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

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Date:	Wed, 7 Mar 2012 21:37:25 +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 Tue, 6 Mar 2012 22:09:04 -0300
> Arnaldo Carvalho de Melo <acme@...hat.com> wrote:
> 
> > Or what kind of such pipe the people who designed snprintf 
> > were using
> > :-(
> 
> I wasn't there for the original 4.4BSD implementation, but I 
> was on the ISO committee when we adopted it, and I regret to 
> say, while the food was lovely, the hosting organization 
> didn't offer us any drugs at all. But I can explain the 
> rationale of the choice.

;-)

> If snprintf returns the size it needed, and you know the size 
> you gave it, you have a choice of what to do, and you have all 
> the information you need to make an informed choice.
> 
> If it returns the amount it wrote, or possibly an error 
> indicator (such as -1) when out of space, you *don't* have the 
> information you need to make an informed choice, and one 
> possible choice ("reallocate with the right amount") is not 
> available to you. We had also seen other functions which made 
> that implementation choice, and consistently, people disliked 
> them more.

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.)

   We *DONT* want to make APIs more fragile just to accomodate a
   rare, esoteric usecase! Doing that you are introducing very
   real bugs in very real code. You are hurting the 99% for the
   sake of the 1%, and needlessly so:

2) It's not even true that should some code want to
   dynamically allocate the 'required' number of bytes is not
   available. Some oddball side API could be added for that 1%:

	size_needed = snprintf_size(...);

So this API could have been designed right but it was messed up 
out of concern for an insane 1% case - FAIL.

This is a case study for how insane semantics are created ...

Thanks,

	Ingo
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