &fs_root);
8e15ce
+  if (err)
8e15ce
+    goto out;
8e15ce
+
8e15ce
+  err = lower_bound(data, &key_in, &key_out, tree,
8e15ce
+                    &elemaddr, &elemsize, &desc, 0);
8e15ce
+
8e15ce
+  if (err)
8e15ce
+    {
8e15ce
+      grub_btrfs_unmount(data);
8e15ce
+      return err;
8e15ce
+    }
8e15ce
+
8e15ce
+  if (key_out.type != GRUB_BTRFS_ITEM_TYPE_ROOT_REF || elemaddr == 0)
8e15ce
+    {
8e15ce
+      r = next(data, &desc, &elemaddr, &elemsize, &key_out);
8e15ce
+    }
8e15ce
+
8e15ce
+  if (key_out.type != GRUB_BTRFS_ITEM_TYPE_ROOT_REF) {
8e15ce
+    err = GRUB_ERR_FILE_NOT_FOUND;
8e15ce
+    grub_error(GRUB_ERR_FILE_NOT_FOUND, N_("can't find root refs"));
8e15ce
+    goto out;
8e15ce
+  }
8e15ce
+
8e15ce
+  do
8e15ce
+    {
8e15ce
+      struct grub_btrfs_root_ref *ref;
8e15ce
+      char *p = NULL;
8e15ce
+
8e15ce
+      if (key_out.type != GRUB_BTRFS_ITEM_TYPE_ROOT_REF)
8e15ce
+        {
8e15ce
+          r = 0;
8e15ce
+          break;
8e15ce
+        }
8e15ce
+
8e15ce
+      if (elemsize > allocated)
8e15ce
+        {
8e15ce
+          grub_free(buf);
8e15ce
+          allocated = 2 * elemsize;
8e15ce
+          buf = grub_malloc(allocated + 1);
8e15ce
+          if (!buf)
8e15ce
+            {
8e15ce
+              r = -grub_errno;
8e15ce
+              break;
8e15ce
+            }
8e15ce
+        }
8e15ce
+      ref = (struct grub_btrfs_root_ref *)buf;
8e15ce
+
8e15ce
+      err = grub_btrfs_read_logical(data, elemaddr, buf, elemsize, 0);
8e15ce
+      if (err)
8e15ce
+        {
8e15ce
+          r = -err;
8e15ce
+          break;
8e15ce
+        }
8e15ce
+        buf[elemsize] = 0;
8e15ce
+
8e15ce
+      find_pathname(data, ref->dirid, fs_root, ref->name, &p);
8e15ce
+
8e15ce
+      if (print)
8e15ce
+        {
8e15ce
+          if (num_only)
8e15ce
+            grub_printf("ID %"PRIuGRUB_UINT64_T"\n", key_out.offset);
8e15ce
+          else if (path_only)
8e15ce
+            grub_printf("%s\n", p);
8e15ce
+          else
8e15ce
+            grub_printf("ID %"PRIuGRUB_UINT64_T" path %s\n", key_out.offset, p);
8e15ce
+        } else {
8e15ce
+          char *old = output;
8e15ce
+          if (num_only)
8e15ce
+            output = grub_xasprintf("%s%"PRIuGRUB_UINT64_T"\n",
8e15ce
+                                    old ?: "", key_out.offset);
8e15ce
+          else if (path_only)
8e15ce
+            output = grub_xasprintf("%s%s\n", old ?: "", p);
8e15ce
+          else
8e15ce
+            output = grub_xasprintf("%sID %"PRIuGRUB_UINT64_T" path %s\n",
8e15ce
+                                    old ?: "", key_out.offset, p);
8e15ce
+
8e15ce
+          if (old)
8e15ce
+            grub_free(old);
8e15ce
+        }
8e15ce
+
8e15ce
+      r = next(data, &desc, &elemaddr, &elemsize, &key_out);
8e15ce
+  } while(r > 0);
8e15ce
+
8e15ce
+  if (output)
8e15ce
+    grub_env_set(varname, output);
8e15ce
+
8e15ce
+out:
8e15ce
+  free_iterator(&desc);
8e15ce
+  grub_btrfs_unmount(data);
8e15ce
+
8e15ce
+  grub_device_close (dev);
8e15ce
+
8e15ce
+  return 0;
8e15ce
+}
8e15ce
+
8e15ce
 static struct grub_fs grub_btrfs_fs = {
8e15ce
   .name = "btrfs",
8e15ce
   .fs_dir = grub_btrfs_dir,
8e15ce
@@ -2205,12 +2641,88 @@ static struct grub_fs grub_btrfs_fs = {
8e15ce
 #endif
8e15ce
 };
8e15ce
 
8e15ce
+static grub_command_t cmd_info;
8e15ce
+static grub_extcmd_t cmd_list_subvols;
8e15ce
+
8e15ce
+static char *
8e15ce
+subvolid_set_env (struct grub_env_var *var __attribute__ ((unused)),
8e15ce
+                  const char *val)
8e15ce
+{
8e15ce
+  unsigned long long result = 0;
8e15ce
+
8e15ce
+  grub_errno = GRUB_ERR_NONE;
8e15ce
+  if (*val)
8e15ce
+    {
8e15ce
+      result = grub_strtoull(val, NULL, 10);
8e15ce
+      if (grub_errno)
8e15ce
+        return NULL;
8e15ce
+    }
8e15ce
+
8e15ce
+  grub_free (btrfs_default_subvol);
8e15ce
+  btrfs_default_subvol = NULL;
8e15ce
+  btrfs_default_subvolid = result;
8e15ce
+  return grub_strdup(val);
8e15ce
+}
8e15ce
+
8e15ce
+static const char *
8e15ce
+subvolid_get_env (struct grub_env_var *var __attribute__ ((unused)),
8e15ce
+                  const char *val __attribute__ ((unused)))
8e15ce
+{
8e15ce
+  if (btrfs_default_subvol)
8e15ce
+    return grub_xasprintf("subvol:%s", btrfs_default_subvol);
8e15ce
+  else if (btrfs_default_subvolid)
8e15ce
+    return grub_xasprintf("%"PRIuGRUB_UINT64_T, btrfs_default_subvolid);
8e15ce
+  else
8e15ce
+    return "";
8e15ce
+}
8e15ce
+
8e15ce
+static char *
8e15ce
+subvol_set_env (struct grub_env_var *var __attribute__ ((unused)),
8e15ce
+                const char *val)
8e15ce
+{
8e15ce
+  grub_free (btrfs_default_subvol);
8e15ce
+  btrfs_default_subvol = grub_strdup (val);
8e15ce
+  btrfs_default_subvolid = 0;
8e15ce
+  return grub_strdup(val);
8e15ce
+}
8e15ce
+
8e15ce
+static const char *
8e15ce
+subvol_get_env (struct grub_env_var *var __attribute__ ((unused)),
8e15ce
+                const char *val __attribute__ ((unused)))
8e15ce
+{
8e15ce
+  if (btrfs_default_subvol)
8e15ce
+    return btrfs_default_subvol;
8e15ce
+  else if (btrfs_default_subvolid)
8e15ce
+    return grub_xasprintf("subvolid:%" PRIuGRUB_UINT64_T,
8e15ce
+                          btrfs_default_subvolid);
8e15ce
+  else
8e15ce
+    return "";
8e15ce
+}
8e15ce
+
8e15ce
 GRUB_MOD_INIT (btrfs)
8e15ce
 {
8e15ce
   grub_fs_register (&grub_btrfs_fs);
8e15ce
+  cmd_info = grub_register_command("btrfs-info", grub_cmd_btrfs_info,
8e15ce
+				   "DEVICE",
8e15ce
+				   "Print BtrFS info about DEVICE.");
8e15ce
+  cmd_list_subvols = grub_register_extcmd("btrfs-list-subvols",
8e15ce
+					 grub_cmd_btrfs_list_subvols, 0,
8e15ce
+					 "[-p|-n] [-o var] DEVICE",
8e15ce
+					 "Print list of BtrFS subvolumes on "
8e15ce
+					 "DEVICE.", options);
8e15ce
+  grub_register_variable_hook ("btrfs_subvol", subvol_get_env,
8e15ce
+                               subvol_set_env);
8e15ce
+  grub_register_variable_hook ("btrfs_subvolid", subvolid_get_env,
8e15ce
+                               subvolid_set_env);
8e15ce
 }
8e15ce
 
8e15ce
 GRUB_MOD_FINI (btrfs)
8e15ce
 {
8e15ce
+  grub_register_variable_hook ("btrfs_subvol", NULL, NULL);
8e15ce
+  grub_register_variable_hook ("btrfs_subvolid", NULL, NULL);
8e15ce
+  grub_unregister_command (cmd_info);
8e15ce
+  grub_unregister_extcmd (cmd_list_subvols);
8e15ce
   grub_fs_unregister (&grub_btrfs_fs);
8e15ce
 }
8e15ce
+
8e15ce
+// vim: si et sw=2:
8e15ce
diff --git a/include/grub/btrfs.h b/include/grub/btrfs.h
b35c50
index 9d93fb6c18..234ad97677 100644
8e15ce
--- a/include/grub/btrfs.h
8e15ce
+++ b/include/grub/btrfs.h
8e15ce
@@ -29,6 +29,7 @@ enum
8e15ce
     GRUB_BTRFS_ITEM_TYPE_ROOT_ITEM = 0x84,
8e15ce
     GRUB_BTRFS_ITEM_TYPE_ROOT_BACKREF = 0x90,
8e15ce
     GRUB_BTRFS_ITEM_TYPE_DEVICE = 0xd8,
8e15ce
+    GRUB_BTRFS_ITEM_TYPE_ROOT_REF = 0x9c,
8e15ce
     GRUB_BTRFS_ITEM_TYPE_CHUNK = 0xe4
8e15ce
   };
8e15ce