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

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Message-Id: <CVYBI76N4PTF.38BQ9KIBIOGEH@seitikki>
Date:   Tue, 03 Oct 2023 01:47:34 +0300
From:   "Jarkko Sakkinen" <jarkko@...nel.org>
To:     "Haitao Huang" <haitao.huang@...ux.intel.com>,
        <dave.hansen@...ux.intel.com>, <tj@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-sgx@...r.kernel.org>,
        <x86@...nel.org>, <cgroups@...r.kernel.org>, <tglx@...utronix.de>,
        <mingo@...hat.com>, <bp@...en8.de>, <hpa@...or.com>,
        <sohil.mehta@...el.com>
Cc:     <zhiquan1.li@...el.com>, <kristen@...ux.intel.com>,
        <seanjc@...gle.com>, <zhanb@...rosoft.com>,
        <anakrish@...rosoft.com>, <mikko.ylinen@...ux.intel.com>,
        <yangjie@...rosoft.com>
Subject: Re: [PATCH v5 01/18] cgroup/misc: Add per resource callbacks for
 CSS events

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@...nel.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