afs: Fix afs_atcell_get_link() to handle RCU pathwalk

The ->get_link() method may be entered under RCU pathwalk conditions (in
which case, the dentry pointer is NULL).  This is not taken account of by
afs_atcell_get_link() and lockdep will complain when it tries to lock an
rwsem.

Fix this by marking net->ws_cell as __rcu and using RCU access macros on it
and by making afs_atcell_get_link() just return a pointer to the name in
RCU pathwalk without taking net->cells_lock or a ref on the cell as RCU
will protect the name storage (the cell is already freed via call_rcu()).

Fixes: 30bca65 ("afs: Make /afs/@cell and /afs/.@cell symlinks")
Reported-by: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20250310094206.801057-2-dhowells@redhat.com/ # v4

Author: dhowells

fs/afs/cell.c

@@ -64,7 +64,8 @@ static struct afs_cell *afs_find_cell_locked(struct afs_net *net,
 		return ERR_PTR(-ENAMETOOLONG);
 
 	if (!name) {
-		cell = net->ws_cell;
+		cell = rcu_dereference_protected(net->ws_cell,
+						 lockdep_is_held(&net->cells_lock));
 		if (!cell)
 			return ERR_PTR(-EDESTADDRREQ);
 		goto found;
@@ -388,8 +389,8 @@ int afs_cell_init(struct afs_net *net, const char *rootcell)
 	/* install the new cell */
 	down_write(&net->cells_lock);
 	afs_see_cell(new_root, afs_cell_trace_see_ws);
-	old_root = net->ws_cell;
-	net->ws_cell = new_root;
+	old_root = rcu_replace_pointer(net->ws_cell, new_root,
+				       lockdep_is_held(&net->cells_lock));
 	up_write(&net->cells_lock);
 
 	afs_unuse_cell(net, old_root, afs_cell_trace_unuse_ws);
@@ -945,8 +946,8 @@ void afs_cell_purge(struct afs_net *net)
 	_enter("");
 
 	down_write(&net->cells_lock);
-	ws = net->ws_cell;
-	net->ws_cell = NULL;
+	ws = rcu_replace_pointer(net->ws_cell, NULL,
+				 lockdep_is_held(&net->cells_lock));
 	up_write(&net->cells_lock);
 	afs_unuse_cell(net, ws, afs_cell_trace_unuse_ws);
 

fs/afs/dynroot.c

@@ -314,12 +314,23 @@ static const char *afs_atcell_get_link(struct dentry *dentry, struct inode *inod
 	const char *name;
 	bool dotted = vnode->fid.vnode == 3;
 
-	if (!net->ws_cell)
+	if (!dentry) {
+		/* We're in RCU-pathwalk. */
+		cell = rcu_dereference(net->ws_cell);
+		if (dotted)
+			name = cell->name - 1;
+		else
+			name = cell->name;
+		/* Shouldn't need to set a delayed call. */
+		return name;
+	}
+
+	if (!rcu_access_pointer(net->ws_cell))
 		return ERR_PTR(-ENOENT);
 
 	down_read(&net->cells_lock);
 
-	cell = net->ws_cell;
+	cell = rcu_dereference_protected(net->ws_cell, lockdep_is_held(&net->cells_lock));
 	if (dotted)
 		name = cell->name - 1;
 	else

fs/afs/internal.h

@@ -287,7 +287,7 @@ struct afs_net {
 
 	/* Cell database */
 	struct rb_root		cells;
-	struct afs_cell		*ws_cell;
+	struct afs_cell __rcu	*ws_cell;
 	struct work_struct	cells_manager;
 	struct timer_list	cells_timer;
 	atomic_t		cells_outstanding;

fs/afs/proc.c

@@ -206,7 +206,7 @@ static int afs_proc_rootcell_show(struct seq_file *m, void *v)
 
 	net = afs_seq2net_single(m);
 	down_read(&net->cells_lock);
-	cell = net->ws_cell;
+	cell = rcu_dereference_protected(net->ws_cell, lockdep_is_held(&net->cells_lock));
 	if (cell)
 		seq_printf(m, "%s\n", cell->name);
 	up_read(&net->cells_lock);
@@ -242,7 +242,7 @@ static int afs_proc_rootcell_write(struct file *file, char *buf, size_t size)
 
 	ret = -EEXIST;
 	inode_lock(file_inode(file));
-	if (!net->ws_cell)
+	if (!rcu_access_pointer(net->ws_cell))
 		ret = afs_cell_init(net, buf);
 	else
 		printk("busy\n");