Re: [PATCH v2] EDAC/mc_sysfs: Add missing newlines when printing {max,dimm}_location

[Xiongfeng Wang]

On 2020/9/18 0:25, Borislav Petkov wrote:
> On Thu, Sep 17, 2020 at 07:38:57PM +0800, Xiongfeng Wang wrote:
>> I am not sure if snprintf will return a value larger than its second input
>> paramter 'size'.
> 
> The comment over snprintf() says
> 
>  * The return value is the number of characters which would be
>  * generated for the given input, excluding the trailing null,
>  * as per ISO C99.  If the return is greater than or equal to
>  * @size, the resulting string is truncated.
> 
> And let's try it, see diff at the end. Now look what that produces:
> 
> [    2.594796] kernel_init: len: 16, str: [A lo]
> 
> it returns 16 for len even though the buffer is 5 chars long. So in our
> patch, we'd increment by 16 which would be wrong.
> 
> Now let's use scnprintf():
> 
> [    2.700142] kernel_init: len: 4, str: [A lo]
> 
> Much better. Lemme do that.
> 
>> Not sure whether we need to check 'len' equals to 0.
>> if (len <= 0) ?
> 
> Yeah, lemme fix that too, so we have now incrementally ontop:


Thansk a lot. I will send another version. Also I will change the 'snprintf' in
'dimmdev_location_show()' to 'scnprintf'

Thanks,
Xiongfeng

> 
> ---
> diff --git a/drivers/edac/edac_mc_sysfs.c b/drivers/edac/edac_mc_sysfs.c
> index fa0551c81e63..c56e0004b39e 100644
> --- a/drivers/edac/edac_mc_sysfs.c
> +++ b/drivers/edac/edac_mc_sysfs.c
> @@ -822,18 +822,17 @@ static ssize_t mci_max_location_show(struct device *dev,
>  	int i, n;
>  
>  	for (i = 0; i < mci->n_layers; i++) {
> -		n = snprintf(p, len, "%s %d ",
> -			     edac_layer_name[mci->layers[i].type],
> -			     mci->layers[i].size - 1);
> -
> +		n = scnprintf(p, len, "%s %d ",
> +			      edac_layer_name[mci->layers[i].type],
> +			      mci->layers[i].size - 1);
>  		len -= n;
> -		if (len < 0)
> +		if (len <= 0)
>  			goto out;
>  
>  		p += n;
>  	}
>  
> -	p += snprintf(p, len, "\n");
> +	p += scnprintf(p, len, "\n");
>  out:
>  	return p - data;
>  }
> ---
> 
> Test diff:
> 
> ---
> diff --git a/init/main.c b/init/main.c
> index ae78fb68d231..e2d6110d3a3d 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1397,7 +1397,8 @@ void __weak free_initmem(void)
>  
>  static int __ref kernel_init(void *unused)
>  {
> -	int ret;
> +	char str[5];
> +	int ret, len;
>  
>  	kernel_init_freeable();
>  	/* need to finish all async __init code before freeing the memory */
> @@ -1419,6 +1420,11 @@ static int __ref kernel_init(void *unused)
>  
>  	do_sysctl_args();
>  
> +	len = snprintf(str, 5, "A longer string\n");
> +
> +	pr_info("%s: len: %d, str: [%s]\n",
> +		__func__, len, str);
> +
>  	if (ramdisk_execute_command) {
>  		ret = run_init_process(ramdisk_execute_command);
>  		if (!ret)
>