afs: Implement @cell substitution handling

Implement @cell substitution handling such that if @cell is seen as a name
in a dynamic root mount, then the name of the root cell for that network
namespace will be substituted for @cell during lookup.

The substitution of @cell for the current net namespace is set by writing
the cell name to /proc/fs/afs/rootcell.  The value can be obtained by
reading the file.

For example:

	# mount -t afs none /kafs -o dyn
	# echo grand.central.org >/proc/fs/afs/rootcell
	# ls /kafs/@cell
	archive/  cvs/  doc/  local/  project/  service/  software/  user/  www/
	# cat /proc/fs/afs/rootcell
	grand.central.org

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

3 files changed, 89 insertions, 1 deletions

diff --git a/fs/afs/cell.c b/fs/afs/cell.c
index 721425b98b31..fdf4c36cff79 100644
--- a/fs/afs/cell.c
+++ b/fs/afs/cell.c
@@ -130,6 +130,8 @@ static struct afs_cell *afs_alloc_cell(struct afs_net *net,
                 _leave(" = -ENAMETOOLONG");
                 return ERR_PTR(-ENAMETOOLONG);
         }
+        if (namelen == 5 && memcmp(name, "@cell", 5) == 0)
+                return ERR_PTR(-EINVAL);
 
         _enter("%*.*s,%s", namelen, namelen, name, vllist);
 
diff --git a/fs/afs/dir.c b/fs/afs/dir.c
index 3ebd741b74d0..b073976db8d2 100644
--- a/fs/afs/dir.c
+++ b/fs/afs/dir.c
@@ -909,6 +909,55 @@ success:
 }
 
 /*
+ * Look up @cell in a dynroot directory.  This is a substitution for the
+ * local cell name for the net namespace.
+ */
+static struct dentry *afs_lookup_atcell(struct dentry *dentry)
+{
+        struct afs_cell *cell;
+        struct afs_net *net = afs_d2net(dentry);
+        struct dentry *ret;
+        unsigned int seq = 0;
+        char *name;
+        int len;
+
+        if (!net->ws_cell)
+                return ERR_PTR(-ENOENT);
+
+        ret = ERR_PTR(-ENOMEM);
+        name = kmalloc(AFS_MAXCELLNAME + 1, GFP_KERNEL);
+        if (!name)
+                goto out_p;
+
+        rcu_read_lock();
+        do {
+                read_seqbegin_or_lock(&net->cells_lock, &seq);
+                cell = rcu_dereference_raw(net->ws_cell);
+                if (cell) {
+                        len = cell->name_len;
+                        memcpy(name, cell->name, len + 1);
+                }
+        } while (need_seqretry(&net->cells_lock, seq));
+        done_seqretry(&net->cells_lock, seq);
+        rcu_read_unlock();
+
+        ret = ERR_PTR(-ENOENT);
+        if (!cell)
+                goto out_n;
+
+        ret = lookup_one_len(name, dentry->d_parent, len);
+
+        /* We don't want to d_add() the @cell dentry here as we don't want to
+         * the cached dentry to hide changes to the local cell name.
+         */
+
+out_n:
+        kfree(name);
+out_p:
+        return ret;
+}
+
+/*
  * Look up an entry in a dynroot directory.
  */
 static struct dentry *afs_dynroot_lookup(struct inode *dir, struct dentry *dentry,
@@ -929,6 +978,10 @@ static struct dentry *afs_dynroot_lookup(struct inode *dir, struct dentry *dentr
                 return ERR_PTR(-ENAMETOOLONG);
         }
 
+        if (dentry->d_name.len == 5 &&
+            memcmp(dentry->d_name.name, "@cell", 5) == 0)
+                return afs_lookup_atcell(dentry);
+
         inode = afs_try_auto_mntpt(dentry, dir);
         if (IS_ERR(inode)) {
                 ret = PTR_ERR(inode);
diff --git a/fs/afs/proc.c b/fs/afs/proc.c
index 15650cd59404..76f6df32cde0 100644
--- a/fs/afs/proc.c
+++ b/fs/afs/proc.c
@@ -334,7 +334,40 @@ inval:
 static ssize_t afs_proc_rootcell_read(struct file *file, char __user *buf,
                                       size_t size, loff_t *_pos)
 {
-        return 0;
+        struct afs_cell *cell;
+        struct afs_net *net = afs_proc2net(file);
+        unsigned int seq = 0;
+        char name[AFS_MAXCELLNAME + 1];
+        int len;
+
+        if (*_pos > 0)
+                return 0;
+        if (!net->ws_cell)
+                return 0;
+
+        rcu_read_lock();
+        do {
+                read_seqbegin_or_lock(&net->cells_lock, &seq);
+                len = 0;
+                cell = rcu_dereference_raw(net->ws_cell);
+                if (cell) {
+                        len = cell->name_len;
+                        memcpy(name, cell->name, len);
+                }
+        } while (need_seqretry(&net->cells_lock, seq));
+        done_seqretry(&net->cells_lock, seq);
+        rcu_read_unlock();
+
+        if (!len)
+                return 0;
+
+        name[len++] = '\n';
+        if (len > size)
+                len = size;
+        if (copy_to_user(buf, name, len) != 0)
+                return -EFAULT;
+        *_pos = 1;
+        return len;
 }
 
 /*