[PATCH v2 3/4] dcache: introduce d_genocide_safe()

[Ondrej Mosnacek <omosnace@xxxxxxxxxx>]

This patch adds a slightly modified variant of d_genocide() that works
safely on live (ramfs-like) trees. This function is needed for a safe
implementation of sel_remove_entries() in selinuxfs.

This new function differs from the original d_genocide in the following:
1. It locks the parent inode when traversing the dentries.
2. It first unhashes the dentry using __d_drop() before dropping the
   refcount and marking the dentry.
3. It does its business in the leave callback so that each dentry is
   unhashed after its children -- otherwise some dentries might never
   get traversed when d_walk() is restarted internally.

The combination of (1.) and (2.) is needed to avoid racing with
dcache_readdir(), which relies on the assumption that any
simple_positive() child dentry will not turn negative without locking
the parent inode for writing.

Signed-off-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
---
 fs/dcache.c            | 32 ++++++++++++++++++++++++++++++++
 include/linux/dcache.h |  1 +
 2 files changed, 33 insertions(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index 70afcb6e6892..f6d667120c1e 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -3142,6 +3142,38 @@ void d_genocide(struct dentry *parent)
 
 EXPORT_SYMBOL(d_genocide);
 
+static enum d_walk_ret d_genocide_safe_enter(void *data, struct dentry *dentry)
+{
+	struct dentry *root = data;
+
+	if (dentry != root && !simple_positive(dentry))
+		return D_WALK_SKIP;
+
+	return D_WALK_CONTINUE;
+}
+
+static void d_genocide_safe_leave(void *data, struct dentry *dentry)
+{
+	struct dentry *root = data;
+
+	if (dentry != root) {
+		__d_drop(dentry);
+
+		if (!(dentry->d_flags & DCACHE_GENOCIDE)) {
+			dentry->d_flags |= DCACHE_GENOCIDE;
+			dentry->d_lockref.count--;
+		}
+	}
+}
+
+void d_genocide_safe(struct dentry *parent)
+{
+	d_walk(parent, true, parent, d_genocide_safe_enter,
+	       d_genocide_safe_leave);
+}
+
+EXPORT_SYMBOL(d_genocide_safe);
+
 void d_tmpfile(struct dentry *dentry, struct inode *inode)
 {
 	inode_dec_link_count(inode);
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 9451011ac014..6d787c26e901 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -253,6 +253,7 @@ extern struct dentry * d_make_root(struct inode *);
 
 /* <clickety>-<click> the ramfs-type tree */
 extern void d_genocide(struct dentry *);
+extern void d_genocide_safe(struct dentry *parent);
 
 extern void d_tmpfile(struct dentry *, struct inode *);
 
-- 
2.21.0