linux-kernel - Re: [PATCH] bitmap: remove explicit newline handling using scnprintf format string

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Message-ID: <20150427161434.GF1499@htj.duckdns.org>
Date:	Mon, 27 Apr 2015 12:14:34 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Sudeep Holla <sudeep.holla@....com>
Cc:	linux-kernel@...r.kernel.org, Pawel Moll <Pawel.Moll@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [PATCH] bitmap: remove explicit newline handling using scnprintf
 format string

Hello, Sudeep.

On Mon, Apr 27, 2015 at 10:46:58AM +0100, Sudeep Holla wrote:
>  int bitmap_print_to_pagebuf(bool list, char *buf, const unsigned long *maskp,
>  			    int nmaskbits)
>  {
> -	ptrdiff_t len = PTR_ALIGN(buf + PAGE_SIZE - 1, PAGE_SIZE) - buf - 2;
> +	ptrdiff_t len = PTR_ALIGN(buf + PAGE_SIZE - 1, PAGE_SIZE) - buf;
>  	int n = 0;
> 
> -	if (len > 1) {
> -		n = list ? scnprintf(buf, len, "%*pbl", nmaskbits, maskp) :
> -			   scnprintf(buf, len, "%*pb", nmaskbits, maskp);
> -		buf[n++] = '\n';
> -		buf[n] = '\0';
> -	}
> +	if (len > 1)
> +		n = list ? scnprintf(buf, len, "%*pbl\n", nmaskbits, maskp) :
> +			   scnprintf(buf, len, "%*pb\n", nmaskbits, maskp);
>  	return n;
>  }
>  EXPORT_SYMBOL(bitmap_print_to_pagebuf);

So, there's one behavior difference stemming from this.  When the
buffer is too small, the original code would still output '\n' at the
end while the new code would just continue to print the formatted
string.  Given that bitmap outputs can be pretty long, this behavior
difference has a minute but still non-zero chance of causing something
surprising.  There are multiple copies of the above function in arch
codes too.  We prolly want to audit the usages to verify that the
passed in buffer is always big enough at which point the above
function and its copies can simply be replaced with direct scnprintf()
calls.  This function doesn't actually add anything.

Thanks.

-- 
tejun
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