Re: [PATCH v2 4/4] overlay: Add rudimentary checking of writeback errseq on volatile remount

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Nov 27, 2020 at 01:20:58AM -0800, Sargun Dhillon 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.
> 
> Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-unionfs@vger.kernel.org
> Cc: Miklos Szeredi <miklos@szeredi.hu>
> Cc: Amir Goldstein <amir73il@gmail.com>
> Cc: Vivek Goyal <vgoyal@redhat.com>
> ---
>  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 7b66fbb20261..5795b28bb4cf 100644
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -1117,6 +1117,12 @@ static int ovl_verify_volatile_info(struct ovl_fs *ofs,
>  		return -EINVAL;
>  	}
>  
> +	err = errseq_check(&volatiledir->d_sb->s_wb_err, info.errseq);

Might be a stupid question. Will ask anyway.

But what protects against wrapping of counter. IOW, Say we stored info.errseq
value as A. It is possible that bunch of errors occurred and at remount
time ->s_wb_err is back to A and we pass the check. (Despite the fact lots
of errors have occurred since we sampled).

Thanks
Vivek