linux-kernel - Re: [PATCH] [v3] warn on performance-impacting configs aka. TAINT

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Date:	Sun, 24 Aug 2014 13:40:58 -0700
From:	Dave Hansen <dave@...1.net>
To:	Ingo Molnar <mingo@...nel.org>
CC:	linux-kernel@...r.kernel.org, dave.hansen@...ux.intel.com,
	peterz@...radead.org, mingo@...hat.com, ak@...ux.intel.com,
	tim.c.chen@...ux.intel.com, akpm@...ux-foundation.org,
	cl@...ux.com, penberg@...nel.org, linux-mm@...ck.org,
	kirill@...temov.name, lauraa@...eaurora.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] [v3] warn on performance-impacting configs aka. TAINT_PERFORMANCE

On 08/24/2014 07:49 AM, Ingo Molnar wrote:
>>>> > >> +	buf_left = buf_len;
>>>> > >> +	for (i = 0; i < ARRAY_SIZE(perfomance_killing_configs); i++) {
>>>> > >> +		buf_written += snprintf(buf + buf_written, buf_left,
>>>> > >> +					"%s%s\n", config_prefix,
>>>> > >> +					perfomance_killing_configs[i]);
>>>> > >> +		buf_left = buf_len - buf_written;
...
>>> > > Also, do you want to check buf_left and break out early from 
>>> > > the loop if it goes non-positive?
>> > 
>> > You're slowly inflating my patch for no practical gain. :)
> AFAICS it's a potential memory corruption and security bug, 
> should the array ever grow large enough to overflow the passed
> in buffer size.

Let's say there is 1 "buf_left" and I attempt a 100-byte snprintf().
Won't snprintf() return 1, and buf_written will then equal buf_len?
buf_left=0 at that point, and will get passed in to the next snprintf()
as the buffer length.  I'm expecting snprintf() to just return 0 when it
gets a 0 for its 'size'.

Exhausting the buffer will, at worst, mean a bunch of useless calls to
snprintf() that do nothing, but I don't think it will run over the end
of the buffer.

Or am I missing something?
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