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From "William A. Rowe Jr." <wr...@rowe-clan.net>
Subject Re: Really big regex results from ap_pregsub
Date Tue, 25 Oct 2011 17:23:03 GMT
On 10/25/2011 12:04 PM, Jim Jagielski wrote:
> 
> On Oct 18, 2011, at 5:59 PM, Stefan Fritsch wrote:
> 
>> On Tue, 18 Oct 2011, Greg Ames wrote:
>>> On Fri, Oct 14, 2011 at 2:34 PM, Stefan Fritsch <sf@sfritsch.de> wrote:
>>>> Btw, isn't the nmatch the number of () pairs in the regex? If so, then
>>>> enforcing the AP_MAX_REG_MATCH limit could introduce behaviour change
>>>> in existing configs: Previously, ap_pregsub on with a regex with more
>>>> than 10 capturing () pairs would replace $1 ... $9 with the first nine
>>>> matches. But now, it would just return the original string.
>>>
>>>
>>> is there some other reasonable way we can return an error or avoid the
>>> problem besides returning the original string?  I would prefer that we
>>> continue to replace $1 ... $9 in the error case in 2.2.x.
>>>
>>>>
>>>> Also, just returning the original string does not allow to detect
>>>> errors. Currently ap_pregsub in 2.2.x always succeeds in that it
>>>> replaces $1 to $9. I am against changing this in a way that may return
>>>> the unchanged string. Maybe it would be more appropriate to enforce
>>>> AP_MAX_REG_MATCH at compile time in ap_regcomp()? Then the errors
>>>> would be more obvious to the user.
>>>>
>>>
>>> the latter sounds good.  I doubt that very many admins have legitimate use
>>> cases for more than AP_MAX_REG_MATCH matches.
>>
>> This problem seems to be much rarer than I thought. In trunk, only ap_rxplus_exec()
(which is not in 2.2.x) passes the number of parens to ap_pregsub. All other callers pass
AP_MAX_REG_MATCH or a smaller constant. While I haven't checked the 2.2 code, it seems the
danger of behaviour changes only exists in third party modules.
> 
> Which would be doing something out-of-the-ordinary if using >10

Well there often a legitimate call to substituting /$1/$2/$3/$1$2$3
(think of the way we break down mod_cache_disk file trees, or how some
/homes/ are organized).

So as a substitution counter, AP_MAX_REG_///MATCH/// is not the right
construct.


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