Question

I’m recompiling a Java program, and I have the following method:

public static native String getMyString(String s);

getMyString is implemented inside mylibrary.so, and looks like follows:

; Scrambler::getMyString(char  const*)
EXPORT _ZN9Scrambler18getMyStringEPKc
_ZN9Scrambler18getMyStringEPKc
LDR             R1, =(mystring - 0x14FC8)
PUSH            {R4-R6,LR}
ADD             R1, PC  ; "mystring"
MOV             R4, R0
BLX             strcasecmp
CMP             R0, #0
BNE.W           loc_150FC

which than ends up in

LDR             R5, =(unk_6AE24 - 0x14FD8)
ADD             R5, PC

The R5 seems to be the return value here.

The unk_6AE24 looks like follows:

                                              27
68 5D 6D 06 3F 10 6D 26  32 10 56 12 06 3F 32 5D
12 27 26 4B 0D 5D 27 4B  6E 0D 2B 06 32 5D 68 24
3F 32 06 24 70 56 00 63  69 4C 1D 06 5A 7F 0C 34
1E 67 3B 63 32 5F 16 1D  34 5F 1E 32 42 69 18 49
3F 51 49 0C 1F 0C 0E 77  2F 44 69 7F 5A 0C 0B 34
0C 34 1C 24 32 5F 7F 00  00 00 00 1D 30 3B 5F 30

IDA suggests, that the string in question is 27 68 5D 6D or “‘h]m”, but I’m having my doubts. When I was using c++ ages ago, the strings used to end with 0, which would make for return value of 27 68 5D 6D 06 3F 10 6D 26 32 10 56 12 06 3F 32 5D 12 27 26 4B 0D 5D 27 4B 6E 0D 2B 06 32 5D 68 24 3F 32 06 24 70 56 . Or we could be talking about java string, which may have its own format.

What kind of string will getMyString return?

Answer 1

What kind of string will getMyString return?

It will return a java.lang.String, you can get that from the declaration.

The actual character data will (probably) be one redirection away from that. (a String is probably a pointer to a char array and a size). Keep in mind that in Java, a char is 16 bits.

What are you trying to do, anyway? Much easier to just set a breakpoint right after getMyString returns and look at the return value in a debugger.