On Fri, Mar 26, 2021 at 08:50:46PM +0800, Qu Wenruo wrote:
> [BUG]
> If restoring dumpped image into a new file, under most cases kernel will
> reject it:
>
> # mkfs.btrfs -f /dev/test/test
> # btrfs-image /dev/test/test /tmp/dump
> # btrfs-image -r /tmp/dump ~/test.img
> # mount ~/test.img /mnt/btrfs
> mount: /mnt/btrfs: wrong fs type, bad option, bad superblock on /dev/loop0,
> missing codepage or helper program, or other error.
> # dmesg -t | tail -n 7
> loop0: detected capacity change from 10592 to 0
> BTRFS info (device loop0): disk space caching is enabled
> BTRFS info (device loop0): has skinny extents
> BTRFS info (device loop0): flagging fs with big metadata feature
> BTRFS error (device loop0): device total_bytes should be at most 5423104 but
> found 10737418240
> BTRFS error (device loop0): failed to read chunk tree: -22
> BTRFS error (device loop0): open_ctree failed
>
> [CAUSE]
> When btrfs-image restores an image into a file, and the source image
> contains only single device, then we don't need to modify the
> chunk/device tree, as we can reuse the existing chunk/dev tree without
> any problem.
>
> This also means, for such restore, we also won't do any target file
> enlarge. This behavior itself is fine, as at that time, kernel won't
> check if the device is smaller than the device size recorded in device
> tree.
>
> But later kernel commit 3a160a933111 ("btrfs: drop never met disk total
> bytes check in verify_one_dev_extent") introduces new check on device
> size at mount time, rejecting any loop file which is smaller than the
> original device size.
>
> [FIX]
> Do extra file enlarge for single device restore.
>
> Reported-by: Nikolay Borisov <nbori...@suse.com>
> Signed-off-by: Qu Wenruo <w...@suse.com>
> ---
> image/main.c | 43 +++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 43 insertions(+)
>
> diff --git a/image/main.c b/image/main.c
> index 24393188e5e3..9933f69d0fdb 100644
> --- a/image/main.c
> +++ b/image/main.c
> @@ -2706,6 +2706,49 @@ static int restore_metadump(const char *input, FILE
> *out, int old_restore,
> close_ctree(info->chunk_root);
> if (ret)
> goto out;
> + } else {
> + struct btrfs_root *root;
> + struct stat st;
> + u64 dev_size;
> +
> + if (!info) {
> + root = open_ctree_fd(fileno(out), target, 0, 0);
> + if (!root) {
> + error("open ctree failed in %s", target);
> + ret = -EIO;
> + goto out;
> + }
> +
> + info = root->fs_info;
> +
> + dev_size = btrfs_stack_device_total_bytes(
> + &info->super_copy->dev_item);
> + close_ctree(root);
> + info = NULL;
> + } else {
> + dev_size = btrfs_stack_device_total_bytes(
> + &info->super_copy->dev_item);
> + }
> +
> + /*
> + * We don't need extra tree modification, but if the output is
> + * a file, we need to enlarge the output file so that
> + * newer kernel won't report error.
> + */
> + ret = fstat(fileno(out), &st);
> + if (ret < 0) {
> + error("failed to stat result image: %m");
> + ret = -errno;
> + goto out;
> + }
> + if (S_ISREG(st.st_mode)) {
> + ret = ftruncate64(fileno(out), dev_size);
This truncates the file unconditionally, so if the file is larger than
required, I don't think it's necessary to do it.
