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

Ingo Molnar
2012-03-08T07:49:15Z

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

> On Wed, 7 Mar 2012 21:37:25 +0100
> Ingo Molnar <mingo@elte.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