linux-kernel - Re: [PATCH -next] cpufreq: Fix reserved space in cpufreq_show

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Date:   Tue, 24 May 2022 12:40:41 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Yi Yang <yiyang13@...wei.com>
Cc:     rafael@...nel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] cpufreq: Fix reserved space in cpufreq_show_cpus()

On 21-05-22, 14:35, Yi Yang wrote:
> Function scnprintf() would reserve space for the trailing '\0' and return 
> value is the number of characters written into buf not including the 
> trailing '\0'. internally meaning the next scnprintf() would write begin 
> the trailing '\0'. The code specifying "PAGE_SIZE - i - 2" here is trying 
> to reserve space for "\n\0" which would cause scnprintf() to reserve an 
> additional byte making the tail of the buf looks like this: "\n\0\0". 
> Thus. we should reserve only the space for one '\0'. passing in 
> "PAGE_SIZE - i - 1".
> 
> Additionally, each iteration would replace the trailing '\0' from the last 
> iteration with a space, and append 4 additional bytes to the string making 
> it a total of 5 additional bytes. That means we should stop printing into 
> the buffer if the remaining size is less than 7 bytes(1 for the ' ', 4 for 
> the %u and 2 for the tailing "\n\0")
> 
> Signed-off-by: Yi Yang <yiyang13@...wei.com>
> ---
>  drivers/cpufreq/cpufreq.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 1f6667ce43bd..60c005c9961e 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -844,9 +844,9 @@ ssize_t cpufreq_show_cpus(const struct cpumask *mask, char *buf)
>  
>  	for_each_cpu(cpu, mask) {
>  		if (i)
> -			i += scnprintf(&buf[i], (PAGE_SIZE - i - 2), " ");
> -		i += scnprintf(&buf[i], (PAGE_SIZE - i - 2), "%u", cpu);
> -		if (i >= (PAGE_SIZE - 5))
> +			i += scnprintf(&buf[i], (PAGE_SIZE - i - 1), " ");
> +		i += scnprintf(&buf[i], (PAGE_SIZE - i - 1), "%u", cpu);
> +		if (i >= (PAGE_SIZE - 6))
>  			break;
>  	}
>  	i += sprintf(&buf[i], "\n");

Acked-by: Viresh Kumar <viresh.kumar@...aro.org>

-- 
viresh