Re: [PATCHv2 7/7] cgroup: mount cgroupns-root when inside non-init cgroupns

On Mon, Nov 3, 2014 at 4:17 PM, Andy Lutomirski [off-list ref] wrote:

On Mon, Nov 3, 2014 at 4:12 PM, Aditya Kali [off-list ref] wrote:

quoted

On Mon, Nov 3, 2014 at 3:48 PM, Andy Lutomirski [off-list ref] wrote:

quoted

On Mon, Nov 3, 2014 at 3:23 PM, Aditya Kali [off-list ref] wrote:

quoted

On Mon, Nov 3, 2014 at 3:15 PM, Andy Lutomirski [off-list ref] wrote:

quoted

On Mon, Nov 3, 2014 at 3:12 PM, Aditya Kali [off-list ref] wrote:

quoted

On Fri, Oct 31, 2014 at 5:07 PM, Andy Lutomirski [off-list ref] wrote:

quoted

On Fri, Oct 31, 2014 at 12:19 PM, Aditya Kali [off-list ref] wrote:

quoted

        if (opts->flags & CGRP_ROOT_SANE_BEHAVIOR) {
                pr_warn("sane_behavior: this is still under development and its behaviors will change, proceed at your own risk\n");
-               if (nr_opts != 1) {
+               if (nr_opts > 1) {
                        pr_err("sane_behavior: no other mount options allowed\n");
                        return -EINVAL;

This looks wrong.  But, if you make the change above, then it'll be right.

It would have been nice if simple 'mount -t cgroup cgroup <mnt>' from
cgroupns does the right thing automatically.

This is a debatable point, but it's not what I meant.  Won't your code
let 'mount -t cgroup -o one_evil_flag cgroup mountpoint' through?

I don't think so. This check "if (nr_opts > 1)" is nested under "if
(opts->flags & CGRP_ROOT_SANE_BEHAVIOR)". So we know that there is
atleast 1 option ('__DEVEL__sane_behavior') present (implicit or not).
Addition of 'one_evil_flag' will make nr_opts = 2 and result in EINVAL
here.

But the implicit __DEVEL__sane_behavior doesn't increment nr_opts, right?

Yes. Hence this change makes sure that we don't return EINVAL when
nr_opts == 0 or nr_opts == 1 :)
That way, both of the following are equivalent when inside non-init cgroupns:

(1) $ mount -t cgroup -o __DEVEL__sane_behavior cgroup mountpoint
(2) $ mount -t cgroup cgroup mountpoint

Any other mount option will trigger the error here.

I still don't get it.  Can you walk me through why mount -o
some_other_option -t cgroup cgroup mountpoint causes -EINVAL?

Argh! You are right. I was totally convinced that this works. But it
clearly doesn't if you specify 1 legit mount option. I wanted to make
it work for both cases (1) and (2) above. But then this check will
have to be changed :(
Sorry about the back and forth. I am just going to make it return
EINVAL if __DEVEL_sane_behavior is not specified as suggested in the
beginning.

--Andy

-- 
Aditya