netdev - Re: [PATCH net-next 1/2] net/iucv: Avoid explicit cpumask var allocation on stack

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Message-ID: <4E49057A4198779C+Zged+hXhxE4GksiL@centos8>
Date: Sat, 30 Mar 2024 13:07:06 +0800
From: Dawei Li <dawei.li@...ngroup.cn>
To: Eric Dumazet <edumazet@...gle.com>
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
	ioana.ciornei@....com, wintera@...ux.ibm.com,
	twinkler@...ux.ibm.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH net-next 1/2] net/iucv: Avoid explicit cpumask var
 allocation on stack

Hi Eric,

On Fri, Mar 29, 2024 at 02:21:28PM +0100, Eric Dumazet wrote:
> On Fri, Mar 29, 2024 at 11:57 AM Dawei Li <dawei.li@...ngroup.cn> wrote:
> >
> > For CONFIG_CPUMASK_OFFSTACK=y kernel, explicit allocation of cpumask
> > variable on stack is not recommended since it can cause potential stack
> > overflow.
> >
> > Instead, kernel code should always use *cpumask_var API(s) to allocate
> > cpumask var in config-neutral way, leaving allocation strategy to
> > CONFIG_CPUMASK_OFFSTACK.
> >
> > Use *cpumask_var API(s) to address it.
> >
> > Signed-off-by: Dawei Li <dawei.li@...ngroup.cn>
> > ---
> >  net/iucv/iucv.c | 37 ++++++++++++++++++++++++++-----------
> >  1 file changed, 26 insertions(+), 11 deletions(-)
> >
> > diff --git a/net/iucv/iucv.c b/net/iucv/iucv.c
> > index a4ab615ca3e3..b51f46ec32f9 100644
> > --- a/net/iucv/iucv.c
> > +++ b/net/iucv/iucv.c
> > @@ -520,14 +520,19 @@ static void iucv_setmask_mp(void)
> >   */
> >  static void iucv_setmask_up(void)
> >  {
> > -       cpumask_t cpumask;
> > +       cpumask_var_t cpumask;
> >         int cpu;
> >
> > +       if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
> > +               return;
> 
> This can not be right. iucv_setmask_up() is not supposed to fail.
> 
> Since iucv_setmask_up() is only called with iucv_register_mutex held,
> you could simply add a 'static' for @cpumask variable.

Correct, iucv_register_mutex is a global lock and can serialize access
on static cpumask var.

I will respin V2 as you suggested.

Thanks,

    Dawei
> 
> 
> 
> > +
> >         /* Disable all cpu but the first in cpu_irq_cpumask. */
> > -       cpumask_copy(&cpumask, &iucv_irq_cpumask);
> > -       cpumask_clear_cpu(cpumask_first(&iucv_irq_cpumask), &cpumask);
> > -       for_each_cpu(cpu, &cpumask)
> > +       cpumask_copy(cpumask, &iucv_irq_cpumask);
> > +       cpumask_clear_cpu(cpumask_first(&iucv_irq_cpumask), cpumask);
> > +       for_each_cpu(cpu, cpumask)
> >                 smp_call_function_single(cpu, iucv_block_cpu, NULL, 1);
> > +
> > +       free_cpumask_var(cpumask);
> >  }
>