esp_elf_pack.py

# esp-bin2elf written by Joel Sandin <jsandin@gmail.com>
#
# MIT licence

# The elffile module provides a pack() function that unfortunately
# has bugs and limitations.
#
# elffile.pack():
#
#   1) Doesn't pack program headers, and instead ignores them silently
#   2) Changes header offsets after they've been set
#   3) Computes the strtab nameoffset incorrectly
#
# esp_elf.py originally contained confusing workarounds for these
# issues, but these workarounds have been removed and a simpler pack
# function with fewer surprises is instead implemented here.

from struct import pack

Elf32_Ehdr = [
    # unsigned char e_ident[EI_NIDENT]
    ('type', '<H'),             # Elf32_Half e_type;
    ('machine', '<H'),          # Elf32_Half e_machine;
    ('version', '<I'),          # Elf32_Word e_version;
    ('entry', '<I'),            # Elf32_Addr e_entry;
    ('phoff', '<I'),            # Elf32_Off e_phoff;
    ('shoff', '<I'),            # Elf32_Off e_shoff;
    ('flags', '<I'),            # Elf32_Word e_flags;
    ('ehsize', '<H'),           # Elf32_Half e_ehsize;
    ('phentsize', '<H'),        # Elf32_Half e_phentsize;
    ('phnum', '<H'),            # Elf32_Half e_phnum;
    ('shentsize', '<H'),        # Elf32_Half e_shentsize;
    ('shnum', '<H'),            # Elf32_Half e_shnum;
    ('shstrndx', '<H')          # Elf32_Half e_shstrndx;
]

Elf32_Shdr = [
    ('nameoffset', '<I'),       # Elf32_Word sh_name;
    ('type', '<I'),             # Elf32_Word sh_type;
    ('flags', '<I'),            # Elf32_Word sh_flags;
    ('addr', '<I'),             # Elf32_Addr sh_addr;
    ('offset', '<I'),           # Elf32_Off sh_offset;
    ('section_size', '<I'),     # Elf32_Word sh_size;
    ('link', '<I'),             # Elf32_Word sh_link;
    ('info', '<I'),             # Elf32_Word sh_info;
    ('addralign', '<I'),        # Elf32_Word sh_addralign;
    ('entsize', '<I')           # Elf32_Word sh_entsize;
]

Elf32_Phdr = [
    ('type', '<I'),             # Elf32_Word p_type;
    ('offset', '<I'),           # Elf32_Off p_offset;
    ('vaddr', '<I'),            # Elf32_Addr p_vaddr;
    ('paddr', '<I'),            # Elf32_Addr p_paddr;
    ('filesz', '<I'),           # Elf32_Word p_filesz;
    ('memsz', '<I'),            # Elf32_Word p_memsz;
    ('flags', '<I'),            # Elf32_Word p_flags;
    ('align', '<I')             # Elf32_Word p_align;
]

Elf32_Sym = [
    ('st_name', '<I'),          # Elf32_Word st_name;
    ('st_value', '<I'),         # Elf32_Addr st_value;
    ('st_size', '<I'),          # Elf32_Word st_size;
    ('st_info', '<B'),          # unsigned char st_info;
    ('st_other', '<B'),         # unsigned char st_other;
    ('st_shndx', '<H')          # Elf32_Half st_shndx;
]

# we handle a few fields manually in pack_ident below:
e_ident = [
    # EI_MAG0-MAG4: File identification
    ('elfClass', '<B'),         # EI_CLASS: File class
    ('elfData', '<B'),          # EI_DATA: Data encoding
    ('fileVersion', '<B')       # EI_VERSION: File version
    # EI_PAD: Start of padding bytes
    # EI_NIDENT: Size of e_ident[]
]


def pack_elf(xtensa_elf):
    packed_elf = pack_ident(xtensa_elf.ident)
    packed_elf += pack_fileheader(xtensa_elf.fileHeader)

    for header in xtensa_elf.sectionHeaders:
        packed_elf += header.content

    for header in xtensa_elf.sectionHeaders:
        packed_elf += pack_section_header(header)

    for header in xtensa_elf.programHeaders:
        packed_elf += pack_program_header(header)

    return packed_elf


def pack_fileheader(file_header):
    return _pack_struct(file_header, Elf32_Ehdr)


def pack_section_header(section_header):
    return _pack_struct(section_header, Elf32_Shdr)


def pack_program_header(program_header):
    return _pack_struct(program_header, Elf32_Phdr)


def pack_symbol(symbol):
    return _pack_struct(symbol, Elf32_Sym)


def pack_ident(ident):
    header = b'\x7fELF'                     # magic number
    header += _pack_struct(ident, e_ident)  # pack fields in e_ident
    return header + b'\x00' * 9             # pad to 16 bytes


def _pack_struct(struct, struct_fields):
    packed = b''

    for (field, size) in struct_fields:
        field_value = getattr(struct, field)
        packed += pack(size, field_value)

    return packed