mythical

mythical bug

Problem I: a.out where are you? (hard)

You received a compiled a.out file from your friend. However, when you try to execute it, you got

zsh: no such file or directory: ./a.out

You are absolutely sure it is right there with

$ ls -lh a.out
-rwxrwxr-x 1 john john 16K Jul 25 04:34 a.out

What is going on?

Click to Solution

1. Googling "zsh: no such file or directory" doesn't give you much info
2. Then you starts random Gooling with "a.out not found" or "file not found but it is there"
3. Luckily, when you Google the complete error message "zsh: no such file or directory: ./a.out"
4. Goole tell you it is related to "dynamic link" and ldd will gives you more info
5. You run ldd a.out, it gives you

linux-vdso.so.1 (0x00007ffff8bfa000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fa3d484c000)
/lib64/ld-linux-x86-64.so.1 => /lib64/ld-linux-x86-64.so.2 (0x00007fa3d4a85000)

6. You notice that there is a strange link trying to link ld-linux-x86-64.so.1 to ld-linux-x86-64.so.2
7. You ask your friend and turns out that this executable is compiled in an old machine with outdated linker

Addtional Challenge Since this executable is crafted by hand, it can actually by repaired through directly modifying a.out with a hex editor. Can you recover it?