pstore: Refactor pstorefs record list removal

The "unlink" handling should perform list removal (which can also make
sure records don't get double-erased), and the "evict" handling should
be responsible only for memory freeing.

Link: https://lore.kernel.org/lkml/20200506152114.50375-8-keescook@chromium.org/Signed-off-by: Kees Cook <keescook@chromium.org>

fs/pstore/inode.c

@@ -177,10 +177,21 @@ static int pstore_unlink(struct inode *dir, struct dentry *dentry)
 {
 	struct pstore_private *p = d_inode(dentry)->i_private;
 	struct pstore_record *record = p->record;
+	int rc = 0;
 
 	if (!record->psi->erase)
 		return -EPERM;
 
+	/* Make sure we can't race while removing this file. */
+	mutex_lock(&records_list_lock);
+	if (!list_empty(&p->list))
+		list_del_init(&p->list);
+	else
+		rc = -ENOENT;
+	mutex_unlock(&records_list_lock);
+	if (rc)
+		return rc;
+
 	mutex_lock(&record->psi->read_mutex);
 	record->psi->erase(record);
 	mutex_unlock(&record->psi->read_mutex);
@@ -193,12 +204,7 @@ static void pstore_evict_inode(struct inode *inode)
 	struct pstore_private	*p = inode->i_private;
 
 	clear_inode(inode);
-	if (p) {
-		mutex_lock(&records_list_lock);
-		list_del(&p->list);
-		mutex_unlock(&records_list_lock);
-		free_pstore_private(p);
-	}
+	free_pstore_private(p);
 }
 
 static const struct inode_operations pstore_dir_inode_operations = {
@@ -417,6 +423,7 @@ static void pstore_kill_sb(struct super_block *sb)
 {
 	kill_litter_super(sb);
 	pstore_sb = NULL;
+	INIT_LIST_HEAD(&records_list);
 }
 
 static struct file_system_type pstore_fs_type = {