linux-kernel - Re: [PATCH v3] smp: Fix a potential usage of stale nr

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Date:   Mon, 27 Jul 2020 13:43:03 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     peterz@...radead.org, tglx@...utronix.de, bigeasy@...utronix.de,
        namit@...are.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] smp: Fix a potential usage of stale nr_cpus


* Muchun Song <songmuchun@...edance.com> wrote:

> When the cmdline of "nr_cpus" is not valid, the @nr_cpu_ids is assigned
> a stale value. The nr_cpus is only valid when get_option() return 1. So
> check the return value to prevent this.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
> changelog in v3:
>  1) Return -EINVAL when the parameter is bogus. 
> 
> changelog in v2:
>  1) Rework the commit log.
>  2) Rework the return value check.
> 
>  kernel/smp.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index a5a66fc28f4e..0dacfcfcf00b 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -772,9 +772,13 @@ static int __init nrcpus(char *str)
>  {
>  	int nr_cpus;
>  
> -	get_option(&str, &nr_cpus);
> +	if (get_option(&str, &nr_cpus) != 1)
> +		return -EINVAL;
> +
>  	if (nr_cpus > 0 && nr_cpus < nr_cpu_ids)
>  		nr_cpu_ids = nr_cpus;
> +	else
> +		return -EINVAL;

Exactly what does 'not valid' mean, and why doesn't get_option() 
return -EINVAL in that case?

Thanks,

	Ingo