Reversing a real-world 249 bytes backdoor

Isn't the entrypoint determined by the operating system (_start on Linux if I remember correctly)

No, it should be easy to get the entry point since it's defined in the ELF file header. The author simply tried to look for the `_start` symbol, which failed because the binary doesn't contain any symbol information (also the entry point doesn't have to be named `_start`, it's just a convention).

Hey I am the author of the blogpost. I have been reversing for a couple of years.

Usually, GDB just parses the ELF header to get the entry point. $ info file; in gdb should give you entrypoint of the binary almost always. In this case however the program has a corrupted section header, due to which gdb is not able to recognize the entry point. I dont exactly know which bytes in the header was corrupted, but apparently radare2 is able to get the entrypoint without much work. Should be fun to investigate more.