linux-kernel - Re: [PATCH 2/3] btrfs: fix suspicious RCU in BTRFS_IOC_DEV

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Date:	Sun, 30 Nov 2014 10:11:41 -0500
From:	Pranith Kumar <bobby.prani+lkml@...il.com>
To:	Omar Sandoval <osandov@...ndov.com>
Cc:	Chris Mason <clm@...com>, Josef Bacik <jbacik@...com>,
	Joe Perches <joe@...ches.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 2/3] btrfs: fix suspicious RCU in BTRFS_IOC_DEV_INFO

On Sun, Nov 30, 2014 at 3:26 AM, Omar Sandoval <osandov@...ndov.com> wrote:
> A naked read of the value of an RCU pointer isn't safe. Put the whole access in
> an RCU critical section, not just the pointer dereference.
>
> Signed-off-by: Omar Sandoval <osandov@...ndov.com>

You can use rcu_access_pointer() in the if() condition check rather
than increasing the read critical section. We should try to keep the
critical section as small as possible.

Also, since we have rcu_str_deref() we can use that instead of
rcu_dereference() on device->name. Thoughts?

> ---
>  fs/btrfs/ioctl.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index ecdf68f..dd55844 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -2706,6 +2706,7 @@ static long btrfs_ioctl_dev_info(struct btrfs_root *root, void __user *arg)
>         struct btrfs_fs_devices *fs_devices = root->fs_info->fs_devices;
>         int ret = 0;
>         char *s_uuid = NULL;
> +       struct rcu_string *name;
>
>         di_args = memdup_user(arg, sizeof(*di_args));
>         if (IS_ERR(di_args))
> @@ -2726,17 +2727,16 @@ static long btrfs_ioctl_dev_info(struct btrfs_root *root, void __user *arg)
>         di_args->bytes_used = btrfs_device_get_bytes_used(dev);
>         di_args->total_bytes = btrfs_device_get_total_bytes(dev);
>         memcpy(di_args->uuid, dev->uuid, sizeof(di_args->uuid));
> -       if (dev->name) {
> -               struct rcu_string *name;
>
> -               rcu_read_lock();
> -               name = rcu_dereference(dev->name);
> +       rcu_read_lock();
> +       name = rcu_dereference(dev->name);
> +       if (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';
>         }
> +       rcu_read_unlock();
>
>  out:
>         mutex_unlock(&fs_devices->device_list_mutex);
> --
> 2.1.3
>
> --
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