_elf_file.py

# -*- Mode:Python; indent-tabs-mode:nil; tab-width:4 -*-
#
# Copyright 2016-2023 Canonical Ltd.
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License version 3 as
# published by the Free Software Foundation.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

"""Helpers to parse and handle ELF binary files."""

import contextlib
import os
import re
import subprocess
from pathlib import Path
from typing import Dict, List, Optional, Set, Tuple, cast

from craft_cli import emit
from elftools.construct import ConstructError
from elftools.elf import constants, dynamic, elffile, gnuversions, sections, segments
from packaging.version import parse as parse_version

from snapcraft import utils

from . import errors

_ElfArchitectureTuple = Tuple[str, str, str]
_SonameCacheDict = Dict[Tuple[_ElfArchitectureTuple, str], Path]

_DEBUG_INFO = ".debug_info"
_DYNAMIC = ".dynamic"
_GNU_VERSION_D = ".gnu.version_d"
_GNU_VERSION_R = ".gnu.version_r"
_INTERP = ".interp"


class _NeededLibrary:
    """Represents an ELF library version."""

    def __init__(self, *, name: str) -> None:
        self.name = name
        self.versions: Set[str] = set()

    def add_version(self, version: str) -> None:
        """Add a new library version."""
        self.versions.add(version)


class SonameCache:
    """A cache for sonames."""

    def __init__(self):
        self._soname_paths: _SonameCacheDict = {}

    def __getitem__(self, key):
        """Obtain cached item."""
        return self._soname_paths[key]

    def __setitem__(self, key: Tuple[_ElfArchitectureTuple, str], item: Path):
        """Add item to cache."""
        # Initial API error checks
        if not isinstance(key, tuple):
            raise EnvironmentError(
                "The key for SonameCache has to be a (arch, soname) tuple."
            )
        if not isinstance(key[0], tuple) or len(key[0]) != 3:
            raise EnvironmentError(
                "The first element of the key needs to of type _ElfArchitectureTuple."
            )
        if not isinstance(key[1], str):
            raise EnvironmentError(
                "The second element of the key needs to be "
                "of type str representing the soname."
            )
        self._soname_paths[key] = item

    def __contains__(self, key) -> bool:
        """Check if key is already in cache."""
        return key in self._soname_paths

    def reset_except_root(self, root: str) -> None:
        """Reset the cache values that aren't contained within root."""
        new_soname_paths: _SonameCacheDict = {}
        for key, value in self._soname_paths.items():
            if value is not None and str(value).startswith(root):
                new_soname_paths[key] = value

        self._soname_paths = new_soname_paths


class _Library:
    """Represents the soname and path to the library.

    :param soname: The library soname.
    :param soname_path: The full path to the version-named library.
    :param search_paths: Library search paths.
    :param base_path: The core base path to search for missing dependencies.
    :param arch_tuple: A tuple that identifies the architecture of the ELF file,
        containing the class, data byte order, and machine instruction set
        (e.g. ``(ELFCLASS64, ELFDATA2LSB, EM_X86_64)``).
    :param soname_cache: The soname cache manager.
    """

    def __init__(  # noqa PLR0913
        self,
        *,
        soname: str,
        soname_path: Path,
        search_paths: List[Path],
        base_path: Optional[Path],
        arch_tuple: _ElfArchitectureTuple,
        soname_cache: SonameCache,
    ) -> None:
        self.soname = soname
        self.soname_path = soname_path
        self.search_paths = search_paths
        self.base_path = base_path
        self.arch_tuple = arch_tuple
        self.soname_cache = soname_cache

        # Resolve path, if possible.
        self.path = self._crawl_for_path()

        if base_path is not None and base_path in self.path.parents:
            self.in_base_snap = True
        else:
            self.in_base_snap = False

        emit.debug(
            f"{soname} with original path {soname_path} found on {str(self.path)!r} "
            f"in base {self.in_base_snap!r}"
        )

    def _update_soname_cache(self, resolved_path: Path) -> None:
        self.soname_cache[self.arch_tuple, self.soname] = resolved_path

    def _is_valid_elf(self, resolved_path: Path) -> bool:
        if not resolved_path.exists() or not ElfFile.is_elf(resolved_path):
            return False

        try:
            elf_file = ElfFile(path=resolved_path)
        except errors.CorruptedElfFile as error:
            # Warn if the ELF file seems corrupted.
            emit.progress(str(error), permanent=True)
            return False

        return elf_file.arch_tuple == self.arch_tuple

    def _crawl_for_path(self) -> Path:
        # Speed things up and return what was already found once.
        if (self.arch_tuple, self.soname) in self.soname_cache:
            return self.soname_cache[self.arch_tuple, self.soname]

        emit.debug(f"Crawling to find soname {self.soname!r}")

        valid_search_paths = [p for p in self.search_paths if p.exists()]
        in_search_paths = any(p in self.soname_path.parents for p in valid_search_paths)

        # Expedite path crawling if we have a valid elf file that lives
        # inside the search paths.
        if in_search_paths and self._is_valid_elf(self.soname_path):
            self._update_soname_cache(self.soname_path)
            return self.soname_path

        for path in valid_search_paths:
            for root, _, files in os.walk(path):
                if self.soname not in files:
                    continue

                file_path = Path(root, self.soname.lstrip("/"))
                if self._is_valid_elf(file_path):
                    self._update_soname_cache(file_path)
                    return file_path

        # Required for libraries on the host and the fetching mechanism.
        self._update_soname_cache(self.soname_path)
        return self.soname_path


class ElfFile:
    """ElfFile represents and elf file on a path and its attributes."""

    def __init__(self, *, path: Path) -> None:
        """Initialize an ElfFile instance.

        :param str path: path to an elf_file within a snapcraft project.
        """
        self.path = path
        self.dependencies: Set[_Library] = set()

        self.arch_tuple: Optional[_ElfArchitectureTuple] = None
        self.interp = ""
        self.soname = ""
        self.versions: Set[str] = set()
        self.needed: Dict[str, _NeededLibrary] = {}
        self.execstack_set = False
        self.is_dynamic = True
        self.build_id = ""
        self.has_debug_info: bool = False

        self._required_glibc = ""

        # String of elf enum type, e.g. "ET_DYN", "ET_EXEC", etc.
        self.elf_type: str = "ET_NONE"

        try:
            emit.debug(f"Extracting ELF attributes: {str(path)!r}")
            self._extract_attributes()
        except (UnicodeDecodeError, AttributeError, ConstructError) as exception:
            emit.debug(f"Extracting ELF attributes exception: {str(exception)}")
            raise errors.CorruptedElfFile(path, exception)

    @classmethod
    def is_elf(cls, path: Path) -> bool:
        """Determine whether the given file is an ELF file.

        :param path: Path to the file to be verified.
        """
        if not path.is_file():
            # ELF binaries are regular files
            return False

        with path.open("rb") as bin_file:
            return bin_file.read(4) == b"\x7fELF"

    # pylint: disable=too-many-branches

    def _extract_attributes(self) -> None:  # noqa: C901
        with self.path.open("rb") as file:
            elf_file = elffile.ELFFile(file)

            # A set of fields to identify the architecture of the ELF file:
            #  EI_CLASS: 32/64 bit (e.g. amd64 vs. x32)
            #  EI_DATA: byte order (e.g. ppc64 vs. ppc64le)
            #  e_machine: instruction set (e.g. x86-64 vs. arm64)
            #
            # For amd64 binaries, this will evaluate to:
            #   ('ELFCLASS64', 'ELFDATA2LSB', 'EM_X86_64')
            self.arch_tuple = (
                elf_file.header.e_ident.EI_CLASS,
                elf_file.header.e_ident.EI_DATA,
                elf_file.header.e_machine,
            )

            # Gather attributes from dynamic sections.
            for section in elf_file.iter_sections():
                if not isinstance(section, dynamic.DynamicSection):
                    continue

                self.is_dynamic = True

                for tag in section.iter_tags():
                    if tag.entry.d_tag == "DT_NEEDED":
                        needed = (
                            tag.needed  # pyright: ignore[reportAttributeAccessIssue]
                        )
                        self.needed[needed] = _NeededLibrary(name=needed)
                    elif tag.entry.d_tag == "DT_SONAME":
                        self.soname = (
                            tag.soname  # pyright: ignore[reportAttributeAccessIssue]
                        )

            for segment in elf_file.iter_segments():
                if segment["p_type"] == "PT_GNU_STACK":
                    # p_flags holds the bit mask for this segment.
                    # See `man 5 elf`.
                    mode = segment["p_flags"]
                    if mode & constants.P_FLAGS.PF_X:
                        self.execstack_set = True
                elif isinstance(segment, segments.InterpSegment):
                    self.interp = segment.get_interp_name()

            build_id_section = elf_file.get_section_by_name(".note.gnu.build-id")
            if (
                isinstance(build_id_section, sections.NoteSection)
                and build_id_section.header["sh_type"] != "SHT_NOBITS"
            ):
                for note in build_id_section.iter_notes():
                    if note.n_name == "GNU" and note.n_type == "NT_GNU_BUILD_ID":
                        self.build_id = note.n_desc

            # If we are processing a detached debug info file, these
            # sections will be present but empty.
            verneed_section = elf_file.get_section_by_name(_GNU_VERSION_R)
            if isinstance(verneed_section, gnuversions.GNUVerNeedSection):
                for library, versions in verneed_section.iter_versions():
                    library_name = cast(str, library.name)
                    # If the ELF file only references weak symbols
                    # from a library, it may be absent from DT_NEEDED
                    # but still have an entry in .gnu.version_r for
                    # symbol versions.
                    if library_name not in self.needed:
                        continue
                    lib = self.needed[library_name]
                    for version in versions:
                        lib.add_version(version.name)

            verdef_section = elf_file.get_section_by_name(_GNU_VERSION_D)
            if isinstance(verdef_section, gnuversions.GNUVerDefSection):
                for _, auxiliaries in verdef_section.iter_versions():
                    for aux in auxiliaries:
                        self.versions.add(aux.name)

            debug_info_section = elf_file.get_section_by_name(_DEBUG_INFO)
            self.has_debug_info = (
                debug_info_section is not None
                and debug_info_section.header.sh_type != "SHT_NOBITS"
            )

            self.elf_type = elf_file.header["e_type"]

    # pylint: enable=too-many-branches

    def is_linker_compatible(self, *, linker_version: str) -> bool:
        """Determine if the linker will work given the required glibc version."""
        version_required = self.get_required_glibc()
        parsed_version_required = parse_version(version_required)
        parsed_linker_version = parse_version(linker_version)
        is_compatible = parsed_version_required <= parsed_linker_version
        emit.debug(
            f"Check if linker {linker_version!r} works with GLIBC_{version_required} "
            f"required by {str(self.path)!r}: {is_compatible}"
        )
        return is_compatible

    def get_required_glibc(self) -> str:
        """Return the required glibc version for this ELF file."""
        if self._required_glibc:
            return self._required_glibc

        version_required = ""
        for lib in self.needed.values():
            for _version in lib.versions:
                if not _version.startswith("GLIBC_"):
                    continue
                version = _version[6:]
                if not version_required or parse_version(version) > parse_version(
                    version_required
                ):
                    version_required = version

        self._required_glibc = version_required
        return version_required

    def load_dependencies(
        self,
        root_path: Path,
        base_path: Optional[Path],
        content_dirs: List[Path],
        arch_triplet: str,
        soname_cache: Optional[SonameCache] = None,
    ) -> Set[Path]:
        """Load the set of libraries that are needed to satisfy elf's runtime.

        This may include libraries contained within the project.
        The object's .dependencies attribute is set after loading.

        :param root_path: the root path to search for missing dependencies.
        :param base_path: the core base path to search for missing dependencies.
        :param content_dirs: list of paths sourced from content snaps.
        :param arch_triplet: architecture triplet of the platform.
        :param soname_cache: a cache of previously search dependencies.

        :returns: a set of paths to the library dependencies of elf.
        """
        if soname_cache is None:
            soname_cache = SonameCache()

        emit.debug(f"Getting dependencies for {str(self.path)!r}")

        search_paths = [root_path, *content_dirs]
        if base_path is not None:
            search_paths.append(base_path)

        ld_library_paths: List[str] = []
        for path in search_paths:
            ld_library_paths.extend(
                utils.get_common_ld_library_paths(path, arch_triplet)
            )

        libraries = _determine_libraries(
            path=self.path, ld_library_paths=ld_library_paths, arch_triplet=arch_triplet
        )
        for soname, soname_path in libraries.items():
            if self.arch_tuple is None:
                raise RuntimeError("failed to parse architecture")

            self.dependencies.add(
                _Library(
                    soname=soname,
                    soname_path=Path(soname_path),
                    search_paths=search_paths,
                    base_path=base_path,
                    arch_tuple=self.arch_tuple,
                    soname_cache=soname_cache,
                )
            )

        # Return the set of dependency paths, minus those found in the base.
        dependencies: Set[Path] = set()
        for library in self.dependencies:
            if not library.in_base_snap:
                dependencies.add(library.path)
        return dependencies


def _get_host_libc_path(arch_triplet) -> Path:
    return Path("/lib") / arch_triplet / "libc.so.6"


def _determine_libraries(
    *, path: Path, ld_library_paths: List[str], arch_triplet: str
) -> Dict[str, str]:
    # Try the usual method with ldd.
    with contextlib.suppress(subprocess.CalledProcessError):
        return _ldd(path, ld_library_paths)

    # Fall back to trying ldd with LD_PRELOAD explicitly loading libc.
    libc_path = _get_host_libc_path(arch_triplet)
    if libc_path.is_file():
        with contextlib.suppress(subprocess.CalledProcessError):
            return _ldd(path, ld_library_paths, ld_preload=str(libc_path))

    # Fall back to trying ld trace method which may fail with permission error
    # for non-executable shared objects, or OSError 8 Exec format error if
    # target is for different arch.
    with contextlib.suppress(PermissionError, OSError, subprocess.CalledProcessError):
        return _ld_trace(path, ld_library_paths)

    emit.progress(
        f"Unable to determine library dependencies for {str(path)!r}", permanent=True
    )

    return {}


def _ldd(
    path: Path, ld_library_paths: List[str], *, ld_preload: Optional[str] = None
) -> Dict[str, str]:
    """Use host ldd to determine library dependencies."""
    ldd = utils.get_host_tool("ldd")  # TODO: use `ld` from the base snap (#4751)
    env = {
        "LD_LIBRARY_PATH": ":".join(ld_library_paths),
    }

    if ld_preload:
        env["LD_PRELOAD"] = ld_preload

    return _parse_ldd_output(_check_output([ldd, str(path)], extra_env=env))


def _ld_trace(path: Path, ld_library_paths: List[str]) -> Dict[str, str]:
    """Use LD_TRACE_LOADED_OBJECTS to determine library dependencies."""
    env = {
        "LD_TRACE_LOADED_OBJECTS": "1",
        "LD_LIBRARY_PATH": ":".join(ld_library_paths),
    }

    return _parse_ldd_output(_check_output([str(path)], extra_env=env))


def _parse_ldd_output(output: str) -> Dict[str, str]:
    """Parse ldd output.

    Example ldd outputs:

    linux-vdso.so.1 =>  (0x00007ffdc13ec000)   <== ubuntu 16.04 ldd
    linux-vdso.so.1 (0x00007ffdc13ec000)       <== newer ldd
    /lib64/ld-linux-x86-64.so.2 (0x00007fb3c5298000)
    libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fb3bef03000)
    libmissing.so.2 => not found

    :returns: Dictionary of dependencies, mapping library name to path.
    """
    libraries: Dict[str, str] = {}
    ldd_lines = output.splitlines()

    for line in ldd_lines:
        # First match against libraries that are found.
        match = re.match(r"\t(.*) => (.*) \(0x", line)

        if not match:
            # Now find those not found, or not providing the address...
            match = re.match(r"\t(.*) => (.*)", line)

        # Ignore ld-linux, linux-vdso, etc. that don't match these regex.
        # As Ubuntu 16.04's ldd provides an empty string for the found
        # path (in group 2) on linux-vdso, check for this and ignore it.
        # See example output above for reference.
        if not match or match.group(2) == "":
            continue

        soname, soname_path = _ldd_resolve(match.group(1), match.group(2))
        libraries[soname] = soname_path

    return libraries


def _ldd_resolve(soname: str, soname_path: str) -> Tuple[str, str]:
    emit.debug(f"_ldd_resolve: {soname!r} {soname_path!r}")

    # If found, resolve the path components.  We can safely determine that
    # ldd found the match if it returns an absolute path.  For additional
    # safety, check that it exists.  See example ldd output in ldd() below.
    # If not found, ldd should use a string like "not found", but we do not
    # really care what that string is with this approach as it has to start
    # with "/" and point to a valid file.
    if soname_path.startswith("/") and os.path.exists(soname_path):
        abs_path = os.path.abspath(soname_path)
        return soname, abs_path

    # Not found, use the soname.
    return soname, soname


def _check_output(cmd: List[str], *, extra_env: Dict[str, str]) -> str:
    env = os.environ.copy()
    env.update(extra_env)

    debug_cmd = [f"{k}={v}" for k, v in extra_env.items()]
    debug_cmd += cmd

    emit.debug(f"executing: {' '.join(debug_cmd)}")
    output = subprocess.check_output(cmd, env=env).decode()

    return output