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POWhat is the gcc ARM equivalent to this x64 assembly
 

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  1. POWhat is the gcc ARM equivalent to this x64 assembly
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    <p>This is the X64 code (I don't know much about assembly, and this code can be compiled by visual studio, don't know which format it is):</p> <pre><code>.code extern mProcs:QWORD myfunc proc jmp mProcs[1*8] myfunc endp </code></pre> <p>The mProcs is an array defined in C code, and the function myfunc simply jmp to the second element in the array. If viewed from C, it is jumping to *(mProcs+1) (1*8 because in x64 a pointer is 8 bytes).</p> <p>In GCC ARM version, I tried to do this:</p> <pre><code>.extern mProcs .global myfunc myfunc: b mProcs+4 </code></pre> <p>(here mProcs+4 because a pointer is 4 bytes)</p> <p>But this code seems no working. In C does it means jump to *(mProcs+1) or jump to mProcs+1? How can I make it *(mProcs+1)? </p> <p>============================================================================</p> <p>After discussion in the comments with Michael, I understood that I need to do the calculation on the register and then use the bx instruction to jump to the target function.</p> <p>However, the problem comes here. Since I'm implementing a thunk (I'm intercepting the function call and do something in the middle), I have no idea how the target routine uses the register.</p> <p><strong>1.</strong> I need to keep the callee save registers before I jmp to the target, else the target function will be preserving the wrong value.</p> <p><strong>2.</strong> I need to keep the argument registers intact before I perform a jump, so that the target function will have the correct arguments. 3. The above 2 points means I can only use caller save but non-argument registers.</p> <p>r0-r3 is the argument register, and r4-r12 are callee save register, r13 onwards are special registers. </p> <p>Which means none of the registers can be used without restoring the value.</p> <p>If the bx instruction can only operate on a register, then there is no chance I can restore that register even if that register is temporally saved on the stack.</p> <p>Any solutions? Or just arm binaries can't be hooked.</p>
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    1. PTQuestion
    1. USMin Lin
    1. USMin Lin
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    1. COShouldn't it be something like `ldr r0,=myfunc` / `ldr r0,[r0,#4]` / `bx r0` ? (note that you might have to use some other register than `r0`)
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      1. POWhat is the gcc ARM equivalent to this x64 assembly
      1. USMichael
    2. CO@Michael not quite clear to me, you don't even have mProcs in the code, can I just do "b [mProcs, #4]"?
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      1. POWhat is the gcc ARM equivalent to this x64 assembly
      1. USMin Lin
    3. COOops. `myfunc` was supposed to be `mProcs` in my example. And no, you can't do `b [mProcs, #4]`, because ARM is load/store-based architecture, so most instructions except `ldr/str` can't have memory operands.
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      1. POWhat is the gcc ARM equivalent to this x64 assembly
      1. USMichael
 

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