Btrfs: use rcu to protect device->name

Al pointed out that we can just toss out the old name on a device and add a
new one arbitrarily, so anybody who uses device->name in printk could
possibly use free'd memory.  Instead of adding locking around all of this he
suggested doing it with RCU, so I've introduced a struct rcu_string that
does just that and have gone through and protected all accesses to
device->name that aren't under the uuid_mutex with rcu_read_lock().  This
protects us and I will use it for dealing with removing the device that we
used to mount the file system in a later patch.  Thanks,

Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <josef@redhat.com>

1 files changed, 56 insertions, 0 deletions

diff --git a/fs/btrfs/rcu-string.h b/fs/btrfs/rcu-string.h
new file mode 100644
index 000000000000..9e111e4576d4
--- /dev/null
+++ b/fs/btrfs/rcu-string.h
@@ -0,0 +1,56 @@
+/*
+ * Copyright (C) 2012 Red Hat.  All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License v2 as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this program; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 021110-1307, USA.
+ */
+
+struct rcu_string {
+        struct rcu_head rcu;
+        char str[0];
+};
+
+static inline struct rcu_string *rcu_string_strdup(const char *src, gfp_t mask)
+{
+        size_t len = strlen(src) + 1;
+        struct rcu_string *ret = kzalloc(sizeof(struct rcu_string) +
+                                         (len * sizeof(char)), mask);
+        if (!ret)
+                return ret;
+        strncpy(ret->str, src, len);
+        return ret;
+}
+
+static inline void rcu_string_free(struct rcu_string *str)
+{
+        if (str)
+                kfree_rcu(str, rcu);
+}
+
+#define printk_in_rcu(fmt, ...) do {    \
+        rcu_read_lock();                \
+        printk(fmt, __VA_ARGS__);       \
+        rcu_read_unlock();              \
+} while (0)
+
+#define printk_ratelimited_in_rcu(fmt, ...) do {        \
+        rcu_read_lock();                                \
+        printk_ratelimited(fmt, __VA_ARGS__);           \
+        rcu_read_unlock();                              \
+} while (0)
+
+#define rcu_str_deref(rcu_str) ({                               \
+        struct rcu_string *__str = rcu_dereference(rcu_str);    \
+        __str->str;                                             \
+})