A little extra info about registers

[GoogleCodeExporter]

It's not unusual to get a crash in objc_msgSend and be left wondering "what 
message was being sent?"  It would be nice if the crash report contained, for 
each register of the crashed thread:

1. The register interpreted as a c string pointer (with a limit on length)
2. [r description], where r is the register interpreted as having type id.

1 seems doable to me, 2 seems tough but gdb can do it somehow so maybe it's 
possible.  

Thanks!

Original issue reported on code.google.com by n8gray@gmail.com on 21 Sep 2011 at 10:29

[GoogleCodeExporter]

#1 is pretty interesting, and could be done reliably and easily, but there is 
the risk of personal user information disclosure.

#2 is not really workable; gdb can get away with it because the failure case is 
considered acceptable during debugging; there's a much more significant 
downside when executing crash reporting on the user's system. It would still be 
possible to fetch the class name, but the risk of user information disclosure 
remains.

The idea of context-aware extended debugging information is very interesting, 
however.

Original comment by landon.j.fuller@gmail.com on 21 Sep 2011 at 10:50

[GoogleCodeExporter]

Created branch landonf-30-register-examine to experiment with this feature.

Original comment by landon.j.fuller@gmail.com on 24 Sep 2011 at 1:12

• Changed state: Accepted

[GoogleCodeExporter]

A very simple hack on ARM is to print r1 and r2 as C strings if the top 
function is objc_msgSend (or r2 and r3 for objc_msgSend_stret), which can be 
done (badly) by assuming the length of objc_msgSend is ~1/2/4K or so. (I don't 
think objc_msgSend touches those registers, but I could be wrong.)

Alternatively, a symbolicatecrash-alike could recover selector names. A really 
simple hack is to just print the contents of all registers interpreted as 
C-strings if they point into a loaded image.

(r2 is nice because it's often a selector, e.g. -performSelector:withObject:... 
or -respondsToSelector: and friends. r3 is a selector for 
addObserver:selector:... and addTarget:action:... but those don't tend to crash)

Original comment by tc...@airsource.co.uk on 28 Sep 2011 at 6:58

[GoogleCodeExporter]

Thanks for giving this some attention.  Your point about personal information 
is well taken.  I'd be just as happy to see a restricted version like described 
in comment 3:  Only objc_msgSend and only a relevant subset of registers.  

Original comment by n8gray@gmail.com on 25 Oct 2011 at 8:46