linux-kernel - Re: [PATCH v1 1/1] s390: Use string_upper() instead of open coded variant

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Date:   Mon, 4 Oct 2021 22:31:46 +0200
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [PATCH v1 1/1] s390: Use string_upper() instead of open coded
 variant

On Fri, Oct 01, 2021 at 04:02:01PM +0300, Andy Shevchenko wrote:
> Use string_upper() from string helper module instead of open coded variant.
> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> ---
>  arch/s390/mm/cmm.c    | 11 ++++-------
>  arch/s390/mm/extmem.c | 21 ++++++++++++---------
>  2 files changed, 16 insertions(+), 16 deletions(-)
...
>  static void
>  dcss_mkname(char *name, char *dcss_name)
>  {
> +	/* Segment name is limited by 8 characters + NUL */
> +	char tmp[8 + 1];
>  	int i;
>  
> -	for (i = 0; i < 8; i++) {
> -		if (name[i] == '\0')
> -			break;
> -		dcss_name[i] = toupper(name[i]);
> -	}
> -	for (; i < 8; i++)
> -		dcss_name[i] = ' ';
> +	/*
> +	 * This snprintf() call does two things:
> +	 * - makes a NUL-terminated copy of the input string
> +	 * - pads it with spaces
> +	 */
> +	snprintf(tmp, sizeof(tmp), "%s        ", name);

I can't say I like code where I have to count spaces in order to
verify if the code is actually correct.

> +	string_upper(dcss_name, tmp);
...
>  static struct dcss_segment *
>  segment_by_name (char *name)
>  {
> -	char dcss_name[9];
> +	char dcss_name[8];

string_upper will copy the terminating NUL-byte. By reducing the size
of dcss_name to 8 bytes this will result in stack corruption.