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Subject[PATCH 31/73] union-mount: Create check_topmost_union_mnt() [ver #2]
Date
From: Valerie Aurora <vaurora@redhat.com>

check_topmost_union_mnt() checks that the topmost layer of a proposed
union mount is read-write, supports fallthrus and whiteouts, and isn't
mounted elsewhere.

Original-author: Valerie Aurora <vaurora@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
---

fs/namespace.c | 40 ++++++++++++++++++++++++++++++++++++++++
1 files changed, 40 insertions(+), 0 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 33aa310..bed9ccd 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1410,6 +1410,46 @@ static int invent_group_ids(struct mount *mnt, bool recurse)
return 0;
}

+/**
+ * check_topmost_union_mnt - mount-time checks for union mount
+ * @topmost_mnt: vfsmount of the topmost union filed system
+ * @mnt_flags: mount flags for the topmost mount
+ *
+ * Our readdir() solution of copying up directory entries requires
+ * that the topmost layer be writeable and support whiteouts and
+ * fallthrus. The topmost file system can't be mounted elsewhere
+ * because it's Too Hard(tm).
+ */
+static int check_topmost_union_mnt(struct vfsmount *topmost_mnt, int mnt_flags)
+{
+ struct super_block *sb = topmost_mnt->mnt_sb;
+
+#ifndef CONFIG_UNION_MOUNT
+ printk(KERN_INFO "union mount: not supported by the kernel\n");
+ return -EINVAL;
+#else
+ if (mnt_flags & MNT_READONLY)
+ return -EROFS;
+
+ if (atomic_read(&sb->s_active) != 1) {
+ printk(KERN_INFO "union mount: topmost fs mounted elsewhere\n");
+ return -EBUSY;
+ }
+
+ if (!(sb->s_flags & MS_WHITEOUT)) {
+ printk(KERN_INFO "union mount: whiteouts not supported by fs\n");
+ return -EINVAL;
+ }
+
+ if (!(sb->s_flags & MS_FALLTHRU)) {
+ printk(KERN_INFO "union mount: fallthrus not supported by fs\n");
+ return -EINVAL;
+ }
+
+ return 0;
+#endif
+}
+
/*
* @source_mnt : mount tree to be attached
* @nd : place the mount tree @source_mnt is attached


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