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Here is the routine. Can anyone explain what I need to modify to make it return TRUE

__text:00086494 ; =============== S U B R O U T I N E =======================================
__text:00086494
__text:00086494 ; BLAH - (char)free
__text:00086494
__text:00086494 ; char __cdecl -[BLAH free](struct BLAH *self, SEL)
__text:00086494 __BLAH_free_                   ; DATA XREF: __objc_const:00BC0CECo
__text:00086494                 MOV             R1, #(_OBJC_IVAR_$_BLAH._free - 0x864A0) ; char _free;
__text:0008649C                 ADD             R1, PC  ; char _free;
__text:0008649E                 LDR             R1, [R1] ; char _free;
__text:000864A0                 LDRSB           R0, [R0,R1]
__text:000864A2                 BX              LR
__text:000864A2 ; End of function -[BLAH free]
Ross Ridge
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totalitarian
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    Reading the [ARM architecture procedure call standard](http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042e/IHI0042E_aapcs.pdf) would probably be a good start. – Michael Sep 14 '14 at 11:46

1 Answers1

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Pretty much any ARM ABI passes the return value in r0, and this code looks to be no exception. In this case, ldrsb r0, [r0, r1] is loading the return value using the self pointer in r0 and the computed offset in r1. Since this has no side effects or other complex behaviour to account for, you could simply replace that instruction depending on what the actual numeric value of TRUE should be, e.g. mov r0, #1, mvn r0, #0, etc.

Notlikethat
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  • Thanks. Could you expand on that a little. How do I know what the numeric value of true is. – totalitarian Sep 14 '14 at 13:05
  • The symbol decoration implies [Objective-C](http://stackoverflow.com/q/541289/3156750) (and LDRSB infers `signed char` to back that up) so the language definition is a good place to start. As a last resort, you can usually look at the disassembly of any callers (or debug step through) to see what kind of values they're testing for. – Notlikethat Sep 14 '14 at 19:01