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, 10 insertions, 3 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 24b776c08d99..c5254dde1f0f 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -52,6 +52,7 @@
 #include "locking.h"
 #include "inode-map.h"
 #include "backref.h"
+#include "rcu-string.h"
 
 /* Mask out flags that are inappropriate for the given type of inode. */
 static inline __u32 btrfs_mask_flags(umode_t mode, __u32 flags)
@@ -1345,8 +1346,9 @@ static noinline int btrfs_ioctl_resize(struct btrfs_root *root,
         do_div(new_size, root->sectorsize);
         new_size *= root->sectorsize;
 
-        printk(KERN_INFO "btrfs: new size for %s is %llu\n",
-                device->name, (unsigned long long)new_size);
+        printk_in_rcu(KERN_INFO "btrfs: new size for %s is %llu\n",
+                      rcu_str_deref(device->name),
+                      (unsigned long long)new_size);
 
         if (new_size > old_size) {
                 trans = btrfs_start_transaction(root, 0);
@@ -2264,7 +2266,12 @@ static long btrfs_ioctl_dev_info(struct btrfs_root *root, void __user *arg)
         di_args->total_bytes = dev->total_bytes;
         memcpy(di_args->uuid, dev->uuid, sizeof(di_args->uuid));
         if (dev->name) {
-                strncpy(di_args->path, dev->name, sizeof(di_args->path));
+                struct rcu_string *name;
+
+                rcu_read_lock();
+                name = rcu_dereference(dev->name);
+                strncpy(di_args->path, name->str, sizeof(di_args->path));
+                rcu_read_unlock();
                 di_args->path[sizeof(di_args->path) - 1] = 0;
         } else {
                 di_args->path[0] = '\0';