[SRU][Xenial][PATCH 1/2] ovl: check if all layers are on the same fs

[Daniel Axtens]

From: Amir Goldstein <amir73il at gmail.com>

BugLink: https://bugs.launchpad.net/bugs/1728489

Some features can only work when all layers are on the same fs.  Test this
condition during mount time, so features can check them later.

Add helper ovl_same_sb() to return the common super block in case all
layers are on the same fs.

Signed-off-by: Amir Goldstein <amir73il at gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi at redhat.com>
(backported from commit 7bcd74b98d7bac3e5149894caaf72de6989af7f0)
Signed-off-by: Daniel Axtens <daniel.axtens at canonical.com>
---
 fs/overlayfs/overlayfs.h |  1 +
 fs/overlayfs/super.c     | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 5ab50cd102af..fdd7ea8b96f3 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -164,6 +164,7 @@ static inline int ovl_do_whiteout(struct inode *dir, struct dentry *dentry,
 }
 
 const struct cred *ovl_override_creds(struct super_block *sb);
+struct super_block *ovl_same_sb(struct super_block *sb);
 enum ovl_path_type ovl_path_type(struct dentry *dentry);
 u64 ovl_dentry_version_get(struct dentry *dentry);
 void ovl_dentry_version_inc(struct dentry *dentry);
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 6c3077473fa8..9d8e5278a199 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -44,6 +44,8 @@ struct ovl_fs {
 	/* pathnames of lower and upper dirs, for show_options */
 	struct ovl_config config;
 	struct cred *mounter_creds;
+	/* sb common to all layers */
+	struct super_block *same_sb;
 };
 
 struct ovl_dir_cache;
@@ -430,6 +432,13 @@ static const struct dentry_operations ovl_reval_dentry_operations = {
 	.d_weak_revalidate = ovl_dentry_weak_revalidate,
 };
 
+struct super_block *ovl_same_sb(struct super_block *sb)
+{
+	struct ovl_fs *ofs = sb->s_fs_info;
+
+	return ofs->same_sb;
+}
+
 static struct ovl_entry *ovl_alloc_entry(unsigned int numlower)
 {
 	size_t size = offsetof(struct ovl_entry, lowerstack[numlower]);
@@ -1157,11 +1166,19 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 
 		ufs->lower_mnt[ufs->numlower] = mnt;
 		ufs->numlower++;
+
+		/* Check if all lower layers are on same sb */
+		if (i == 0)
+			ufs->same_sb = mnt->mnt_sb;
+		else if (ufs->same_sb != mnt->mnt_sb)
+			ufs->same_sb = NULL;
 	}
 
 	/* If the upper fs is nonexistent, we mark overlayfs r/o too */
 	if (!ufs->upper_mnt)
 		sb->s_flags |= MS_RDONLY;
+	else if (ufs->upper_mnt->mnt_sb != ufs->same_sb)
+		ufs->same_sb = NULL;
 
 	if (remote)
 		sb->s_d_op = &ovl_reval_dentry_operations;
-- 
2.11.0