Re: [PATCH 0/1] ns: introduce proc_get_ns_by_fd()

From: Oleg Nesterov
Date: Tue Sep 29 2015 - 12:46:37 EST

On 09/28, Eric W. Biederman wrote:
> Oleg Nesterov <oleg@xxxxxxxxxx> writes:
> > Honestly, I do not really like the new helper... I understand this
> > is subjective, so I won't insist. But how about 1/1? We do not need
> > fd/file at all. With this patch your sys_getvpid() can just use
> > proc_get_ns_by_fd(fd, CLONE_NEWPID) and put_pid_ns().
> >
> > Eric, what do you think?
> At some level I don't care this is not exposed to userspace.

I agree, this is minor. But you know, the kernel is already complicated
too much, we should try to simplify/cleanup everything we can ;)

> Of the existing uses several of them sleep, which unfortunately means we
> can not use rcu locking for everything. The network namespace ones wind
> up taking a reference to struct net because the have the legacy pid case
> to deal with. Which makes we can not use fdget for all callers either.

And that is why 1/1 adds proc_get_ns_by_fd/get_type which can also be used
by the network namespace.

> For this translate_pid rcu locking is sufficient, rcu locking is easy
> and doing any more than rcu locking just seems silly. So let me
> respectfully suggest.
> struct ns_common *ns_by_fd_rcu(int fd, int type)
> {
> struct files_struct *files = current->files;
> struct file *file;
> struct ns_common *ns;
> void *ret;
> file = fcheck_files(files, fd);
> if (!file)
> return ERR_PTR(-EBADF);
> if (file->f_mode & FMODE_PATH)
> return ERR_PTR(-EINVAL);
> if (file->f_op != &ns_file_operations)
> return ERR_PTR(-EINVAL);
> ns = get_proc_ns(file_inode(file));
> if (ns->ops->type != type)
> return ERR_PTR(-EINVAL);
> return ns;
> }

OK, I won't insist, this too looks better to me than proc_ns_fdget(&fd_ref).

And in any case fcheck_files() makes more sense than fdget(), somehow I did
not think about this when I sent 1/1.

Hmm. and after the quick look at cleanup_net() I can't understand whether
get_net_ns_by_fd() can use ns_by_fd_rcu() + maybe_get_net(to_net_ns()) or
not... Can it?


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