Re: [RFC][PATCH 2/5] pid: Generalize task_active_pid_ns

Nadia Derbey
2008-11-27T13:11:33Z

On Tue, 2008-11-25 at 19:45 -0800, Sukadev Bhattiprolu wrote:
> >From 7f7caaa9d9014d7230dc0b1e0f75536f0b6ccdbf Mon Sep 17 00:00:00 2001
> From: Eric W. Biederman <ebiederm@xmission.com>
> Date: Mon, 10 Nov 2008 19:12:02 -0800
> Subject: [PATCH 2/5] pid: Generalize task_active_pid_ns
>
> Currently task_active_pid_ns is not safe to call after a
> task becomes a zombie and exit_task_namespaces is called,
> as nsproxy becomes NULL. By reading the pid namespace from
> the pid of the task we can trivially solve this problem at
> the cost of one extra memory read in what should be the
> same cacheline as we read the namespace from.
>
> When moving things around I have made task_active_pid_ns
> out of line because keeping it in pid_namespace.h would
> require adding includes of pid.h and sched.h that I
> don't think we want.
>
> This change does make task_active_pid_ns unsafe to call during
> copy_process until we attach a pid on the task_struct which
> seems to be a reasonable trade off.
>
> Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
> ---
> include/linux/pid_namespace.h | 6 +-----
> kernel/fork.c | 4 ++--
> kernel/pid.c | 6 ++++++
> 3 files changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
> index d82fe82..38d1032 100644
> --- a/include/linux/pid_namespace.h
> +++ b/include/linux/pid_namespace.h
> @@ -79,11 +79,7 @@ static inline void zap_pid_ns_processes(struct pid_namespace *ns)
> }
> #endif /* CONFIG_PID_NS */
>
> -static inline struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
> -{
> - return tsk->nsproxy->pid_ns;
> -}
> -
> +extern struct pid_namespace *task_active_pid_ns(struct task_struct *tsk);
> void pidhash_init(void);
> void pidmap_init(void);
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index f608356..28be39a 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1111,12 +1111,12 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>

Suka,

I'm wondering if it is still safe to keep the call to
task_active_pid_ns() in create_new_namespaces(): copy_namespaces() is
called a couple of lines above this sequence and it calls
create_new_namespaces(). So I don't see why you're now referencing
p->nsproxy->pid_ns here and not in create_new_namespaces()?

Regards,
Nadia

> if (pid != &init_struct_pid) {
> retval = -ENOMEM;
> - pid = alloc_pid(task_active_pid_ns(p));
> + pid = alloc_pid(p->nsproxy->pid_ns);
> if (!pid)
> goto bad_fork_cleanup_io;
>
> if (clone_flags & CLONE_NEWPID) {
> - retval = pid_ns_prepare_proc(task_active_pid_ns(p));
> + retval = pid_ns_prepare_proc(p->nsproxy->pid_ns);
> if (retval < 0)
> goto bad_fork_free_pid;
> }
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 064e76a..c5513fe 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -474,6 +474,12 @@ pid_t task_session_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
> }
> EXPORT_SYMBOL(task_session_nr_ns);
>
> +struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
> +{
> + return ns_of_pid(task_pid(tsk));
> +}
> +EXPORT_SYMBOL_GPL(task_active_pid_ns);
> +
> /*
> * Used by proc to find the first pid that is greater then or equal to nr.
> *
--
Nadia Derbey <Nadia.Derbey@bull.net>