After reading about shellcode in Chapter 5 of Hacking: The Art of Exploitation, I wanted to go back through some of the examples and try them out. The first example was a simple Hello World program in Intel assembly. I followed along in the book and had no problems reproducing results on a 32 bit Linux VM using nasm with elf file format and ld for linking.
Then I decided I wanted to try something similar but with a little bit of a challenge: write a Mac OS X 64 bit “hello world” program using the new fast ‘syscall’ instruction instead of the software interrupt based (
int 0x80 ) system call, this is where things got interesting. First and foremost, the version of Nasm that comes with Mac OS X is a really old version. If you want to assemble macho64 code, you’ll need to download the lastest version.
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I figured I could replace the extended registers with the 64 bit registers and the
int 0x80 call with a syscall instruction so my first attempt was something like this
After assembling and linking, I got this
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Belkin sxuptp driver windows 10. Apparently Mac OS X doesn’t use
_start for linking, instead it just uses start . After removing the underscore prefix from start, I was able to link but after running, I got this Fifa online 3 mac download.
![]() Nasm Download Windows 10
I was pretty stumped at this point so I headed off to Google to figure out how I was supposed to use the
syscall instruction. After a bunch of confusion, I stumbled upon the documentation and realized that x86_64 uses entirely different registers for passing arguments. From the documentation:
So I tweaked the code with this new information
https://omegagreat.weebly.com/blog/tina-free-download. Adobe reader for mac. And with high hopes that I’d see “Hello World!” on the console, I still got the exact same ‘Bus error’ after assembling and linking.
Back to Google to see if others had tried a write syscall on Mac OS X. I found a few posts of people having success with the syscall number
0x2000004 so I thought I’d give it a try. Similarly, the exit syscall number was 0x2000001 . I tweaked the code and BINGO! I was now able to see “Hello World” output on my console but I was seriously confused at this point; what was this magic number 0x200000 that is being added to the standard syscall numbers?
I looked in syscall.h to see if this was some sort of padding (for security?) I greped all of
/usr/include for 0x2000000 with no hints what-so-ever. I looked into the Mach-o file format to see if it was related to that with no luck.
After about an hour and a half of looking, I spotted what I was looking for in ‘syscall_sw.h’ https://comptechte.hatenablog.com/entry/2020/10/13/225623.
Mac OS X or likely BSD has split up the system call numbers into several different “classes.” The upper order bits of the syscall number represent the class of the system call, in the case of write and exit, it’s
SYSCALL_CLASS_UNIX and hence the upper order bits are 2! Minecraft funland 3 download mac. Thus, every Unix system call will be (0×2000000 + unix syscall #) .
Armed with this information, here’s the final x86_64 Mach-o “Hello World” Gta for mac download.
And here’s the output
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