On Sat, 6 Oct 2012 23:56:33 +0400
Andrew Vagin <[email protected]> wrote:
> Here is a stack trace of recursion:
> free_pid_ns(parent)
> put_pid_ns(parent)
> kref_put(&ns->kref, free_pid_ns);
> free_pid_ns
>
> This patch turns recursion into loops.
>
> pidns can be nested many times, so in case of recursion
> a simple user space program can provoke a kernel panic
> due to exceed of a kernel stack.
So we should backport this into earlier kernels.
> --- a/include/linux/kref.h
> +++ b/include/linux/kref.h
> @@ -95,6 +95,18 @@ static inline int kref_put(struct kref *kref, void
> (*release)(struct kref *kref)
> return kref_sub(kref, 1, release);
> }
>
> +/**
> + * kref_put - decrement refcount for object.
> + * @kref: object.
> + *
> + * Decrement the refcount.
> + * Return 1 if refcount is zero.
> + */
> +static inline int __kref_put(struct kref *kref)
> +{
> + return atomic_dec_and_test(&kref->refcount);
> +}
Greg might be interested in this.
It's a pretty specialised thing and perhaps it needs some stern words
in the description explaining when and why it should and shouldn't be
used.
I wonder if people might (ab)use this to avoid the "doesn't
have a release function" warning.
> static inline int kref_put_mutex(struct kref *kref,
> void (*release)(struct kref *kref),
> struct mutex *lock)
> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
> index 6144bab..b051fa6 100644
> --- a/kernel/pid_namespace.c
> +++ b/kernel/pid_namespace.c
> @@ -138,11 +138,19 @@ void free_pid_ns(struct kref *kref)
>
> ns = container_of(kref, struct pid_namespace, kref);
>
> - parent = ns->parent;
> - destroy_pid_namespace(ns);
> + while (1) {
> + parent = ns->parent;
> + destroy_pid_namespace(ns);
>
> - if (parent != NULL)
> - put_pid_ns(parent);
> + if (parent == &init_pid_ns)
> + break;
> +
> + /* kref_put cannot be used for avoiding recursion */
> + if (__kref_put(&parent->kref) == 0)
> + break;
> +
> + ns = parent;
> + }
> }
>
> void zap_pid_ns_processes(struct pid_namespace *pid_ns)
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
