Re: +core_pattern-set-core-helpers-root-and-namespace-to-crashing-process.patch added to -mm tree

From: Oleg Nesterov
Date: Mon Dec 17 2012 - 11:03:59 EST

On 12/17, Neil Horman wrote:
> On Mon, Dec 17, 2012 at 01:34:28PM +0100, Oleg Nesterov wrote:
> > @@ -455,6 +468,14 @@ static int umh_pipe_setup(struct subproc
> > /* and disallow core files too */
> > current->signal->rlim[RLIMIT_CORE] = (struct rlimit){1, 1};
> >
> > +
> > + if (cp->switch_ns) {
> > + get_fs_root(cp->cprocess->fs, &root);
> > + set_fs_root(current->fs, &root);
> > + switch_task_namespaces(current, cp->cprocess->nsproxy);
> >
> > How? You can't simply change ->nsproxy this way.
> >
> Why not? This is exactly how fork, exit, and setns use this call.

No. exit() does switch_task_namespaces(NULL), this is different.
fork() doesn't do this, and unshare/setns carefully creates the new ns.

> > If nothing else this breaks sys_getpid(), no?
> >
> hmm, I think you're inferring here that there is a chance that a pid allocated
> in the init namespace might conflict with another process who holds the same pid
> in another namespace?

No, I meant that sys_getpid() should always return 0 after this
switch_task_namespaces() if the coredumping task is not from the root

> Is there a way to switch all namespaces, except for the pid
> namespace?

Which exactly namespaces you want to change?

To be honest, I do not understand this patch at all. It seems that
you need to do something like sys_setns(). But if we do this, then
why we can't make core_pattern per-namespace?

Anyway, please ask Pavel and Eric, they should know better ;)

> > And a lot more problems, afaics. For example, this thread can continue
> > to run after, say, this cprocess->nsproxy->pid_ns was already destroyed.
> > zap_pid_ns_processes() obviously won't see this thread.
> >
> Hmm, I don't think so. The crashing process won't exit until the pipe reader is
> done, so the reference on the namespace should never decrement to zero.
> Actually I take that back. switch_task_namespaces doesn't add a ref count to
> the name space being switched to. So if the pipe reader doesn't exit
> immediately after closing the pipe, it may live on after the namespace is
> destroyed.


> It would seem a get_nsproxy call is needed here to hold an
> additional reference. Or do you think more is necessecary?

This can only pin ->nsproxy itself, this is not enough iirc.

Note that the exiting sub-init assumes that nobody else can use
ns->proc_mnt after zap_pid_ns_processes().


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