I haven't yet put it up on the J wiki, but we had a segment at this month's NYCJUG about generating random J phrases. I won't go into the why and how but here's some things I uncovered with this exercise.
DON'T TRY THIS AT HOME ==================== If you want to use up all your memory, you may be able to do this (on a 3 GB RAM machine): *.^:x:23 but this probably will not work *.^:x:24 and this won't (unless - maybe - you have more than 16GB): *.^:x:29 If you look at the shape of this expression with a number smaller than 23, you'll get an idea of what the problem is. Don't try these at all: _.*.1 1:H.0:_. *f.H.1:_. 1:H.}:_. ,:/a. (these all lock up J so that it has to be killed or eventually use up all memory, at least on the two or three machines where I've tried it). I have a longer list of valid - but very bad - J phrases that I have to filter out from my randomly-generated phrases but this will have to wait until I have time to put up the longer essay. In any case, there are potentially dangerous J phrases you may not be able to filter out. On Wed, Nov 27, 2013 at 2:22 PM, Pascal Jasmin <[email protected]>wrote: > I'm not positive what you mean, > > w rexec 'do_z_';2;3;4 > ┌─┬───────────────────────────────────────────────────────────────────┐ > │4│|ill-formed name: do_z__testcommands_ | w rexec'do_z_';2;3;4 │ > └─┴───────────────────────────────────────────────────────────────────┘ > > the sanitize function picks the first word (;:) from cmd. As long as the > "whitelist" provided by the testcommands locale does not include something > like do or memw, then I don't think its possible to run a locative (?) > > I did find an error in sanitize, which assumed cmd is a string. Easiest, > atm, to preguard for that. > > w rexec 2;3;4 > |domain error: sanitize > | w rexec 2;3;4 > > > ________________________________ > From: Henry Rich <[email protected]> > To: [email protected] > Sent: Wednesday, November 27, 2013 1:11:57 PM > Subject: Re: [Jprogramming] safe remote execution > > > What stops cmd from being a locative? > > Henry Rich > > > On 11/27/2013 10:37 AM, Pascal Jasmin wrote: > > These routines should allow for "safe" remote code execution from a not > completely trusted user, but I am posting here in case I missed anything. > Safe means should not crash, and should not execute any command outside the > sandbox locale. User passes a noun that consists of 'cmd';monadic y > arguments (maybe multiple boxes). > > > > mkerr_z_ =: ((0&;)@:) ( :: ((13!:11 ; 13!:12)@:(''"_))) > > chkerr_z_ =: ;@:}. ^:(0 = >@{.) > > sanitize_z_ =: (0&pick)@:;: > > > > > > rexec_z_ =: (sanitize@:>@{.@:] loc [) apply mkerr }.@:] > > > > cocurrent 'testcommands' > > (18!:5 '') 18!:2 ] 18!:5 '' NB. removes _z_ > > double =: +:@:; > > echo =: ] > > > > in immediate window, (or use cocurrent 'base' if typed above > > > > w =. <'testcommands' > > > > > > w rexec 'double';2;3;4 > > ┌─┬─────┐ > > │0│4 6 8│ > > └─┴─────┘ > > chkerr w rexec 'double';2;3;4 > > 4 6 8 > > > > w rexec 'do_z';2;3;4 > > ┌──┬─────────────────────────────────────────────────────┐ > > │21│|value error: do_z_testcommands_ | (<0),<x u y │ > > └──┴─────────────────────────────────────────────────────┘ > > > > any improvements or security failure? > > > > Note the first line of 'testcommands' locale, is there a way to set the > search path to nothing (instead of self as done there) > > > > ---------------------------------------------------------------------- > > For information about J forums see http://www.jsoftware.com/forums.htm > > > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm > -- Devon McCormick, CFA ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
