Re: CONSTANTS patch - LKBIT

[HyperHacker <hyperhacker@...>]

On Tue, Sep 28, 2010 at 07:36, Nilson <nilson.brazil@gmail.com> wrote:
> On Tue, Sep 28, 2010 at 5:51 AM, Juris Kalnins <juris@mt.lv> wrote:
>>
>> You have to account for the overhead it will cause for all code
>> that doesn't use your const values.
>>
> I agree. I estimate a 6 to 10 additional basic native machine codes
> operations for each assignment  of a non constant value.
> Please, read the next answer.
>
>>> The implementation of selective constants in tables using metamethods
>>> requires LUA's ifs (much more time) or a table of hashed constants
>>> (much more space and more time).
>>
>> I can't imagine it being 25 times
>> faster than something like the following simple metatable based
>> read only object (just a sketch, this is not a tested code):
>>
>> local t = { ... } --
>> local read_only = setmetatable({}, { __index = t, __newindex = bark_fn,
>> __metatable = bark_fn })
>>
>
> First, CONST patch is not a read-only table implementation. The goal
> is to turn chosen  table fields into unchangeable fields. It is
> different. I wrote about this before.
>
> When you read the lvm.c code  you see that each VM OPERATION in
> interpreted executing many native machine instructions. I estimate
> from 60 to 200 per interpreted instruction. A metamethod
> implementation will require from 5 to 20 VM OPs. So making
> calculations: 60 x 5 = 300 to 200 x 5 = 1000. Considering that CONST
> patch requires 6 / 10 extra instructions, calculation again: from
> 300/10 to 1000/10  results in 30 to 100 times faster. Applying a 20%
> discount = 25 times.
>
> About the overhead: When Lua executes a = 1000 command, it generates a
> SETGLOBAL VM OP that needs some C language ifs, some comparisons, a
> couple of function calls and some assigments to be interpreted. I
> estimated to add 2 ifs and 4 masks operation per aassignment. Well, 8
> extra instructions in 64 gives 12,5% of overheard on writes. *** I
> really need benchmarks to measure the overall performance impact.  ***
> This is one of the reasons because I asked for some help. I'm a newbie
> in Lua internals ...
>
>>> Login = const(Login) -- Avoids ManInTheMiddle internal attack
>>
>> As HyperHacker very well explained in another reply,
>> this does not really prevent internal attack.
>>
> I didn't see that.
>
> But if you code  " Login = const( function () ... end) " the value
> stored inside Login field cannot be changed using normal Lua anymore
> and I cannot see a way to alter the original value before storing it
> in Login field using regular Lua. I would be confident on value
> integrity. And if you make "  Login2 = const(Login) ", Login2 will
> store the same unchangeable value too.
>
> Any way, the const(Login) solution would be - at least - as good as
> the metamethod solution but it is easier to use. After all, It is a
> simple function call.
>
>> Actually, I cannot imagine doing it in Lua at all. The WoW people
>> tried to create some form of access policy. And while it works, it's
>> neither simple, nor generic.
>>
> Well, the Login stuff was just an example and I was talking about
> internal (Lua code) "Man in the middle".
>
>> See this for a well thought-out example of
>> language-level access control (this one is capability based):
>>
>> http://www.erights.org/elang/
>>
> Thanks. I will read this later.
>
> --
> Nilson
>
>

mhm, but what if before your Login = const(...), I were to do something like:
const = function(n) return n end
then I'd be free to change anything I like.

-- 
Sent from my toaster.