[SRU][Zesty][PATCH 01/10] 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/ovl_entry.h | 2 ++
 fs/overlayfs/super.c     | 8 ++++++++
 fs/overlayfs/util.c      | 7 +++++++
 4 files changed, 18 insertions(+)

diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index f14fb7b56b09..ee05d77ad7cd 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -154,6 +154,7 @@ int ovl_want_write(struct dentry *dentry);
 void ovl_drop_write(struct dentry *dentry);
 struct dentry *ovl_workdir(struct dentry *dentry);
 const struct cred *ovl_override_creds(struct super_block *sb);
+struct super_block *ovl_same_sb(struct super_block *sb);
 struct ovl_entry *ovl_alloc_entry(unsigned int numlower);
 bool ovl_dentry_remote(struct dentry *dentry);
 bool ovl_dentry_weird(struct dentry *dentry);
diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
index d14bca1850d9..1894ce31c20b 100644
--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h
@@ -27,6 +27,8 @@ struct ovl_fs {
 	struct ovl_config config;
 	/* creds of process who forced instantiation of super block */
 	const struct cred *creator_cred;
+	/* sb common to all layers */
+	struct super_block *same_sb;
 };
 
 /* private information held for every overlayfs dentry */
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index ed2607904a1a..18c8bb3baff5 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -865,11 +865,19 @@ 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;
diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 952286f4826c..9889daaae78b 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -39,6 +39,13 @@ const struct cred *ovl_override_creds(struct super_block *sb)
 	return override_creds(ofs->creator_cred);
 }
 
+struct super_block *ovl_same_sb(struct super_block *sb)
+{
+	struct ovl_fs *ofs = sb->s_fs_info;
+
+	return ofs->same_sb;
+}
+
 struct ovl_entry *ovl_alloc_entry(unsigned int numlower)
 {
 	size_t size = offsetof(struct ovl_entry, lowerstack[numlower]);
-- 
2.11.0