From: Ingo Molnar <mingo@elte.hu>
To: Peter Seebach <peter.seebach@windriver.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>,
Anton Blanchard <anton@samba.org>,
paulus@samba.org, peterz@infradead.org, dsahern@gmail.com,
fweisbec@gmail.com, yanmin_zhang@linux.intel.com,
emunson@mgebm.net, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] perf: Incorrect use of snprintf results in SEGV
Date: Wed, 7 Mar 2012 21:37:25 +0100 [thread overview]
Message-ID: <20120307203725.GA4333@elte.hu> (raw)
In-Reply-To: <20120306192912.59811e3e@wrlaptop>
* Peter Seebach <peter.seebach@windriver.com> wrote:
> On Tue, 6 Mar 2012 22:09:04 -0300
> Arnaldo Carvalho de Melo <acme@redhat.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
next prev parent reply other threads:[~2012-03-07 20:37 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-03-07 0:42 [PATCH] perf: Incorrect use of snprintf results in SEGV Anton Blanchard
2012-03-07 0:49 ` Peter Seebach
2012-03-07 1:09 ` Arnaldo Carvalho de Melo
2012-03-07 1:29 ` Peter Seebach
2012-03-07 18:44 ` Nick Bowler
2012-03-07 20:24 ` Peter Seebach
2012-03-07 20:37 ` Ingo Molnar [this message]
2012-03-07 20:59 ` Peter Zijlstra
2012-03-07 21:28 ` Peter Seebach
2012-03-08 7:34 ` Ingo Molnar
2012-03-08 8:51 ` Peter Seebach
2012-03-07 21:19 ` Peter Seebach
2012-03-08 0:58 ` Arnaldo Carvalho de Melo
2012-03-08 7:48 ` Ingo Molnar
2012-03-08 7:52 ` Ingo Molnar
2012-03-09 19:00 ` Peter Seebach
2012-03-14 19:59 ` [tip:perf/urgent] perf tools: " tip-bot for Anton Blanchard
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