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February 2018

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draft PEP: manylinux2
by Mark Williams 01 Feb '18

01 Feb '18

Hi everyone! The manylinux1 platform tag has been tremendously useful, but unfortunately it's showing its age: https://mail.python.org/pipermail/distutils-sig/2017-April/030360.html https://mail.python.org/pipermail/wheel-builders/2016-December/000239.html Nathaniel identified a list of things to do for its successor, manylinux2: https://mail.python.org/pipermail/distutils-sig/2017-April/030361.html Please find below a draft PEP for manylinux2 that attempts to address these issues. I've also opened a PR against python/peps: https://github.com/python/peps/pull/565 Docker images for x86_64 and i686 are available to test drive: https://hub.docker.com/r/markrwilliams/manylinux2/tags/ Thanks! ---- PEP: 9999 Title: The manylinux2 Platform Tag Version: $Revision$ Last-Modified: $Date$ Author: Mark Williams <mrw(a)enotuniq.org> BDFL-Delegate: Nick Coghlan <ncoghlan(a)gmail.com> Discussions-To: Distutils SIG <distutils-sig(a)python.org> Status: Active Type: Informational Content-Type: text/x-rst Created: Post-History: Resolution: Abstract ======== This PEP proposes the creation of a ``manylinux2`` platform tag to succeed the ``manylinux1`` tag introduced by PEP 513 [1]_. It also proposes that PyPI and ``pip`` both be updated to support uploading, downloading, and installing ``manylinux2`` distributions on compatible platforms. Rationale ========= True to its name, the ``manylinux1`` platform tag has made the installation of binary extension modules a reality on many Linux systems. Libraries like ``cryptography`` [2]_ and ``numpy`` [3]_ are more accessible to Python developers now that their installation on common architectures does not depend on fragile development environments and build toolchains. ``manylinux1`` wheels achieve their portability by allowing the extension modules they contain to link against only a small set of system-level shared libraries that export versioned symbols old enough to benefit from backwards-compatibility policies. Extension modules in a ``manylinux1`` wheel that rely on ``glibc``, for example, must be built against version 2.5 or earlier; they may then be run systems that provide more recent ``glibc`` version that still export the required symbols at version 2.5. PEP 513 drew its whitelisted shared libraries and their symbol versions from CentOS 5.11, which was the oldest supported CentOS release at the time of its writing. Unfortunately, CentOS 5.11 reached its end-of-life on March 31st, 2017 with a clear warning against its continued use. [4]_ No further updates, such as security patches, will be made available. This means that its packages will remain at obsolete versions that hamper the efforts of Python software packagers who use the ``manylinux1`` Docker image. CentOS 6.9 is now the oldest supported CentOS release, and will receive maintenance updates through November 30th, 2020. [5]_ We propose that a new PEP 425-style [6]_ platform tag called ``manylinux2`` be derived from CentOS 6.9 and that the ``manylinux`` toolchain, PyPI, and ``pip`` be updated to support it. The ``manylinux2`` policy ========================= The following criteria determine a ``linux`` wheel's eligibility for the ``manylinux2`` tag: 1. The wheel may only contain binary executables and shared objects compiled for one of the two architectures supported by CentOS 6.9: x86_64 or i686. [5]_ 2. The wheel's binary executables or shared objects may not link against externally-provided libraries except those in the following whitelist: :: libgcc_s.so.1 libstdc++.so.6 libm.so.6 libdl.so.2 librt.so.1 libcrypt.so.1 libc.so.6 libnsl.so.1 libutil.so.1 libpthread.so.0 libresolv.so.2 libX11.so.6 libXext.so.6 libXrender.so.1 libICE.so.6 libSM.so.6 libGL.so.1 libgobject-2.0.so.0 libgthread-2.0.so.0 libglib-2.0.so.0 This list is identical to the externally-provided libraries whitelisted for ``manylinux1``, minus ``libncursesw.so.5`` and ``libpanelw.so.5``. [7]_ ``libpythonX.Y`` remains ineligible for inclusion for the same reasons outlined in PEP 513. On Debian-based systems, these libraries are provided by the packages: ============ ======================================================= Package Libraries ============ ======================================================= libc6 libdl.so.2, libresolv.so.2, librt.so.1, libc.so.6, libpthread.so.0, libm.so.6, libutil.so.1, libcrypt.so.1, libnsl.so.1 libgcc1 libgcc_s.so.1 libgl1 libGL.so.1 libglib2.0-0 libgobject-2.0.so.0, libgthread-2.0.so.0, libglib-2.0.so.0 libice6 libICE.so.6 libsm6 libSM.so.6 libstdc++6 libstdc++.so.6 libx11-6 libX11.so.6 libxext6 libXext.so.6 libxrender1 libXrender.so.1 ============ ======================================================= On RPM-based systems, they are provided by these packages: ============ ======================================================= Package Libraries ============ ======================================================= glib2 libglib-2.0.so.0, libgthread-2.0.so.0, libgobject-2.0.so.0 glibc libresolv.so.2, libutil.so.1, libnsl.so.1, librt.so.1, libcrypt.so.1, libpthread.so.0, libdl.so.2, libm.so.6, libc.so.6 libICE libICE.so.6 libX11 libX11.so.6 libXext: libXext.so.6 libXrender libXrender.so.1 libgcc: libgcc_s.so.1 libstdc++ libstdc++.so.6 mesa libGL.so.1 ============ ======================================================= 3. If the wheel contains binary executables or shared objects linked against any whitelisted libraries that also export versioned symbols, they may only depend on the following maximum versions:: GLIBC_2.12 CXXABI_1.3.3 GLIBCXX_3.4.13 GCC_4.3.0 As an example, ``manylinux2`` wheels may include binary artifacts that require ``glibc`` symbols at version ``GLIBC_2.4``, because this an earlier version than the maximum of ``GLIBC_2.12``. 4. If a wheel is built for any version of CPython 2 or CPython versions 3.0 up to and including 3.2, it *must* include a CPython ABI tag indicating its Unicode ABI. A ``manylinux2`` wheel built against Python 2, then, must include either the ``cpy27mu`` tag indicating it was built against an interpreter with the UCS-4 ABI or the ``cpy27m`` tag indicating an interpeter with the UCS-2 ABI. *[Citation for UCS ABI tags?]* 5. A wheel *must not* require the ``PyFPE_jbuf`` symbol. This is achieved by building it against a Python compiled *without* the ``--with-fpectl`` ``configure`` flag. Compilation of Compliant Wheels =============================== Like ``manylinux1``, the ``auditwheel`` tool adds ```manylinux2`` platform tags to ``linux`` wheels built by ``pip wheel`` or ``bdist_wheel`` a ``manylinux2`` Docker container. Docker Images ------------- ``manylinux2`` Docker images based on CentOS 6.9 x86_64 and i686 are provided for building binary ``linux`` wheels that can reliably be converted to ``manylinux2`` wheels. [8]_ These images come with a full compiler suite installed (``gcc``, ``g++``, and ``gfortran`` 4.8.2) as well as the latest releases of Python and ``pip``. Compatibility with kernels that lack ``vsyscall`` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A Docker container assumes that its userland is compatible with its host's kernel. Unfortunately, an increasingly common kernel configuration breaks breaks this assumption for x86_64 CentOS 6.9 Docker images. Versions 2.14 and earlier of ``glibc`` require the kernel provide an archaic system call optimization known as ``vsyscall`` on x86_64. [9]_ To effect the optimization, the kernel maps a read-only page of frequently-called system calls -- most notably ``time(2)`` -- into each process at a fixed memory location. ``glibc`` then invokes these system calls by dereferencing a function pointer to the appropriate offset into the ``vsyscall`` page and calling it. This avoids the overhead associated with invoking the kernel that affects normal system call invocation. ``vsyscall`` has long been deprecated in favor of an equivalent mechanism known as vDSO, or "virtual dynamic shared object", in which the kernel instead maps a relocatable virtual shared object containing the optimized system calls into each process. [10]_ The ``vsyscall`` page has serious security implications because it does not participate in address space layout randomization (ASLR). Its predictable location and contents make it a useful source of gadgets used in return-oriented programming attacks. [11]_ At the same time, its elimination breaks the x86_64 ABI, because ``glibc`` versions that depend on ``vsyscall`` suffer from segmentation faults when attempting to dereference a system call pointer into a non-existent page. As a compromise, Linux 3.1 implemented an "emulated" ``vsyscall`` that reduced the executable code, and thus the material for ROP gadgets, mapped into the process. [12]_ ``vsyscall=emulated`` has been the default configuration in most distribution's kernels for many years. Unfortunately, ``vsyscall`` emulation still exposes predicatable code at a reliable memory location, and continues to be useful for return-oriented programming. [13]_ Because most distributions have now upgraded to ``glibc`` versions that do not depend on ``vsyscall``, they are beginning to ship kernels that do not support ``vsyscall`` at all. [14]_ CentOS 5.11 and 6.9 both include versions of ``glibc`` that depend on the ``vsyscall`` page (2.5 and 2.12.2 respectively), so containers based on either cannot run under kernels provided with many distribution's upcoming releases. [15]_ Continuum Analytics faces a related problem with its conda software suite, and as they point out, this will pose a significant obstacle to using these tools in hosted services. [16]_ If Travis CI, for example, begins running jobs under a kernel that does not provide the ``vsyscall`` interface, Python packagers will not be able to use our Docker images there to build ``manylinux`` wheels. [17]_ We have derived a patch from the ``glibc`` git repository that backports the removal of all dependencies on ``vsyscall`` to the version of ``glibc`` included with our ``manylinux2`` image. [18]_ Rebuilding ``glibc``, and thus building ``manylinux2`` image itself, still requires a host kernel that provides the ``vsyscall`` mechanism, but the resulting image can be both run on hosts that provide it and those that do not. Because the ``vsyscall`` interface is an optimization that is only applied to running processes, the ``manylinux2`` wheels built with this modified image should be identical to those built on an unmodified CentOS 6.9 system. Also, the ``vsyscall`` problem applies only to x86_64; it is not part of the i686 ABI. Auditwheel ---------- The ``auditwheel`` tool has also been updated to produce ``manylinux2`` wheels. [19]_ Its behavior and purpose are otherwise unchanged from PEP 513. Platform Detection for Installers ================================= Platforms may define a ``manylinux2_compatible`` boolean attribute on the ``_manylinux`` module described in PEP 513. A platform is considered incompatible with ``manylinux2`` if the attribute is ``False``. Backwards compatibility with ``manylinux1`` wheels ================================================== As explained in PEP 513, the specified symbol versions for ``manylinux1`` whitelisted libraries constitute an *upper bound*. The same is true for the symbol versions defined for ``manylinux2`` in this PEP. As a result, ``manylinux1`` wheels are considered ``manylinux2`` wheels. A ``pip`` that recognizes the ``manylinux2`` platform tag will thus install ``manylinux1`` wheels for ``manylinux2`` platforms -- even when explicitly set -- when no ``manylinux2`` wheels are available. [20]_ PyPI Support ============ PyPI should permit wheels containing the ``manylinux2`` platform tag to be uploaded in the same way that it permits ``manylinux1``. It should not attempt to verify the compatibility of ``manylinux2`` wheels. References ========== .. [1] PEP 513 -- A Platform Tag for Portable Linux Built Distributions (https://www.python.org/dev/peps/pep-0513/) .. [2] pyca/cryptography (https://cryptography.io/) .. [3] numpy (https://numpy.org) .. [4] CentOS 5.11 EOL announcement (https://lists.centos.org/pipermail/centos-announce/2017-April/022350.html) .. [5] CentOS Product Specifications (https://web.archive.org/web/20180108090257/https://wiki.centos.org/About/Pr…) .. [6] PEP 425 -- Compatibility Tags for Built Distributions (https://www.python.org/dev/peps/pep-0425/) .. [7] ncurses 5 -> 6 transition means we probably need to drop some libraries from the manylinux whitelist (https://github.com/pypa/manylinux/issues/94) .. [8] manylinux2 Docker images (https://hub.docker.com/r/markrwilliams/manylinux2/) .. [9] On vsyscalls and the vDSO (https://lwn.net/Articles/446528/) .. [10] vdso(7) (http://man7.org/linux/man-pages/man7/vdso.7.html) .. [11] Framing Signals -- A Return to Portable Shellcode (http://www.cs.vu.nl/~herbertb/papers/srop_sp14.pdf) .. [12] ChangeLog-3.1 (https://www.kernel.org/pub/linux/kernel/v3.x/ChangeLog-3.1) .. [13] Project Zero: Three bypasses and a fix for one of Flash's Vector.<*> mitigations (https://googleprojectzero.blogspot.com/2015/08/three-bypasses-and-fix-for-o…) .. [14] linux: activate CONFIG_LEGACY_VSYSCALL_NONE ? (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=852620) .. [15] [Wheel-builders] Heads-up re: new kernel configurations breaking the manylinux docker image (https://mail.python.org/pipermail/wheel-builders/2016-December/000239.html) .. [16] Due to glibc 2.12 limitation, static executables that use time(), cpuinfo() and maybe a few others cannot be run on systems that do not support or use `vsyscall=emulate` (https://github.com/ContinuumIO/anaconda-issues/issues/8203) .. [17] Travis CI (https://travis-ci.org/) .. [18] remove-vsyscall.patch https://github.com/markrwilliams/manylinux/commit/e9493d55471d153089df3aafc… .. [19] auditwheel manylinux2 branch (https://github.com/markrwilliams/auditwheel/tree/manylinux2) .. [20] pip manylinux2 branch https://github.com/markrwilliams/pip/commits/manylinux2 Copyright ========= This document has been placed into the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: -- Mark Williams mrw(a)enotuniq.org

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