Re: [PATCH] cpuset: fix cpuset_print_task_mems_allowed() vs rename() race

Al Viro
2013-01-25T07:26:40Z

On Fri, Jan 25, 2013 at 03:09:48PM +0800, Li Zefan wrote:
> rename() will change dentry->d_name. The result of this race can
> be worse than seeing partially rewritten name, but we might access
> a stale pointer because rename() will re-allocate memory to hold
> a longer name.
>
> It's safe in the protection of dentry->d_lock.
>
> Signed-off-by: Li Zefan <lizefan@huawei.com>
> ---
> kernel/cpuset.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
> index 16be7c9..b2476c2 100644
> --- a/kernel/cpuset.c
> +++ b/kernel/cpuset.c
> @@ -2606,8 +2606,12 @@ void cpuset_print_task_mems_allowed(struct task_struct *tsk)
>
> dentry = task_cs(tsk)->css.cgroup->dentry;
> spin_lock(&cpuset_buffer_lock);
> +
> + spin_lock(&dentry->d_lock);
> snprintf(cpuset_name, CPUSET_NAME_LEN,
> dentry ? (const char *)dentry->d_name.name : "/");

Ahem... Can dentry actually be NULL here? If not, this conditional
is bogus; otherwise, spin_lock(&dentry->d_lock) is going to blow up...