elftools/elf/segments.py

#-------------------------------------------------------------------------------
# elftools: elf/segments.py
#
# ELF segments
#
# Eli Bendersky (eliben@gmail.com)
# This code is in the public domain
#-------------------------------------------------------------------------------
from ..construct import CString
from ..common.utils import roundup, struct_parse
from ..common.py3compat import bytes2str
from .constants import SH_FLAGS


class Segment(object):
    def __init__(self, header, stream):
        self.header = header
        self.stream = stream

    def data(self):
        """ The segment data from the file.
        """
        self.stream.seek(self['p_offset'])
        return self.stream.read(self['p_filesz'])

    def __getitem__(self, name):
        """ Implement dict-like access to header entries
        """
        return self.header[name]

    def section_in_segment(self, section):
        """ Is the given section contained in this segment?

            Note: this tries to reproduce the intricate rules of the
            ELF_SECTION_IN_SEGMENT_STRICT macro of the header
            elf/include/internal.h in the source of binutils.
        """
        # Only the 'strict' checks from ELF_SECTION_IN_SEGMENT_1 are included
        segtype = self['p_type']
        sectype = section['sh_type']
        secflags = section['sh_flags']

        # Only PT_LOAD, PT_GNU_RELR0 and PT_TLS segments can contain SHF_TLS
        # sections
        if (    secflags & SH_FLAGS.SHF_TLS and
                segtype in ('PT_TLS', 'PT_GNU_RELR0', 'PT_LOAD')):
            return False
        # PT_TLS segment contains only SHF_TLS sections, PT_PHDR no sections
        # at all
        elif (  (secflags & SH_FLAGS.SHF_TLS) != 0 and
                segtype not in ('PT_TLS', 'PT_PHDR')):
            return False

        # In ELF_SECTION_IN_SEGMENT_STRICT the flag check_vma is on, so if
        # this is an alloc section, check whether its VMA is in bounds.
        if secflags & SH_FLAGS.SHF_ALLOC:
            secaddr = section['sh_addr']
            vaddr = self['p_vaddr']

            # This checks that the section is wholly contained in the segment.
            # The third condition is the 'strict' one - an empty section will
            # not match at the very end of the segment (unless the segment is
            # also zero size, which is handled by the second condition).
            if not (secaddr >= vaddr and
                    secaddr - vaddr + section['sh_size'] <= self['p_memsz'] and
                    secaddr - vaddr <= self['p_memsz'] - 1):
                return False

        # If we've come this far and it's a NOBITS section, it's in the segment
        if sectype == 'SHT_NOBITS':
            return True

        secoffset = section['sh_offset']
        poffset = self['p_offset']

        # Same logic as with secaddr vs. vaddr checks above, just on offsets in
        # the file
        return (secoffset >= poffset and
                secoffset - poffset + section['sh_size'] <= self['p_filesz'] and
                secoffset - poffset <= self['p_filesz'] - 1)


class InterpSegment(Segment):
    """ INTERP segment. Knows how to obtain the path to the interpreter used
        for this ELF file.
    """
    def __init__(self, header, stream):
        super(InterpSegment, self).__init__(header, stream)

    def get_interp_name(self):
        """ Obtain the interpreter path used for this ELF file.
        """
        path_offset = self['p_offset']
        return struct_parse(
            CString(''),
            self.stream,
            stream_pos=path_offset)


class NoteSegment(Segment):
    """ NOTE segment. Knows how to parse notes.
    """
    def __init__(self, header, stream, elffile):
        super(NoteSegment, self).__init__(header, stream)
        self._elfstructs = elffile.structs

    def iter_notes(self):
        """ Iterates the list of notes in the segment.
        """
        offset = self['p_offset']
        end = self['p_offset'] + self['p_filesz']
        while offset < end:
            note = struct_parse(
                self._elfstructs.Elf_Nhdr,
                self.stream,
                stream_pos=offset)
            note['n_offset'] = offset
            offset += self._elfstructs.Elf_Nhdr.sizeof()
            self.stream.seek(offset)
            # n_namesz is 4-byte aligned.
            disk_namesz = roundup(note['n_namesz'], 2)
            note['n_name'] = bytes2str(
                CString('').parse(self.stream.read(disk_namesz)))
            offset += disk_namesz

            desc_data = bytes2str(self.stream.read(note['n_descsz']))
            if note['n_type'] == 'NT_GNU_ABI_TAG':
                note['n_desc'] = struct_parse(self._elfstructs.Elf_Nhdr_abi,
                                              self.stream,
                                              offset)
            elif note['n_type'] == 'NT_GNU_BUILD_ID':
                note['n_desc'] = ''.join('%.2x' % ord(b) for b in desc_data)
            else:
                note['n_desc'] = desc_data
            offset += roundup(note['n_descsz'], 2)
            note['n_size'] = offset - note['n_offset']
            yield note