Rename Symbols Using __asm__

on assembly, symbol, obfuscation, C, and low-level

objc source code has code below __asm__("_objc_retain"), what is this __asm__?

Discovery

Source code as below

objc source code download https://opensource.apple.com/tarballs/objc4/

After searching, discovered can rename symbol, write test code:

#include <stdio.h>
void foo(void) __asm__("_objc_release");
void foo(void)  {
    printf("hello world");
}
int main(int argc, const char * argv[]) {
    foo();
    return 0;
}

Debug to see

From figure above can know, symbol in callstack is not foo, but _objc_release.

Variables

From link below also can know, can rename variable’s symbol.

int counter __asm__("counter_v1") = 0;

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491f/Cacgegch.html

Summary

Seems can use to do simple export symbol obfuscation for dylib?

References

  • http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.kui0097a/armcc_bcfggcdh.htm
  • http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491f/Cacgegch.html
  • https://stackoverflow.com/questions/1034852/adding-leading-underscores-to-assembly-symbols-with-gcc-on-win32

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