[PATCH 037/112] fs: support different root directories

[Ahmad Fatoum]

A root directory is a directory that has itself as the parent.
There is only one such directory, but for chroot support we will want to
stop path resolution at a non-root directory as well.

We do this by giving struct nameidata a d_root field, which serves as a
root directory for the lookup. As we initialize it to the global root
directory, this introduces no functional change.

Signed-off-by: Ahmad Fatoum <a.fat...@pengutronix.de>
---
 fs/fs.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/fs/fs.c b/fs/fs.c
index 54f38aa2e63b..7fc638451f13 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -1623,6 +1623,7 @@ struct nameidata {
        } *stack, internal[EMBEDDED_LEVELS];
        struct filename *name;
        struct inode    *link_inode;
+       struct dentry   *d_root;
 };
 
 struct filename {
@@ -1635,6 +1636,7 @@ static void set_nameidata(struct nameidata *p, struct 
filename *name)
        p->stack = p->internal;
        p->name = name;
        p->total_link_count = 0;
+       p->d_root = d_root;
 }
 
 static void path_get(const struct path *path)
@@ -1820,16 +1822,16 @@ static int lookup_fast(struct nameidata *nd, struct 
path *path)
  * Return 1 if we went up a level and 0 if we were already at the
  * root.
  */
-static int follow_up(struct path *path)
+static int follow_up(struct nameidata *nd)
 {
-       struct vfsmount *parent, *mnt = path->mnt;
+       struct path *path = &nd->path;
+       struct vfsmount *mnt = path->mnt;
        struct dentry *mountpoint;
 
-       parent = mnt->parent;
-       if (parent == mnt)
+       if (nd->d_root == path->dentry)
                return 0;
 
-       mntget(parent);
+       mntget(mnt->parent);
        mountpoint = dget(mnt->mountpoint);
        dput(path->dentry);
        path->dentry = mountpoint;
@@ -1871,7 +1873,7 @@ static int follow_dotdot(struct nameidata *nd)
                        break;
                }
 
-               if (!follow_up(&nd->path))
+               if (!follow_up(nd))
                        break;
        }
 
-- 
2.39.2