linux-kernel - Re: [PATCH 6/6] perf_counter: kerneltop: output event support

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Date:	Sat, 04 Apr 2009 11:10:13 -0700
From:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Paul Mackerras <paulus@...ba.org>,
	Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH 6/6] perf_counter: kerneltop: output event support

Peter Zijlstra wrote:
> On Fri, 2009-04-03 at 17:21 -0700, Corey Ashford wrote:
>> As I was stealing code from kerneltop today to use in the PAPI profiling 
>> implementation, I ran across the code below:
>>
>> Peter Zijlstra wrote:
>>> Teach kerneltop about the new output ABI.
>>>
>>> XXX: anybody fancy integrating the PID/TID data into the output?
>>>
>>> Bump the mmap_data pages a little because we bloated the output and
>>> have to be more careful about overruns with structured data.
>>>
>>> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>>> ---
>> [snip]
>>
>>> @@ -1147,28 +1152,75 @@ static void mmap_read(struct mmap_data *
>>>  	unsigned int head = mmap_read_head(md);
>>>  	unsigned int old = md->prev;
>>>  	unsigned char *data = md->base + page_size;
>>> +	int diff;
>>>
>>>  	gettimeofday(&this_read, NULL);
>>>
>>> -	if (head - old > md->mask) {
>>> +	/*
>>> +	 * If we're further behind than half the buffer, there's a chance
>>> +	 * the writer will bite our tail and screw up the events under us.
>>> +	 *
>>> +	 * If we somehow ended up ahead of the head, we got messed up.
>>> +	 *
>>> +	 * In either case, truncate and restart at head.
>>> +	 */
>>> +	diff = head - old;
>>> +	if (diff > md->mask / 2 || diff < 0) {
>>>  		struct timeval iv;
>>>  		unsigned long msecs;
>>>
>>>  		timersub(&this_read, &last_read, &iv);
>>>  		msecs = iv.tv_sec*1000 + iv.tv_usec/1000;
>>>
>>> -		fprintf(stderr, "WARNING: failed to keep up with mmap data.  Last read %lu msecs ago.\n", msecs);
>>> +		fprintf(stderr, "WARNING: failed to keep up with mmap data."
>>> +				"  Last read %lu msecs ago.\n", msecs);
>>>
>> [snip]
>>
>> The test for diff < 0 looks incorrect to me.  This shouldn't be an 
>> error, because it will frequently be the case that the head has wrapped 
>> around back to the beginning of the mmap'd pages, while old is near the end.
>>
>> What it needs to find out, I think, is if the modulo distance between 
>> old and head is greater than 1/2 of the span of the mmap'd pages. 
>> Here's a suggested change:
>>
>> -	diff = head - old;
>> +	diff = (head - old) & md->mask;
>> -	if (diff > md->mask / 2 || diff < 0) {
>> +	if (diff > md->mask / 2) {
>>
>>
>> What do you think?
> 
> head and old are both u32 and are monotonically incremented, that means
> that (s32)(head - old) < 0 will only be true if old is ahead (or more
> than 2^31 behind) of head.
> 
> Since mask will be smaller than 2^31 this seemed like a reasonable
> integrity test.

Ah, I see.  I assumed (but did not check) that there was logic to wrap 
the old and head back to the beginning when they hit the end of the 
mmap'd pages.

Thanks for your reply.

- Corey

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/