linux-kernel - Re: [PATCH 2/2] drivers/rtc/rtc-ds1685.c: don't try to micromanage sysfs output size

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Message-ID: <20151130190627.GO22136@piout.net>
Date:	Mon, 30 Nov 2015 20:06:27 +0100
From:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Joshua Kinard <kumba@...too.org>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	rtc-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] drivers/rtc/rtc-ds1685.c: don't try to micromanage
 sysfs output size

On 24/11/2015 at 14:51:24 +0100, Rasmus Villemoes wrote :
> ...and don't do it wrong.
> 
> "not ok or N/A" has length 13. Add the trailing newline, and the
> snprintf return value will be 14. However, we lied to snprintf and
> told it that only 13 bytes were available. Hence snprintf has only
> written "not ok or N/" and a trailing '\0' to the buffer. Next we
> continue lying, this time to the upper sysfs layer, claiming that we
> wrote 14 meaningful bytes to the buffer. That'll make the upper layer
> copy "not ok or N/" plus two nul bytes to user space (one nul byte
> from snprintf, the other since sysfs takes care to clear the buffer
> before giving it to the ->show method).
> 
> In the other cases, the claimed buffer size is closer to sufficient,
> but we'll still get a nul byte instead of a newline written to user
> space.  There's absolutely no reason to try to predict the output
> size, and there's plenty of room in the buffer, so just use sprintf.
> 
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
>  drivers/rtc/rtc-ds1685.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
Applied, thanks.

-- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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