linux-kernel - Re: [PATCH] clocksource/drivers/pistachio: Fix wrong calculated clocksource read value

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

Message-ID: <56713DB1.9070301@linaro.org>
Date:	Wed, 16 Dec 2015 11:32:17 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Jisheng Zhang <jszhang@...vell.com>, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] clocksource/drivers/pistachio: Fix wrong calculated
 clocksource read value

On 12/16/2015 10:33 AM, Russell King - ARM Linux wrote:
> On Wed, Dec 16, 2015 at 10:21:55AM +0100, Daniel Lezcano wrote:
>> On 12/16/2015 08:36 AM, Jisheng Zhang wrote:
>>> And in fact, clocksource_mmio_readw_down() also has similar issue, but it masks
>>> with c->mask before return, the c->mask is less than 32 bit (because the
>>> clocksource_mmio_init think number of valid bits > 32 or < 16 is invalid.)
>>> the higher 32 bits are masked off, so we never saw such issue. But we'd better
>>> to fix that, what's your opinion?
>>
>> I think we should have a look to this portion closely.
>
> There is no need to return more bits than are specified.  If you have
> a N-bit counter, then the high (64-N)-bits can be any value, because:
>
> static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask)
> {
>          return (now - last) & mask;
> }
>
> where 'now' is the current value returned from the clock source read
> function, 'last' is a previously returned value, and 'mask' is the
> bit mask.  This has the effect of ignoring the high order bits.

I think this approach is perfectly sane. When I said we should look at 
this portion closely, I meant we should double check the bitwise-nor 
order regarding the explicit cast. The clocksource's mask makes sense 
and must stay untouched.



-- 
  <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs

Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
<http://www.linaro.org/linaro-blog/> Blog

--
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/