Re: [PATCH v5 01/18] cgroup/misc: Add per resource callbacks for CSS events

Jarkko Sakkinen
2023-10-02T22:47:45Z

On Wed Sep 27, 2023 at 4:56 AM EEST, Haitao Huang wrote:
> On Tue, 26 Sep 2023 08:13:18 -0500, Jarkko Sakkinen <jarkko@kernel.org>
> wrote:
>
> ...
> >> > >> /**
> >> > >> @@ -410,7 +429,14 @@ misc_cg_alloc(struct cgroup_subsys_state
> >> > >> *parent_css)
> >> > >> */
> >> > >> static void misc_cg_free(struct cgroup_subsys_state *css)
> >> > >> {
> >> > >> - kfree(css_misc(css));
> >> > >> + struct misc_cg *cg = css_misc(css);
> >> > >> + enum misc_res_type i;
> >> > >> +
> >> > >> + for (i = 0; i < MISC_CG_RES_TYPES; i++)
> >> > >> + if (cg->res[i].free)
> >> > >> + cg->res[i].free(cg);
> >> > >> +
> >> > >> + kfree(cg);
> >> > >> }
> >> > >>
> >> > >> /* Cgroup controller callbacks */
> >> > >> --
> >> > >> 2.25.1
> >> > >
> >> > > Since the only existing client feature requires all callbacks,
> >> should
> >> > > this not have that as an invariant?
> >> > >
> >> > > I.e. it might be better to fail unless *all* ops are non-nil (e.g.
> >> to
> >> > > catch issues in the kernel code).
> >> > >
> >> >
> >> > These callbacks are chained from cgroup_subsys, and they are defined
> >> > separately so it'd be better follow the same pattern. Or change
> >> together
> >> > with cgroup_subsys if we want to do that. Reasonable?
> >>
> >> I noticed this one later:
> >>
> >> It would better to create a separate ops struct and declare the instance
> >> as const at minimum.
> >>
> >> Then there is no need for dynamic assigment of ops and all that is in
> >> rodata. This is improves both security and also allows static analysis
> >> bit better.
> >>
> >> Now you have to dynamically trace the struct instance, e.g. in case of
> >> a bug. If this one done, it would be already in the vmlinux.
> >I.e. then in the driver you can have static const struct declaration
> > with *all* pointers pre-assigned.
> >
> > Not sure if cgroups follows this or not but it is *objectively*
> > better. Previous work is not always best possible work...
> >
>
> IIUC, like vm_ops field in vma structs. Although function pointers in
> vm_ops are assigned statically, but you still need dynamically assign
> vm_ops for each instance of vma.
>
> So the code will look like this:
>
> if (parent_cg->res[i].misc_ops && parent_cg->res[i].misc_ops->alloc)
> {
> ...
> }
>
> I don't see this is the pattern used in cgroups and no strong opinion
> either way.
>
> TJ, do you have preference on this?

I do have strong opinion on this. In the client side we want as much
things declared statically as we can because it gives more tools for
statical analysis.

I don't want to see dynamic assignments in the SGX driver, when they
are not actually needed, no matter things are done in cgroups.

> Thanks
> Haitao

BR, Jarkko