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In demerphq's own words:

bulk88> I disagree there is a measurable cost to the current implementation of

bulk88> REHASH. If the rehash code is so rare, maybe it should be removed from

bulk88> hv_common and placed in its own func, but looking at the asm, the

bulk88> overhead is so tiny I dont think the rehash code even matters compared

bulk88> to the rest of the design of hv_common. I don't see any performance

bulk88> gains by removing it.

demerphq Yes, I am unable to show any actual performance gains either.

So, not for performance (gain) reasoning.

Also in his own words:

demerphq So I think that the current rehash mechanism is about as secure as the random hash seed proposal.


demerphqPersonally I dont think its worth the effort of doing much more than thinking about this until someone demonstrates at least a laboratory grade attack. IMO in a real world environment with multi-host web sites, web servers being restarted, load balancers, and etc, that simple hash randomization is sufficient. Seems like any attack would require large numbers of fetch/response cycles and in general would just not be effective in a real production environment. I would assume that the administrators would notice the weird request pattern before an attacker could discover enough information to cause damage. Same argument for non-web services IMO.

And it doesn't make anything more secure.

And Dave_the_m said:

Indeed, based on the thread starting at

Message-ID: <>

it looks like the primary motivation for moving to rehash was to restore the binary compatibility within the 5.8.x branch inadvertently broken by 5.8.1.

I'm not particularly keen on having hashes always randomised - it makes debugging harder, and reproducing a reported issue nigh-on impossible; but if Yves can show a measurable performance gain without the rehash checks, then I'll approve, as long as the hash seed can still be initialised with the env var PERL_HASH_SEED=0 - otherwise debugging becomes impossible.

Which mirrors the OPs objections.

So, significant consequences

So, why? What did Perl gain?

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

In reply to Re^3: Truly randomized keys() in perl 5.17 - a challenge for testing? by BrowserUk
in thread Truly randomized keys() in perl 5.17 - a challenge for testing? by saintmike

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