Re: [PATCH] libdw: Add overflow checking to __libdw_form_val_len.

[Josh Stone <jistone at redhat dot com>]

On 12/04/2014 01:00 PM, Mark Wielaard wrote:
> This solves a couple of crashers reported by Alexander.
> 
> This will probably have some performance impact, but I haven't measured
> it yet. It would be good to have some performance tests. We also need
> some overflow check for leb128 reading.
> 
> Josh, which tests did you use last time when you did the performance
> improvements of this code? I am afraid I won't be able to keep it as fast,
> but it would be good to know how much this slows things down.

I couldn't remember, but I found my original intro:
https://lists.fedorahosted.org/pipermail/elfutils-devel/2013-December/003549.html

So the numbers I reported were with "tests/varlocs -k >/dev/null".

The good news is that your patch is barely slower, 81.64s -> 82.34s.
That's almost noise, but it's consistently a little slower over several
attempts.  That's to be expected with more checks though.

The bad news is that these numbers in the 80s are back in the
neighborhood of where I was before those optimization patches.  That's
not an apples-to-apples comparison, since I'm now on F21 gcc and kernel,
but that's still a lot to slip back. :(

I'll see if I can grab that old kernel debuginfo to do a more direct
comparison.