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

[Sargun Dhillon <sargun@sargun.me>]

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);
+	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 a8ee3ba4ebbd..2e473f8c75dd 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