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

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Date:	Mon, 27 Apr 2015 17:26:16 +0100
From:	Sudeep Holla <sudeep.holla@....com>
To:	Tejun Heo <tj@...nel.org>
CC:	Sudeep Holla <sudeep.holla@....com>,
	"linux-kernel@...r.kernel.org" <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



On 27/04/15 17:14, Tejun Heo wrote:
> 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.

Completely agree and in-fact we did discuss that internally too.
But since this function deals only with page size buffers, we thought
it's highly unlikely to hit that corner case.

> 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.

I assumed that I had consolidated most of them in commit 5aaba36318e5
("cpumask: factor out show_cpumap into separate helper function").
I might have missed, will have a look at it again.

> 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.
>

Ah, right that would be much simpler.

Regards,
Sudeep
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