Modify segment permissions.

modify.py

#!/usr/bin/env python3

# Use `readelf -l <file>` to check segment no

import sys
import struct


def main():
    if len(sys.argv) != 4:
        print(f"Usage: {sys.argv[0]} <file> <segment no> <permissions (rwx)>")
        sys.exit(1)

    filename = sys.argv[1]
    try:
        seg_no = int(sys.argv[2])
    except ValueError:
        print("Error: segment no must be an integer")
        sys.exit(1)

    perms_str = sys.argv[3].lower()
    perms = 0
    if "r" in perms_str:
        perms |= 4
    if "w" in perms_str:
        perms |= 2
    if "x" in perms_str:
        perms |= 1

    with open(filename, "rb+") as f:
        ident = f.read(16)
        if ident[:4] != b"\x7fELF":
            print("Error: Not an ELF file")
            sys.exit(1)

        elf_class = ident[4]  # 1 for 32-bit, 2 for 64-bit
        is_64 = elf_class == 2

        f.seek(0x1C if not is_64 else 0x20)
        phoff = struct.unpack(
            "<I" if not is_64 else "<Q", f.read(4 if not is_64 else 8)
        )[0]

        f.seek(0x2A if not is_64 else 0x36)
        phentsize = struct.unpack("<H", f.read(2))[0]
        phnum = struct.unpack("<H", f.read(2))[0]

        if seg_no >= phnum:
            print(f"Error: Segment index {seg_no} out of range (0-{phnum - 1})")
            sys.exit(1)

        seg_offset = phoff + (seg_no * phentsize)

        if is_64:
            # Elf64_Phdr: type(4), flags(4), offset(8), vaddr(8), ...
            # p_flags is at offset 4 within the phdr
            f.seek(seg_offset + 4)
        else:
            # Elf32_Phdr: type(4), offset(4), vaddr(4), paddr(4), filesz(4), memsz(4), flags(4), align(4)
            # p_flags is at offset 24 within the phdr
            f.seek(seg_offset + 24)

        f.write(struct.pack("<I", perms))
        print(
            f"Successfully changed segment {seg_no} permissions to {perms_str} ({perms})"
        )


if __name__ == "__main__":
    main()