linux-kernel - Re: [PATCH RESEND] smp: Fix a potential usage of stale nr

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Message-ID: <87k0z1rj89.fsf@nanos.tec.linutronix.de>
Date:   Fri, 17 Jul 2020 22:15:34 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Muchun Song <songmuchun@...edance.com>, peterz@...radead.org,
        mingo@...nel.org, bigeasy@...utronix.de, namit@...are.com
Cc:     linux-kernel@...r.kernel.org,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH RESEND] smp: Fix a potential usage of stale nr_cpus

Muchun,

Muchun Song <songmuchun@...edance.com> writes:

> The get_option() maybe return 0, it means that the nr_cpus is
> not initialized.

Good catch, but see below.

> Then we will use the stale nr_cpus to initialize

We use nothing. Please describe your changes in technical neutral
language.

> the nr_cpu_ids. So fix it.

'So fix it.' is not much valuable information. What about:

    Check the return value to prevent this.

Hmm?

> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  kernel/smp.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/kernel/smp.c b/kernel/smp.c
> index 472c2b274c65..2a9a04acf123 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -634,8 +634,7 @@ static int __init nrcpus(char *str)
>  {
>  	int nr_cpus;
>  
> -	get_option(&str, &nr_cpus);
> -	if (nr_cpus > 0 && nr_cpus < nr_cpu_ids)
> +	if (get_option(&str, &nr_cpus) && nr_cpus > 0 && nr_cpus < nr_cpu_ids)
>  		nr_cpu_ids = nr_cpus;
>  
>  	return 0;

get_option() can return 0 - 3:

 *      0 - no int in string
 *      1 - int found, no subsequent comma
 *      2 - int found including a subsequent comma
 *      3 - hyphen found to denote a range

For this parameter exists only one valid format: '1 - int found, no comma',
right?

So why fixing it just half and why returning '0' aka success if the
parameter is bogus?

Thanks,

        tglx