Re: [PATCH testsuite] tests/task_setscheduler: add cgroup v2 case for moving proc to root cgroup

[Stephen Smalley]

On Tue, Jul 30, 2024 at 4:15 AM Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote:
>
> On Mon, Jul 29, 2024 at 1:55 PM Stephen Smalley
> <stephen.smalley.work@xxxxxxxxx> wrote:
> >
> > On Mon, Jul 29, 2024 at 6:29 AM Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote:
> > >
> > > On Sat, Jul 27, 2024 at 4:55 AM Gong Ruiqi <gongruiqi1@xxxxxxxxxx> wrote:
> > > >
> > > >
> > > > On 2024/07/26 21:43, Ondrej Mosnacek wrote:
> > > > > On Thu, Jul 18, 2024 at 2:34 PM Gong Ruiqi <gongruiqi1@xxxxxxxxxx> wrote:
> > > > >>
> > > > >>
> > > > >> On 2024/07/18 0:17, Paul Moore wrote:
> > > > >>> ...
> > > > >>>
> > > > >>> Where (what distribution, version, etc.) did you see this problem?
> > > > >>
> > > > >> The problem occurred when I ran the testsuite on Fedora 40 with v6.6
> > > > >> kernel, and it was the only failed testcase.
> > > > >
> > > > > Sorry for the delay... For some reason the test passes for me even
> > > > > with cgroup v2 only and without the patch (also when run from a
> > > > > regular user account with sudo). Do you happen to know what
> > > > > circumstances are needed for it to fail when the cgroup is not
> > > > > switched?
> > > > >
> > > >
> > > > As the comment in the script says, a process need to be in the root
> > > > cgroup in order to switch its scheduler policy to SCHED_{RR,FIFO}. So
> > > > maybe in your case the shell process is already in the root cgroup?
> > > >
> > > > In my case I need to ssh to a Fedora VM, and that makes my shell process
> > > > to be in a sub cgroup called /user.slice/.../XXX.scope (looks like some
> > > > systemd stuff). And since /sys/fs/cgroup/cpu/tasks doesn't exit in the
> > > > system with cgroup v2 only, the script skips moving the target process
> > > > to the root cgroup, and therefore the subsequent test fails.
> > >
> > > In my case I ssh as root and end up in
> > > /user.slice/user-0.slice/session-1.scope cgroup,
> > > /sys/fs/cgroup/cpu/tasks also doesn't exist, and yet the test passes.
> > > The same also happens when I ssh as a regular user (with cgroup
> > > /user.slice/user-1000.slice/session-3.scope) and run the testsuite
> > > with sudo. So there must be something more to it... maybe some kernel
> > > config or sysctl setting?
> >
> > As a further data point, I also have been unable to reproduce the
> > reported behavior.
> > That said, since tasks doesn't exist, isn't the passing test a false
> > negative (i.e. it isn't truly testing as intended)?
>
> I don't think it is. The test wants to verify that it is possible to
> change the scheduling policy with the SELinux permission and that it
> is not possible without it - and it tests basically identical
> conditions with the permission allowed and denied, so it indeed
> verifies it correctly. The cgroup switch is just a preparation step so
> that changing the policy to realtime policies can succeed at all. When
> the test fully passes without switching the cgroup, then it just means
> that the switch wasn't necessary for whatever reason.

Ok, thank you for clarifying.