[Devel] [PATCH rh7 2/4] overlayfs: introduce d_select_inode dentry operation

[Maxim Patlasov]

The patch is simplified defensive backport of upstream commit
4bacc9c9234c7c8eec44f5ed4e960d9f96fa0f01:

>    overlayfs: Make f_path always point to the overlay and f_inode to the underlay
>
>    Make file->f_path always point to the overlay dentry so that the path in
>    /proc/pid/fd is correct and to ensure that label-based LSMs have access to the
>    overlay as well as the underlay (path-based LSMs probably don't need it).
>    ...
>    Signed-off-by: David Howells <dhowells at redhat.com>
>    Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>

Original patch is prone to errors because other parts of linux kernel are not
prepared to such a change of semantics of f_path and f_inode. So, we only backport
simplified d_select_inode keeping f_path and f_inode intact.

https://jira.sw.ru/browse/PSBM-47981

Signed-off-by: Maxim Patlasov <mpatlasov at virtuozzo.com>
---
 fs/dcache.c              |    5 ++++-
 fs/overlayfs/inode.c     |   15 +++++++++++++++
 fs/overlayfs/overlayfs.h |    1 +
 fs/overlayfs/super.c     |    2 ++
 include/linux/dcache.h   |    2 ++
 5 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 6433814..7db8aef 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1635,7 +1635,8 @@ void d_set_d_op(struct dentry *dentry, const struct dentry_operations *op)
 				DCACHE_OP_COMPARE	|
 				DCACHE_OP_REVALIDATE	|
 				DCACHE_OP_WEAK_REVALIDATE	|
-				DCACHE_OP_DELETE ));
+				DCACHE_OP_DELETE	|
+				DCACHE_OP_SELECT_INODE));
 	dentry->d_op = op;
 	if (!op)
 		return;
@@ -1651,6 +1652,8 @@ void d_set_d_op(struct dentry *dentry, const struct dentry_operations *op)
 		dentry->d_flags |= DCACHE_OP_DELETE;
 	if (op->d_prune)
 		dentry->d_flags |= DCACHE_OP_PRUNE;
+	if (op->d_select_inode)
+		dentry->d_flags |= DCACHE_OP_SELECT_INODE;
 
 }
 EXPORT_SYMBOL(d_set_d_op);
diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 1d2c32f..5fe7acf 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -368,6 +368,21 @@ out:
 	return err;
 }
 
+struct inode *ovl_d_select_inode(struct dentry *dentry)
+{
+	struct path realpath;
+
+	if (d_is_dir(dentry))
+		return d_backing_inode(dentry);
+
+	ovl_path_real(dentry, &realpath);
+
+	if (realpath.dentry->d_flags & DCACHE_OP_SELECT_INODE)
+		return realpath.dentry->d_op->d_select_inode(realpath.dentry);
+
+	return d_backing_inode(realpath.dentry);
+}
+
 static const struct inode_operations_wrapper ovl_file_inode_operations = {
 	.ops = {
 	.setattr	= ovl_setattr,
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 45d183b..8da9684 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -173,6 +173,7 @@ ssize_t ovl_getxattr(struct dentry *dentry, const char *name,
 		     void *value, size_t size);
 ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size);
 int ovl_removexattr(struct dentry *dentry, const char *name);
+struct inode *ovl_d_select_inode(struct dentry *dentry);
 
 struct inode *ovl_new_inode(struct super_block *sb, umode_t mode,
 			    struct ovl_entry *oe);
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index d5c57b4..24ec90b 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -316,10 +316,12 @@ static int ovl_dentry_weak_revalidate(struct dentry *dentry, unsigned int flags)
 
 static const struct dentry_operations ovl_dentry_operations = {
 	.d_release = ovl_dentry_release,
+	.d_select_inode = ovl_d_select_inode,
 };
 
 static const struct dentry_operations ovl_reval_dentry_operations = {
 	.d_release = ovl_dentry_release,
+	.d_select_inode = ovl_d_select_inode,
 	.d_revalidate = ovl_dentry_revalidate,
 	.d_weak_revalidate = ovl_dentry_weak_revalidate,
 };
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 2f6e4d8..267dbc6 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -160,6 +160,7 @@ struct dentry_operations {
 	char *(*d_dname)(struct dentry *, char *, int);
 	struct vfsmount *(*d_automount)(struct path *);
 	int (*d_manage)(struct dentry *, bool);
+	struct inode *(*d_select_inode)(struct dentry *);
 } ____cacheline_aligned;
 
 /*
@@ -221,6 +222,7 @@ struct dentry_operations {
 #define DCACHE_FILE_TYPE		0x04000000 /* Other file type */
 
 #define DCACHE_MAY_FREE			0x00800000
+#define DCACHE_OP_SELECT_INODE		0x20000000 /* Unioned entry: dcache op selects inode */
 
 extern seqlock_t rename_lock;