Why `sub rsp, 8` is nessesary while executing a shellcode

The instruction sub rsp, 8 (or sub esp, 8 in 32-bit code) is often used in shellcode for stack alignment purposes. It adjusts the stack pointer to ensure that the stack is properly aligned before executing the rest of the shellcode.

In x86/x64 architectures, the stack pointer (rsp or esp) needs to be aligned to a multiple of 16 bytes for efficient memory access. This alignment requirement is due to the way some instructions, particularly SIMD (Single Instruction, Multiple Data) instructions, access memory.

By subtracting a small value (8 in this case) from the stack pointer, the shellcode aligns the stack to a multiple of 16 bytes, meeting the alignment requirement.

It's worth noting that the specific value subtracted from the stack pointer may vary depending on the context and requirements of the shellcode. Some shellcodes may require different adjustments to the stack pointer based on the number of bytes they push onto the stack or other factors.

Stack alignment is critical to ensure correct execution and prevent crashes or unexpected behavior. Therefore, shellcode developers include stack alignment instructions to maintain the proper stack alignment before executing the payload.

It's important to exercise caution when working with shellcode, as it often involves low-level programming and can have security implications. Always handle shellcode responsibly and avoid executing untrusted or malicious code.