Refactor ELF parsing logic to standalone class

This makes it easier to reuse the same code for manylinux, musllinux,
and test code.

packaging/_elffile.py

@@ -0,0 +1,108 @@
+"""ELF file parser.
+
+This provides a class ``ElfFile`` that parses an ELF executable in a similar
+interface to ``ZipFile``. Only the read interface is implemented.
+
+Based on: https://gist.github.com/lyssdod/f51579ae8d93c8657a5564aefc2ffbca
+ELF header: https://refspecs.linuxfoundation.org/elf/gabi4+/ch4.eheader.html
+"""
+
+import enum
+import os
+import struct
+from typing import IO, Any, ContextManager, Optional, Tuple
+
+
+class ElfInvalid(ValueError):
+    pass
+
+
+class EIClass(enum.IntEnum):
+    C32 = 1
+    C64 = 2
+
+
+class EIData(enum.IntEnum):
+    Lsb = 1
+    Msb = 2
+
+
+class EMachine(enum.IntEnum):
+    I386 = 3
+    S390 = 22
+    Arm = 40
+    X8664 = 62
+    AArc64 = 183
+
+
+class ElfFile(ContextManager["ElfFile"]):
+    """Representation of an ELF executable."""
+
+    def __init__(self, f: IO[bytes]) -> None:
+        self._f = f
+
+        try:
+            ident = self._read("16B")
+        except struct.error:
+            raise ElfInvalid
+        if bytes(ident[:4]) != b"\x7fELF":  # Invalid magic, not ELF.
+            raise ElfInvalid
+
+        self.capacity = ident[4]  # Format for program header (bitness).
+        self.encoding = ident[5]  # Data structure encoding (endianess).
+
+        try:
+            # e_fmt: Format for program header.
+            # p_fmt: Format for section header.
+            # p_idx: Indexes to find p_type, p_offset, and p_filesz.
+            e_fmt, self._p_fmt, self._p_idx = {
+                (1, 1): ("<HHIIIIIHHH", "<IIIIIIII", (0, 1, 4)),  # 32-bit LSB.
+                (1, 2): (">HHIIIIIHHH", ">IIIIIIII", (0, 1, 4)),  # 32-bit MSB.
+                (2, 1): ("<HHIQQQIHHH", "<IIQQQQQQ", (0, 2, 5)),  # 64-bit LSB.
+                (2, 2): (">HHIQQQIHHH", ">IIQQQQQQ", (0, 2, 5)),  # 64-bit MSB.
+            }[(self.capacity, self.encoding)]
+        except KeyError:
+            raise ElfInvalid
+
+        try:
+            (
+                _,
+                self.machine,  # Architecture type.
+                _,
+                _,
+                self._e_phoff,  # Offset of program header.
+                _,
+                self.flags,  # Processor-specific flags.
+                _,
+                self._e_phentsize,  # Size of section.
+                self._e_phnum,  # Number of sections.
+            ) = self._read(e_fmt)
+        except struct.error:
+            raise ElfInvalid
+
+    def __enter__(self) -> "ElfFile":
+        return self
+
+    def __exit__(self, typ: Any, val: Any, tb: Any) -> None:
+        self.close()
+
+    def _read(self, fmt: str) -> Tuple[int, ...]:
+        return struct.unpack(fmt, self._f.read(struct.calcsize(fmt)))
+
+    def close(self) -> None:
+        self._f.close()
+
+    @property
+    def interpreter(self) -> Optional[str]:
+        """Path recorded in the ``PT_INTERP`` section header."""
+        for index in range(self._e_phnum):
+            self._f.seek(self._e_phoff + self._e_phentsize * index)
+            try:
+                data = self._read(self._p_fmt)
+            except struct.error:
+                continue
+            if data[self._p_idx[0]] != 3:  # Not PT_INTERP.
+                continue
+            self._f.seek(data[self._p_idx[1]])
+            return os.fsdecode(self._f.read(data[self._p_idx[2]])).strip("\0")
+        return None

packaging/_manylinux.py

@@ -2,120 +2,47 @@
 import functools
 import os
 import re
-import struct
 import sys
 import warnings
-from typing import IO, Dict, Iterator, NamedTuple, Optional, Tuple
-
-
-# Python does not provide platform information at sufficient granularity to
-# identify the architecture of the running executable in some cases, so we
-# determine it dynamically by reading the information from the running
-# process. This only applies on Linux, which uses the ELF format.
-class _ELFFileHeader:
-    # https://en.wikipedia.org/wiki/Executable_and_Linkable_Format#File_header
-    class _InvalidELFFileHeader(ValueError):
-        """
-        An invalid ELF file header was found.
-        """
-
-    ELF_MAGIC_NUMBER = 0x7F454C46
-    ELFCLASS32 = 1
-    ELFCLASS64 = 2
-    ELFDATA2LSB = 1
-    ELFDATA2MSB = 2
-    EM_386 = 3
-    EM_S390 = 22
-    EM_ARM = 40
-    EM_X86_64 = 62
-    EF_ARM_ABIMASK = 0xFF000000
-    EF_ARM_ABI_VER5 = 0x05000000
-    EF_ARM_ABI_FLOAT_HARD = 0x00000400
-
-    def __init__(self, file: IO[bytes]) -> None:
-        def unpack(fmt: str) -> int:
-            try:
-                data = file.read(struct.calcsize(fmt))
-                result: Tuple[int, ...] = struct.unpack(fmt, data)
-            except struct.error:
-                raise _ELFFileHeader._InvalidELFFileHeader()
-            return result[0]
-
-        self.e_ident_magic = unpack(">I")
-        if self.e_ident_magic != self.ELF_MAGIC_NUMBER:
-            raise _ELFFileHeader._InvalidELFFileHeader()
-        self.e_ident_class = unpack("B")
-        if self.e_ident_class not in {self.ELFCLASS32, self.ELFCLASS64}:
-            raise _ELFFileHeader._InvalidELFFileHeader()
-        self.e_ident_data = unpack("B")
-        if self.e_ident_data not in {self.ELFDATA2LSB, self.ELFDATA2MSB}:
-            raise _ELFFileHeader._InvalidELFFileHeader()
-        self.e_ident_version = unpack("B")
-        self.e_ident_osabi = unpack("B")
-        self.e_ident_abiversion = unpack("B")
-        self.e_ident_pad = file.read(7)
-        format_h = "<H" if self.e_ident_data == self.ELFDATA2LSB else ">H"
-        format_i = "<I" if self.e_ident_data == self.ELFDATA2LSB else ">I"
-        format_q = "<Q" if self.e_ident_data == self.ELFDATA2LSB else ">Q"
-        format_p = format_i if self.e_ident_class == self.ELFCLASS32 else format_q
-        self.e_type = unpack(format_h)
-        self.e_machine = unpack(format_h)
-        self.e_version = unpack(format_i)
-        self.e_entry = unpack(format_p)
-        self.e_phoff = unpack(format_p)
-        self.e_shoff = unpack(format_p)
-        self.e_flags = unpack(format_i)
-        self.e_ehsize = unpack(format_h)
-        self.e_phentsize = unpack(format_h)
-        self.e_phnum = unpack(format_h)
-        self.e_shentsize = unpack(format_h)
-        self.e_shnum = unpack(format_h)
-        self.e_shstrndx = unpack(format_h)
-
-
-def _get_elf_header() -> Optional[_ELFFileHeader]:
-    try:
-        with open(sys.executable, "rb") as f:
-            elf_header = _ELFFileHeader(f)
-    except (OSError, TypeError, _ELFFileHeader._InvalidELFFileHeader):
-        return None
-    return elf_header
+from typing import Dict, Iterator, NamedTuple, Optional, Tuple
+
+from ._elffile import EIClass, EIData, ElfFile, EMachine
 
+EF_ARM_ABIMASK = 0xFF000000
+EF_ARM_ABI_VER5 = 0x05000000
+EF_ARM_ABI_FLOAT_HARD = 0x00000400
 
-def _is_linux_armhf() -> bool:
+
+def _is_linux_armhf(f: ElfFile) -> bool:
     # hard-float ABI can be detected from the ELF header of the running
     # process
     # https://static.docs.arm.com/ihi0044/g/aaelf32.pdf
-    elf_header = _get_elf_header()
-    if elf_header is None:
+    if f.capacity != EIClass.C32:
+        return False
+    if f.encoding != EIData.Lsb:
+        return False
+    if f.machine != EMachine.Arm:
         return False
-    result = elf_header.e_ident_class == elf_header.ELFCLASS32
-    result &= elf_header.e_ident_data == elf_header.ELFDATA2LSB
-    result &= elf_header.e_machine == elf_header.EM_ARM
-    result &= (
-        elf_header.e_flags & elf_header.EF_ARM_ABIMASK
-    ) == elf_header.EF_ARM_ABI_VER5
-    result &= (
-        elf_header.e_flags & elf_header.EF_ARM_ABI_FLOAT_HARD
-    ) == elf_header.EF_ARM_ABI_FLOAT_HARD
-    return result
-
-
-def _is_linux_i686() -> bool:
-    elf_header = _get_elf_header()
-    if elf_header is None:
+    if f.flags & EF_ARM_ABIMASK != EF_ARM_ABI_VER5:
         return False
-    result = elf_header.e_ident_class == elf_header.ELFCLASS32
-    result &= elf_header.e_ident_data == elf_header.ELFDATA2LSB
-    result &= elf_header.e_machine == elf_header.EM_386
-    return result
+    if f.flags & EF_ARM_ABI_FLOAT_HARD != EF_ARM_ABI_FLOAT_HARD:
+        return False
+    return True
+
 
+def _is_linux_i686(f: ElfFile) -> bool:
+    return (
+        f.capacity == EIClass.C32
+        and f.encoding == EIData.Lsb
+        and f.machine == EMachine.I386
+    )
 
-def _have_compatible_abi(arch: str) -> bool:
+
+def _have_compatible_abi(f: ElfFile, arch: str) -> bool:
     if arch == "armv7l":
-        return _is_linux_armhf()
+        return _is_linux_armhf(f)
     if arch == "i686":
-        return _is_linux_i686()
+        return _is_linux_i686(f)
     return arch in {"x86_64", "aarch64", "ppc64", "ppc64le", "s390x"}
 
 
@@ -141,10 +68,10 @@ def _glibc_version_string_confstr() -> Optional[str]:
     # platform module.
     # https://github.com/python/cpython/blob/fcf1d003bf4f0100c/Lib/platform.py#L175-L183
     try:
-        # os.confstr("CS_GNU_LIBC_VERSION") returns a string like "glibc 2.17".
-        version_string = os.confstr("CS_GNU_LIBC_VERSION")
+        # Should be a string like "glibc 2.17".
+        version_string: str = getattr(os, "confstr")("CS_GNU_LIBC_VERSION")
         assert version_string is not None
-        _, version = version_string.split()
+        _, version = version_string.rsplit()
     except (AssertionError, AttributeError, OSError, ValueError):
         # os.confstr() or CS_GNU_LIBC_VERSION not available (or a bad value)...
         return None
@@ -211,8 +138,8 @@ def _parse_glibc_version(version_str: str) -> Tuple[int, int]:
     m = re.match(r"(?P<major>[0-9]+)\.(?P<minor>[0-9]+)", version_str)
     if not m:
         warnings.warn(
-            "Expected glibc version with 2 components major.minor,"
-            " got: %s" % version_str,
+            f"Expected glibc version with 2 components major.minor,"
+            f" got: {version_str}",
             RuntimeWarning,
         )
         return -1, -1
@@ -265,7 +192,13 @@ def _is_compatible(name: str, arch: str, version: _GLibCVersion) -> bool:
 
 
 def platform_tags(linux: str, arch: str) -> Iterator[str]:
-    if not _have_compatible_abi(arch):
+    try:
+        with open(sys.executable, "rb") as f:
+            with ElfFile(f) as ef:
+                abi_compatible = _have_compatible_abi(ef, arch)
+    except (OSError, TypeError, ValueError):
+        abi_compatible = False
+    if not abi_compatible:
         return
     # Oldest glibc to be supported regardless of architecture is (2, 17).
     too_old_glibc2 = _GLibCVersion(2, 16)

packaging/_musllinux.py

@@ -4,70 +4,13 @@
 linked against musl, and what musl version is used.
 """
 
-import contextlib
 import functools
-import operator
-import os
 import re
-import struct
 import subprocess
 import sys
-from typing import IO, Iterator, NamedTuple, Optional, Tuple
+from typing import Iterator, NamedTuple, Optional
 
-
-def _read_unpacked(f: IO[bytes], fmt: str) -> Tuple[int, ...]:
-    return struct.unpack(fmt, f.read(struct.calcsize(fmt)))
-
-
-def _parse_ld_musl_from_elf(f: IO[bytes]) -> Optional[str]:
-    """Detect musl libc location by parsing the Python executable.
-
-    Based on: https://gist.github.com/lyssdod/f51579ae8d93c8657a5564aefc2ffbca
-    ELF header: https://refspecs.linuxfoundation.org/elf/gabi4+/ch4.eheader.html
-    """
-    f.seek(0)
-    try:
-        ident = _read_unpacked(f, "16B")
-    except struct.error:
-        return None
-    if ident[:4] != tuple(b"\x7fELF"):  # Invalid magic, not ELF.
-        return None
-    f.seek(struct.calcsize("HHI"), 1)  # Skip file type, machine, and version.
-
-    try:
-        # e_fmt: Format for program header.
-        # p_fmt: Format for section header.
-        # p_idx: Indexes to find p_type, p_offset, and p_filesz.
-        e_fmt, p_fmt, p_idx = {
-            (1, 1): ("<IIIIHHH", "<IIIIIIII", (0, 1, 4)),  # 32-bit LSB.
-            (1, 2): (">IIIIHHH", ">IIIIIIII", (0, 1, 4)),  # 32-bit MSB.
-            (2, 1): ("<QQQIHHH", "<IIQQQQQQ", (0, 2, 5)),  # 64-bit LSB.
-            (2, 2): (">QQQIHHH", ">IIQQQQQQ", (0, 2, 5)),  # 64-bit MSB.
-        }[(ident[4], ident[5])]
-    except KeyError:
-        return None
-    else:
-        p_get = operator.itemgetter(*p_idx)
-
-    # Find the interpreter section and return its content.
-    try:
-        _, e_phoff, _, _, _, e_phentsize, e_phnum = _read_unpacked(f, e_fmt)
-    except struct.error:
-        return None
-    for i in range(e_phnum + 1):
-        f.seek(e_phoff + e_phentsize * i)
-        try:
-            p_type, p_offset, p_filesz = p_get(_read_unpacked(f, p_fmt))
-        except struct.error:
-            return None
-        if p_type != 3:  # Not PT_INTERP.
-            continue
-        f.seek(p_offset)
-        interpreter = os.fsdecode(f.read(p_filesz)).strip("\0")
-        if "musl" not in interpreter:
-            return None
-        return interpreter
-    return None
+from ._elffile import ElfFile
 
 
 class _MuslVersion(NamedTuple):
@@ -97,13 +40,14 @@ def _get_musl_version(executable: str) -> Optional[_MuslVersion]:
         Version 1.2.2
         Dynamic Program Loader
     """
-    with contextlib.ExitStack() as stack:
-        try:
-            f = stack.enter_context(open(executable, "rb"))
-        except OSError:
-            return None
-        ld = _parse_ld_musl_from_elf(f)
-    if not ld:
+    try:
+        with open(executable, "rb") as f:
+            with ElfFile(f) as ef:
+                ld = ef.interpreter
+    except (OSError, TypeError, ValueError):
+        return None
+
+    if ld is None or "musl" not in ld:
         return None
     proc = subprocess.run([ld], stderr=subprocess.PIPE, universal_newlines=True)
     return _parse_musl_version(proc.stderr)