I'm attempting to figure out the structure of a "God object".

I found where it's being initialized, but I've never seen calloc used like this before:

god = (God *)calloc(1,(size_t)&god_size_marker?);

Where God is the structure that I'm currently attempting to figure out, and god_size_marker? is the name I gave to the pointer location.

The bizarre thing is, it's passing a pointer to the second argument of calloc. If I have Ghidra follow god_size_marker?, I see:

                     god_size_marker?                                XREF[2]:     create_instance:0005b0ac(*), 
00700438 7d              db         7Dh

So the address is 0x700438. If I manually set the structure to have a size of 0x700438, that appears to fix the issue in my last question, but that's ridiculous. What significance could a pointer into the .rodata section possibly have? This also apparently has the bizarre consequence of some field offsets in the struct coinciding with global variable addresses:

puVar4 = (uint *)&DAT_007003e8;  // These lines are right beside each other. Note the names.
bVar20 = first_counter <= *(uint *)&god->field_0x7003e8;

Is it actually reasonable that they've initialized a struct's size using a pointer? To me, that suggests that Ghidra is misinterpreting something, and that there's something that I need to fix. I wouldn't even know where to start though from this behavior alone.

For reference, here is the relevant disassembly of the calls to calloc and __aebi_memclr8:

    0005b0ac 05 f1 70 01     add.w      r1=>god_size_marker?,r5,#0x70                    = 7Dh
    0005b0b0 93 46           mov        r11,r2
    0005b0b2 00 68           ldr        r0,[r0,#0x0]=>->__stack_chk_guard                = 01dbe014
    0005b0b4 4f f0 01 0a     mov.w      r10,#0x1
    0005b0b8 00 68           ldr        r0,[r0,#0x0]=>__stack_chk_guard                  = ??
    0005b0ba 19 90           str        r0,[sp,#local_3c]
    0005b0bc 01 20           movs       r0,#0x1
    0005b0be f8 f7 2c ec     blx        <EXTERNAL>::calloc                               void * calloc(size_t __nmemb, si...
    0005b0c2 40 f2 38 41     movw       r1,#0x438
    0005b0c6 04 46           mov        r4,god
    0005b0c8 c0 f2 70 01     movt       r1=>god_size_marker?,#0x70                       = 7Dh
    0005b0cc f8 f7 ee eb     blx        <EXTERNAL>::__aeabi_memclr8                      undefined __aeabi_memclr8()

Unfortunately, while I can read x86 assembly, my knowledge of ARM is fairly limited. Any insight as to what might be going on here would be appreciated.

1 Answer 1

0005b0c2 40 f2 38 41     movw       r1,#0x438
0005b0c6 04 46           mov        r4,god
0005b0c8 c0 f2 70 01     movt       r1=>god_size_marker?,#0x70 

The movw and movt together will set r1 to 0x700438 (0x70<<16 + 0x438) so this is the amount being cleared and likely the amount being allocated. It seems that Ghidra replaces it by the address expression just because there happens to be a variable at that address. You'll probably have to contact Ghidra support to figure out if there's a way to treat it as a number instead of address.

  • indeed, it should be 0x700438 (0x70<<16 + 0x438), I'll fix it.
    – Igor Skochinsky
    Jul 3, 2021 at 18:34
  • Beautiful, thank you. I was hoping I wouldn't need to learn ARM for this project, but I see it may be necessary to figure out what Ghidra is showing. Jul 3, 2021 at 18:35

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