Re: [PATCH v1 3/3] overlay: Add rudimentary checking of writeback errseq on volatile remount

[Amir Goldstein <amir73il@gmail.com>]

On Wed, Nov 25, 2020 at 12:46 PM Sargun Dhillon <sargun@sargun.me> wrote:
>
> Volatile remounts validate the following at the moment:
>  * Has the module been reloaded / the system rebooted
>  * Has the workdir been remounted
>
> This adds a new check for errors detected via the superblock's
> errseq_t. At mount time, the errseq_t is snapshotted to disk,
> and upon remount it's re-verified. This allows for kernel-level
> detection of errors without forcing userspace to perform a
> sync and allows for the hidden detection of writeback errors.
>

Looks fine as long as you verify that the reuse is also volatile.

Care to also add the alleged issues that Vivek pointed out with existing
volatile mount to the documentation? (unless Vivek intends to do fix those)

> Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> ---
>  fs/overlayfs/overlayfs.h | 1 +
>  fs/overlayfs/readdir.c   | 6 ++++++
>  fs/overlayfs/super.c     | 1 +
>  3 files changed, 8 insertions(+)
>
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index de694ee99d7c..e8a711953b64 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -85,6 +85,7 @@ struct ovl_volatile_info {
>          */
>         uuid_t          ovl_boot_id;    /* Must stay first member */
>         u64             s_instance_id;
> +       errseq_t        errseq; /* Implemented as a u32 */
>  } __packed;
>
>  /*
> diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> index 4e3e2bc3ea43..2bb0641ecbbd 100644
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -1109,6 +1109,12 @@ static int ovl_verify_volatile_info(struct ovl_fs *ofs,
>                 return -EINVAL;
>         }
>
> +       err = errseq_check(&volatiledir->d_sb->s_wb_err, info.errseq);
> +       if (err) {
> +               pr_debug("Workdir filesystem reports errors: %d\n", err);
> +               return -EINVAL;
> +       }
> +
>         return 1;
>  }
>
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 9a1b07907662..49dee41ec125 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -1248,6 +1248,7 @@ static int ovl_set_volatile_info(struct ovl_fs *ofs, struct dentry *volatiledir)
>         int err;
>         struct ovl_volatile_info info = {
>                 .s_instance_id = volatiledir->d_sb->s_instance_id,
> +               .errseq = errseq_sample(&volatiledir->d_sb->s_wb_err),
>         };
>
>         uuid_copy(&info.ovl_boot_id, &ovl_boot_id);
> --
> 2.25.1
>