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Picking Apart the Crashing iOS String

manishearth.github.io

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Re: Picking Apart the Crashing iOS String

#91

Post by Someone1234 »

pkaler wrote: post #67
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…

The crash seems to be in CoreText. CoreText is embedded/linked in Messages, Spotlight, Springboard, etc. CoreText is written in C. The fix would be to rewrite CoreText in a memory safe language like Swift. This would be “hard”. Or put CoreText in an XPC container. This would both be “hard” and result in terrible performance. For more details on how hard C, memory management, systems programming, and operating system…

Or just move CoreText into its own process, and restart it when it crashes.

The big issue is that when CoreText crashes right now the kernel panics and the device restarts. If CoreText itself could crash safely, get restarted, and the OS continue running then these bugs would go from "significant" to "annoying." Even if CoreText crashing caused individual apps to also crash, that would be a big improvement the current situation.

Obviously we'd all like bug free fonts and text rendering, but if we call that goal aspirational (read: impossible), the best we can hope for today is handling the fault cases better than they're handled today. Bootloops are a pretty lame user experience.

Re: Picking Apart the Crashing iOS String

#92

Post by bhasi »

Manishearth wrote: post #51
kens wrote: post #38

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#Conso…

I can answer your question for Devanagari: - You normally think of clusters as single "letters". The word for them in Marathi is "joined letter". This notion of "letter" may not be totally in line with what English speakers would expect. - You still think of clusters as having component parts that make it up. However, क्ष/ज्ञ and sometimes त्र are thought of as their own "fundamental" consonants even though they're c…

I'm no linguist, but Malayalam has a more predictable stacking rule than Devanagari, more so in the modern alphabet/sandhi scheme than the old. For instance, one can barely tell apart ह्म and ह्य, but their Malayalam equivalents are pretty distinct: ഹ്മ (old scheme) and ഹ്‌മ (modern scheme) for the first, and ഹ്യ for the second.

Re: Picking Apart the Crashing iOS String

#93

Post by acheron »

glibgil wrote: post #61
stcredzero wrote: post #37

Earlier quoted context omitted.

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?

If Apple people chime in I think they get beheaded. It never happens

“The bomb says no, Brian.”

https://www.penny-arcade.com/comic/2007/05/23/

Re: Picking Apart the Crashing iOS String

#94

Post by Someone1234 »

Manishearth wrote: post #51
kens wrote: post #38

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#Conso…

I can answer your question for Devanagari: - You normally think of clusters as single "letters". The word for them in Marathi is "joined letter". This notion of "letter" may not be totally in line with what English speakers would expect. - You still think of clusters as having component parts that make it up. However, क्ष/ज्ञ and sometimes त्र are thought of as their own "fundamental" consonants even though they're c…

As someone who can read both Devanagari and English/Latin script, I'm just curious, when you read Devanagari does the text size need to be larger for good comprehension?

There seems more subtlety with the word structure, like tiny little flicks which seem to have significant meaning. While Latin-like script has characters like the comma/full-stop, and modifiers like the dot on the i, these are far rarer than Devanagari's modifiers from what I have seen.

Another related question is, because Devanagari's information density seems higher, does that mean shorter text for the equivalent information?

PS - Sorry if anything I said was offensive, that wasn't my intent. I am just an ignorant idiot wanting to learn someone's experience with a different language script, nothing more.

Re: Picking Apart the Crashing iOS String

#95

Post by lloeki »

Someone1234 wrote: post #91
pkaler wrote: post #67

Earlier quoted context omitted.

The crash seems to be in CoreText. CoreText is embedded/linked in Messages, Spotlight, Springboard, etc. CoreText is written in C. The fix would be to rewrite CoreText in a memory safe language like Swift. This would be “hard”. Or put CoreText in an XPC container. This would both be “hard” and result in terrible performance. For more details on how hard C, memory management, systems programming, and operating system…

Or just move CoreText into its own process, and restart it when it crashes. The big issue is that when CoreText crashes right now the kernel panics and the device restarts. If CoreText itself could crash safely, get restarted, and the OS continue running then these bugs would go from "significant" to "annoying." Even if CoreText crashing caused individual apps to also crash, that would be a big improvement the curren…

From what I gathered so far this doesn't hit the kernel but the process. It seems to turn out that on iOS one of such processes happens to be Springboard, hence the UI (but not the kernel) gets a kick and restarts.

Maybe I missed something though.

Re: Picking Apart the Crashing iOS String

#96

Post by ladberg »

enzanki_ars wrote: post #65
gsich wrote: post #60

Earlier quoted context omitted.

Use Hackintosh or a VM.

Given that Apple does not design their OS to work on anything but their hardware, the pain I would incur trying to set up either option is just not worth it. Unless someone has published a stable and updated pre-built VM for macOS, it just seems like nothing but trouble.

You should look up if your hardware has been used for Hackintosh by anyone else. I lucked out and my PC has nearly identical specs to this /r/hackintosh post and my install went flawlessly: https://www.reddit.com/r/hackintosh/comments/2mgq9e/everythi...

Re: Picking Apart the Crashing iOS String

#97

Post by callalex »

kens wrote: post #38

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#Conso…

An interesting debate that may be closer to home for English speakers: is ñ a letter? The Spanish alphabet considers it to be a different letter than n, but é is not a different letter than e. https://en.m.wikipedia.org/wiki/Ñ

Re: Picking Apart the Crashing iOS String

#98

Post by AceJohnny2 »

glibgil wrote: post #61
stcredzero wrote: post #37

Earlier quoted context omitted.

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?

If Apple people chime in I think they get beheaded. It never happens

Absolutely. It's drilled and repeated on a regular schedule: Shut the fuck up.

Re: Picking Apart the Crashing iOS String

#99

Post by crumbshot »

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…

Someone posted a crash report: https://gist.github.com/steipete/a759b56dc1b5d94ba5b3c03ddd0...

Looks like a heap corruption, leading to a failure when allocating memory later on:

    Application Specific Information:
    abort() called
    *** error for object 0x604000673e00: Invalid pointer dequeued from free list
    
    Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
    0   libsystem_kernel.dylib        	0x00007fff73a30e3e __pthread_kill + 10
    1   libsystem_pthread.dylib       	0x00007fff73b6f150 pthread_kill + 333
    2   libsystem_c.dylib             	0x00007fff7398d312 abort + 127
    3   libsystem_malloc.dylib        	0x00007fff73a9bdbf nanozone_error + 502
    4   libsystem_malloc.dylib        	0x00007fff73a8fdac _nano_malloc_check_clear + 410
    5   libsystem_malloc.dylib        	0x00007fff73a901d7 nano_calloc + 72
    6   libsystem_malloc.dylib        	0x00007fff73a8acc0 malloc_zone_calloc + 87
    7   libsystem_malloc.dylib        	0x00007fff73a8b5d6 calloc + 30
Probably caused by an out of bounds write on some heap buffer. If this bug is also present on OS X, it would be interesting to see where it crashes with some of the malloc debugging flags enabled (https://developer.apple.com/library/content/documentation/Pe...), hopefully to get a crash a bit closer to the root cause.

Re: Picking Apart the Crashing iOS String

#100

Post by raincom »

kens wrote: post #38

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#Conso…

To understand Telugu and similar scripts, one has to think in terms of syllables. Only when we look at these scripts from Roman alphabet, do we see the discussion of consonant and vowel graphemes in the script.

/peɪ/ is a syllable in English, but can be represented by these grapheme clusters: , , . The similar syllable is represented in Telugu or Devanagari this way: పే (Telugu script) or पे (Hindi/Devangari script); there are phonetic differences in the way this syllable represented: (a) /p/ is aspirated in English, but not in the telugu/devangari syllable; (b) the diphthon /eɪ/ is realized as a monophthong in Telugu/Devanagari scripts.

sequence plays a major role in Sanskrit sandhi. In Sanskrit, in assimilatory contexts, a voiced palatal affricate /dʒ/ + a palatal nasal /ñ/ + some open vowel, leads to this sequence . Since this assimilation occurs in many contexts, the resultant grapheme is taught as part of alphabet in this special set of clusters: kSa, tra, jna ( క్ష త్ర జ్ఞ) or (क्ष त्र ज्ञ). In fact, there was a poet in Telugu, whose name was ksEtrajna (combo of these three graphemes with ligatures). https://en.wikipedia.org/wiki/Kshetrajna (in general) https://en.wikipedia.org/wiki/Kshetrayya (Telugu poet)

The context of jna in Sanskrit: for instance, declension of the masuclar stem: rAjan http://littleredbook.vedicsociety.org.in/noun-tables/rajan--...

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