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Picking Apart the Crashing iOS String

manishearth.github.io

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Re: Picking Apart the Crashing iOS String

#31

Post by jrochkind1 »

What I'm really curious about is how bugs/errors in the iOS typesetting algorithm result in a crash, rather than just wrong or nonsense typesetting -- and how the last time this happened, they appearently just fixed the specific case, but not the ability of bugs/errors in the typesetting algorithm to crash the system.

I am not surprised there will be errors/bugs in the typesetting algorithm, as the OP demonstrates, this stuff is extraordinarily complicated to do for every language/all of unicode. But if you have something so complicated that bugs/errors aren't surprising -- you'd think you'd want to make sure they didn't cause hard crashes.

Re: Picking Apart the Crashing iOS String

#32

Post by thephyber »

enzanki_ars wrote: post #25
kibwen wrote: post #12

Has anyone yet traced this to a specific syscall? Seems like the perfect opportunity for whipping up a fuzzer to spit arbitrary unicode into the system and see what else crashes.

Would that even be possible on iOS for non Apple employees? My idea would be just to create a program that continually creates Unicode strings and display them on screen with a pause before creating and displaying the next. Record the screen and see if any thing exciting happens. Would never end, if there was a SETI @ Home style program for it with insentives for running it would be cool.

> Would that even be possible on iOS for non Apple employees?

iOS emulator (part of XCode) might be enough.

Re: Picking Apart the Crashing iOS String

#33

Post by zokier »

nikanj wrote: post #10
spaceywilly wrote: post #5

Earlier quoted context omitted.

My question was, why was there not a unit test for this? It seems like it would be trivial to step through every character combination for each language they support and make sure it doesn't cause a crash.

Unicode allows for 17 planes, each of 65,536 possible characters (or 'code points'). This gives a total of 1,114,112. This crashing sequence is five characters long. Your unit tests would have to go through 1.71650179e30 sequences to be guaranteed to catch this one. At a test rate of 1 millisecond per sequence, that's just 4×10^9 × the age of the universe, according to wolfram alpha.

> Unicode allows for 17 planes, each of 65,536 possible characters (or 'code points'). This gives a total of 1,114,112.

Allows for 17 planes, but only small portion of those are actually used. According to Wikipedia[1], currently Unicode has 148944 codepoints + 128k private use ones (which might, or might not make sense to include in unit tests). So your time estimate is off by mere 5 orders of magnitude.

Re: Picking Apart the Crashing iOS String

#35

Post by gwern »

minimaxir wrote: post #30
brian-armstrong wrote: post #4

The broader question is, I think, how can any textual sequence cause a crash. Text should be regarded very skeptically by the text renderer. What could this be doing that it invokes a crash? Where else is this team not being careful? It reeks of process problems.

A good example of test sequences causing weird behavior/crashes are RTL languages (like Hebrew and Arabic) and CJK characters (Chinese/Japanese/Korean), which have UI implications as well. While working at Apple in Software QA, both of these cases in part inspired me to create the Big List of Naughty Strings years ago: https://github.com/minimaxir/big-list-of-naughty-strings

Are you adding OP's strings to the list?

Re: Picking Apart the Crashing iOS String

#36

Post by jrochkind1 »

TazeTSchnitzel wrote: post #7
spaceywilly wrote: post #5

Earlier quoted context omitted.

My question was, why was there not a unit test for this? It seems like it would be trivial to step through every character combination for each language they support and make sure it doesn't cause a crash.

There are more possible character sequences than there are stars in the sky.

Googling, "If you tabulate all stars visible down to magnitude 6.5, thought to be the faintest stars still visible to the unaided eye, the entire sky contains some 9,000 stars. Since you can only see half the sky at any time, that means there are as many as 4,500 stars visible in your sky tonight."

Apparently not a very high bar.

Re: Picking Apart the Crashing iOS String

#37

Post by stcredzero »

jrochkind1 wrote: post #31

What I'm really curious about is how bugs/errors in the iOS typesetting algorithm result in a crash, rather than just wrong or nonsense typesetting -- and how the last time this happened, they appearently just fixed the specific case, but not the ability of bugs/errors in the typesetting algorithm to crash the system. I am not surprised there will be errors/bugs in the typesetting algorithm, as the OP demonstrates, t…

you'd think you'd want to make sure they didn't cause hard crashes.

Any Apple people want to chime in? Why wasn't the general bug -- where there could be even the possibility of a "text crash" -- fixed?

Re: Picking Apart the Crashing iOS String

#38

Post by kens »

Not directly related to the crash, but I have a question about Telugu and similar scripts: How do their speakers think about the structure of the script? Do they consider each vowel and consonant a separate "thing" that just happens to get written as a complex grapheme, or is the grapheme the unit you think about and it just happens to be made up of smaller parts? I.e https://en.wikipedia.org/wiki/Telugu_script#Consonant_Conjun...

Also, do you learn the large number of graphemes separately? Or are they the "obvious" way to write the consonant and vowel? Or is there a set of rules you learn?

Re: Picking Apart the Crashing iOS String

#39

Post by blackflame7000 »

jrochkind1 wrote: post #31

What I'm really curious about is how bugs/errors in the iOS typesetting algorithm result in a crash, rather than just wrong or nonsense typesetting -- and how the last time this happened, they appearently just fixed the specific case, but not the ability of bugs/errors in the typesetting algorithm to crash the system. I am not surprised there will be errors/bugs in the typesetting algorithm, as the OP demonstrates, t…

I bet you a dollar its related to Auto-Correct trying to interpret some malformed word.
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