3 ms·

This project is a rewrite of a Bash script that I started to teach myself a little bit of x86 assembly. I still don't know much, but I did learn that ELF execut

This project is a rewrite of a Bash script that I started to teach myself a little bit of x86 assembly. I still don't know much, but I did learn that ELF executables can be surprisingly small and simple for a very basic program. A 64-byte file header and a single 56-byte program header are the only "overhead" that is really mandatory (and it's even possible to make those overlap a little). But it's difficult to stop the linker from adding extra stuff, which is why I eventually specified the headers (and three addresses) by hand in the assembly file and stopped using a linker.

samatman 3y ago
The installation instructions made me smile. "turn this Base64 string into an executable" is the new "curl | sh"! Why did you decide to build the string backward?
- meribold 3y ago
  I don't remember if it's why I initially did it, but it allows hardcoding the final characters of the output string ("%)\n") somewhere into the ELF header where they don't do damage ahead of execution.
LegionMammal978 3y ago
Good luck on your x86-golfing journey! If it helps, I have a nice template for overlapping the two headers into 80 bytes, and stuffing up to ~24 bytes of instructions into them. It's included near the top of my article on the smallest x86-64 ELF Hello World [0]. (In the same article, I have a 73-byte template that's a bit shorter but trickier to use.) You may find some of the other tricks in the article helpful, but it might be hard to follow depending on your level of experience with assembly. My general advice would be that 'push' and 'pop' are your two best friends if you want to move around 64-bit values. [0] https://tmpout.sh/3/22.html https://tmpout.sh/3/22.html
- meribold 3y ago
  I'll have a look. Thanks! I'll avoid using potentially transient details of Linux's ELF loader, though, since I use `btry` daily and don't want to tempt fate to have it suddenly break following a kernel update too much. (Perhaps that's a bit silly given the hardcoded `/sys` path and syscall numbers.)
  - LegionMammal978 3y ago
    You're in the clear with the syscall numbers, at least. Linux treats them as part of the stable public API for each platform. If they want to update a syscall, they have to make a new version with its own number, and keep the old version around for as long as x86-64 is supported.

samatman 3y ago

The installation instructions made me smile. "turn this Base64 string into an executable" is the new "curl | sh"! Why did you decide to build the string backward?

meribold 3y ago
I don't remember if it's why I initially did it, but it allows hardcoding the final characters of the output string ("%)\n") somewhere into the ELF header where they don't do damage ahead of execution.

LegionMammal978 3y ago

Good luck on your x86-golfing journey! If it helps, I have a nice template for overlapping the two headers into 80 bytes, and stuffing up to ~24 bytes of instructions into them. It's included near the top of my article on the smallest x86-64 ELF Hello World [0]. (In the same article, I have a 73-byte template that's a bit shorter but trickier to use.) You may find some of the other tricks in the article helpful, but it might be hard to follow depending on your level of experience with assembly. My general advice would be that 'push' and 'pop' are your two best friends if you want to move around 64-bit values. [0] https://tmpout.sh/3/22.html https://tmpout.sh/3/22.html

meribold 3y ago
I'll have a look. Thanks! I'll avoid using potentially transient details of Linux's ELF loader, though, since I use `btry` daily and don't want to tempt fate to have it suddenly break following a kernel update too much. (Perhaps that's a bit silly given the hardcoded `/sys` path and syscall numbers.)
- LegionMammal978 3y ago
  You're in the clear with the syscall numbers, at least. Linux treats them as part of the stable public API for each platform. If they want to update a syscall, they have to make a new version with its own number, and keep the old version around for as long as x86-64 is supported.