afs: Display manually added cells in dynamic root mount

Alter the dynroot mount so that cells created by manipulation of
/proc/fs/afs/cells and /proc/fs/afs/rootcell and by specification of a root
cell as a module parameter will cause directories for those cells to be
created in the dynamic root superblock for the network namespace[*].

To this end:

 (1) Only one dynamic root superblock is now created per network namespace
     and this is shared between all attempts to mount it.  This makes it
     easier to find the superblock to modify.

 (2) When a dynamic root superblock is created, the list of cells is walked
     and directories created for each cell already defined.

 (3) When a new cell is added, if a dynamic root superblock exists, a
     directory is created for it.

 (4) When a cell is destroyed, the directory is removed.

 (5) These directories are created by calling lookup_one_len() on the root
     dir which automatically creates them if they don't exist.

[*] Inasmuch as network namespaces are currently supported here.

Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 29 insertions, 0 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 186bd2464fd5..2e0a1c5729f1 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2464,6 +2464,35 @@ static int lookup_one_len_common(const char *name, struct dentry *base,
 }
 
 /**
+ * try_lookup_one_len - filesystem helper to lookup single pathname component
+ * @name:       pathname component to lookup
+ * @base:       base directory to lookup from
+ * @len:        maximum length @len should be interpreted to
+ *
+ * Look up a dentry by name in the dcache, returning NULL if it does not
+ * currently exist.  The function does not try to create a dentry.
+ *
+ * Note that this routine is purely a helper for filesystem usage and should
+ * not be called by generic code.
+ *
+ * The caller must hold base->i_mutex.
+ */
+struct dentry *try_lookup_one_len(const char *name, struct dentry *base, int len)
+{
+        struct qstr this;
+        int err;
+
+        WARN_ON_ONCE(!inode_is_locked(base->d_inode));
+
+        err = lookup_one_len_common(name, base, len, &this);
+        if (err)
+                return ERR_PTR(err);
+
+        return lookup_dcache(&this, base, 0);
+}
+EXPORT_SYMBOL(try_lookup_one_len);
+
+/**
  * lookup_one_len - filesystem helper to lookup single pathname component
  * @name:       pathname component to lookup
  * @base:       base directory to lookup from