linux-kernel - Re: [PATCH] perf: Incorrect use of snprintf results in SEGV

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Message-ID: <20120308074837.GE20784@elte.hu>
Date:	Thu, 8 Mar 2012 08:48:37 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Seebach <peter.seebach@...driver.com>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Anton Blanchard <anton@...ba.org>, paulus@...ba.org,
	peterz@...radead.org, dsahern@...il.com, fweisbec@...il.com,
	yanmin_zhang@...ux.intel.com, emunson@...bm.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: Incorrect use of snprintf results in SEGV

* Peter Seebach <peter.seebach@...driver.com> wrote:

> On Wed, 7 Mar 2012 21:37:25 +0100
> Ingo Molnar <mingo@...e.hu> wrote:
> 
> > You are missing two important aspects:
> > 
> > 1) Dynamic reallocation on snprintf() failure is an utterly rare
> >    thing - it is used in less than 1% of snprintf() invocations. 
> >    (Yes, I just checked a couple of codebases.)
> 
> I would agree that it's very rare.  But then, using the return 
> value at all isn't especially common in my experience -- the 
> only interesting part, most of the time, is "we're sure this 
> didn't overrun the buffer".

Erm. Doing:

	+= snprintf(...);

is a *very* common pattern within the kernel. It occurs more 
than a thousand times - i.e. about 25% of all snprintf uses 
(~5000 instances) within the kernel does care about the return 
value.

I found only a single case that did a reallocation if the buffer 
did not fit. Lets assume that I missed some and there's 4 
altogether.

I.e. the API usage proportion, within the kernel project, looks 
like this, approximately:

  snprintf() call site that:

   does not care about the return value:          75.0%
   uses the return value as a 'written' count:    24.9%
   wants to dynamically reallocate:                0.1%

> >    We *DONT* want to make APIs more fragile just to accomodate a
> >    rare, esoteric usecase!
> 
> I would view snprintf as an API which already exists.

Changing it is obviously not possible anymore.

I was just countering your justification for it - which is still 
wrong. People might read that and use it to justify newly 
introduced, crappy APIs.

The 0.1% usecase is absolutely not a valid excuse to make an API 
less robust - *especially* when a separate API could serve that 
0.1% case just fine.

When designing APIs it is of utmost importance how average 
developers intuitively *think* it works - not how the designer 
thinks it should work ... Any severe mismatch between the two is 
a serious design FAIL that should not be repeated in new code.

Thanks,

	Ingo
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