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[PAuthABIELF64] Use .note.gnu.property section as ELF marking scheme.
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Make the alternative .note.gnu.property section the default ELF
marking scheme for ELF executables and shared libraries. Preserve
the original SHT_NOTE section as an alternative for legacy
compatibility.

Extract the common parts of all of the ELF marking schemes
into "Core Information". The ELF marking schemes now describe
how they encode the "Core Information". The compatibility model
is now written in terms of the "Core Information".

The default PAuth vendor and version for ELF files with no ELF
marking is (0,0) which is incompatible with everything. This is
the safest default as there is no information about what, if any,
signing schema is used by that ELF file.

Implementations can change this default in the presence of
supplemental information, such as a linker command line option.
For example there could be the equivalent of the -z force-bti
which declares that all ELF objects without the BTI property are
considered to be compatible.
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smithp35 committed Mar 15, 2024
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205 changes: 121 additions & 84 deletions pauthabielf64/pauthabielf64.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@
.. _CPPABI64: https://github.com/ARM-software/abi-aa/releases
.. _LSB: https://refspecs.linuxfoundation.org/LSB_1.2.0/gLSB/noteabitag.html
.. _SCO-ELF: http://www.sco.com/developers/gabi/
.. _SYSVABI64: https://github.com/ARM-software/abi-aa/releases
.. _TLSDESC: http://www.fsfla.org/~lxoliva/writeups/TLS/paper-lk2006.pdf
.. _LINUX_ABI: https://github.com/hjl-tools/linux-abi/wiki
.. footer::
Expand Down Expand Up @@ -234,6 +235,8 @@ changes to the content of the document for that release.
| 2023Q3 | 6\ :sup:`th` October 2023 | Update tags in `Dynamic Section`_ to avoid conflict with |
| | | DT_AARCH64_VARIANT_PCS. |
+------------+-----------------------------+------------------------------------------------------------------+
| 2024Q1 | 29\ :sup:`th` January 2024 | Update preferred ELF marking scheme to be GNU property based |
+------------+-----------------------------+------------------------------------------------------------------+

References
----------
Expand All @@ -257,11 +260,14 @@ This document refers to, or is referred to by, the following documents.
+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+
| SCO-ELF_ | http://www.sco.com/developers/gabi/ | System V Application Binary Interface – DRAFT |
+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+
| SYSVABI64_ | sysvabi64 | System V Application Binary Interface (ABI) for the Arm 64-bit |
| | | Architecture |
+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+
| TLSDESC_ | http://www.fsfla.org/~lxoliva/writeups/TLS/paper-lk2006.pdf | TLS Descriptors for Arm. Original proposal document |
+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+
| `GABI_SHT_RELR <https://groups.google.com/d/msg/generic-abi/bX460iggiKg/YT2RrjpMAwAJ>`_ | ELF GABI Google Groups | Proposal for a new section type SHT_RELR |
+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+
| LINUX_ABI_ | https://github.com/hjl-tools/linux-abi/wiki | Linux Extensions to gABI |
| `LINUX_ABI`_ | https://github.com/hjl-tools/linux-abi/wiki | Linux Extensions to gABI |
+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+

Terms and Abbreviations
Expand Down Expand Up @@ -841,31 +847,27 @@ language to signing schema is expected to evolve over time. Even if
the low-level ELF extensions remain constant, a change to the
high-level language mapping may result in incompatible ELF files.

This document defines a default ELF marking schema and a base
compatibility model. Platforms may define their own ELF marking and
compatibility model that replace or extend the default ones. `Appendix
Alternative ELF Marking Using GNU Program Properties`_ defines an
alternative marking schema for platforms that support the
``.note.gnu.property`` section.
This document defines the core information that any ELF marking
scheme must contain and the base compatibility model that uses that
information.

Default Marking Schema
----------------------
The default ELF marking scheme uses the Program Property note format
defined in (`LINUX_ABI`_). An alternative encoding that uses a Arm
defined Note section called ``.note.AARCH64-PAUTH-ABI-tag`` is defined
for platforms that do not support Program Properties, or have legacy
binaries from earlier versions of this specification. This is described
in `Appendix Alternative ELF Marking Using SHT_NOTE section`_.

A new section named ``.note.AARCH64-PAUTH-ABI-tag`` of type
``SHT_NOTE`` is defined. This section is structured as a note section
as documented in SCO-ELF_, and its attribute flag ``SHF_ALLOC`` must
be set.
Core information
----------------

The name field (``namesz`` / ``name``) contains the string "ARM". The
type field shall be 1, and the ``descsz`` field must be at least 16.
The first 16 bytes of the description must contain 2 64-bit words, with
the first 64-bit word being a platform identifier, and the second
64-bit word being a version number for the ABI for the platform
identified for the first word. When ``descsz`` is larger than 16 the
remainder of the contents of desc are defined by the (platform id,
version number).
The core information used by the base compatibility model is made up
of two values, both of which must be present.

The following values of the platform id are reserved:
* ``platform identifier`` is a 64-bit value that specifies the platform vendor. The
values of the ``platform identifier`` in the table below are reserved:

.. table:: Reserved id

+-----------+-----------+
| Platform | Hex value |
Expand All @@ -875,56 +877,100 @@ The following values of the platform id are reserved:
| Baremetal | 0x1 |
+-----------+-----------+

The version number in ``.note.AARCH64-PAUTH-ABI-tag`` is not directly
related to the version number of this document. It is controlled by
the object-producer based on the signing schema that has been used for
pointers.
* ``version number`` is a 64-bit value that identifies the signing
schema used by the ELF file. The meaning of the value is determined
by the platform vendor identified by the ``platform identifier``
above.

If a file contains a section named ``.note.AARCH64-PAUTH-ABI-tag``,
it must observe the entirety of the rules in this default marking
schema. Generating such section with a platform-specific schema is
forbidden.
The tuple of (``platform identifier``, ``version number``) equal to
(0,0) is reserved to mean an ELF file incompatible with the PAuth ABI
Extension to ELF for the Arm® 64-bit Architecture (AArch64).

Default Marking Schema
----------------------

The default ELF marking scheme for executables and shared-libraries
uses the ``.note.gnu.property`` section. The format of this section is
defined in (`LINUX_ABI`_).

The following processor-specific program property types are defined:

+----------------------------------------+------------+
| Name | Value |
+========================================+============+
| GNU\_PROPERTY\_AARCH64\_FEATURE\_PAUTH | 0xc0000001 |
+----------------------------------------+------------+

Other processor-specific program property types defined by the 64-bit
ABI for the Arm Architecture are defined in (SYSVABI64_).

The format of the data in ``pr_data`` is two 64-bit words. With the
first 64-bit word being the ``platform identifier``, and the second
64-bit word being the ``version number``. Both of these form the
information required in `Core Information`_ above.

``.note.gnu.property`` sections can be used as ELF marking for
relocatable objects as well as executables and shared libraries. Arm
intends to use standardize build attributes for all relocatable-object
ELF marking. When this change occurs the default ELF marking for
relocatable objects will be updated to use build attributes.

Base Compatibility Model
------------------------

A per-ELF file marking scheme permits a coarse way of reasoning about
compatibility.

* The absence of a ``.note.AARCH64-PAUTH-ABI-tag`` section means no
information on how pointers are signed is available for this ELF
file.

* The presence of a ``.note.AARCH64-PAUTH-ABI-tag`` means that, if the
file contains pointers, they were signed in a compatible way with
the default signing rules for tuple (platform id, version number).

* The result of a successful combination of
``.note.AArch64-PAUTH-ABI-tag`` sections is a single
``.note.AArch64-PAUTH-ABI-tag`` section containing the (platform id,
version number) tuple. The result of an unsuccesful combination must
be either a single ``.note.AArch64-PAUTH-ABI-tag`` section containing
a platform id with value Invalid, or no ``.note.AArch64-PAUTH-ABI-tag``
section written to the output file.

* The static linker may fault the combination of relocatable
objects that contain ``.note.AARCH64-PAUTH-ABI-tag`` sections with
incompatible (platform id, version number) tuples.
* All reasoning about compatibility is done using the `Core Information`_.
This permits an ELF file using the ``.note.gnu.property`` ELF marking to
be compared to an ELF file using the ``.note.AARCH64-PAUTH-ABI-tag`` ELF
marking.

* If an ELF file contains multiple ELF markings of the `Core
Information`_, for example it contains both a ``.note.gnu.property``
section and a ``.note.AARCH64-PAUTH-ABI-tag`` section, then all
must encode the same `Core Information`_.

* The absence of any ELF marking means no information on how pointers
are signed is available for this ELF file. When used in combination
with ELF files that contain ELF marking, then by default the file is
assigned the (``platform identifer``, ``version number``) of (0,0).
Implementations may use additional information supplementary to the
ELF file, such as linker command-line options, to provide an
implementation defined `Core Information`_ for ELF files with no ELF
marking.

* The presence of ELF marking means that, if the file contains
pointers, they were signed in a compatible way with the schema
identified in the (platform identifier, version number). `Core
Information`_.

* A combination of `Core Information`_ from two ELF files is
successful if the ``platform identifier`` values match and the
``version numbers`` values match. All other combinations are
unsuccessful.

* The result of a successful combination of `Core Information`_ is a
single ELF Marking containing the `Core Information`_. The result of
an unsuccesful combination must be either a single ELF marking with
(``platform identifer``, ``version number``) of (0,0), or no ELF
marking is written to the output file.

* The static linker may choose to fault the unsuccessful combination
of `Core Information`_.

* A dynamic loader may disable pointer authentication, or fault the
program with an error message, in case it encounters an incompatible
ELF file. A file is incompatible with the loader in any of the
following cases:

* If section ``.note.AARCH64-PAUTH-ABI-tag`` is missing.
ELF file. A dynamic loader may consider the ELF file to be
incompatible with the PAuthABI in any of the following cases:

* If the (platform id, version number) tuple is not recognized.
* If no ELF marking is present.

* If the platform id is Invalid.
* If the (platform identifier, version number) from the `Core
Information`_ is not recognized by the loader.

The combination of relocatable objects with
``.note.AARCH64-PAUTH-ABI-tag`` and relocatable objects without a
``.note.AARCH64-PAUTH-ABI-tag`` is not defined by this ABI.
* If the platform identifier from the `Core Information`_ is the
reserved Invalid value 0.

Platforms may replace the base compatibility model with a
platform-specific model.
Expand Down Expand Up @@ -1236,33 +1282,24 @@ Some observations:
pointer signing information in a custom encoding understood by the
start-up code used.

Appendix Alternative ELF Marking Using GNU Program Properties
=============================================================
Appendix Alternative ELF Marking Using SHT_NOTE section
=======================================================

A new section named ``.note.AARCH64-PAUTH-ABI-tag`` of type
``SHT_NOTE`` is defined. This section is structured as a note section
as documented in SCO-ELF_, and its attribute flag ``SHF_ALLOC`` must
be set.

The ``namesz`` field shall be 4

If a platform supports section ``.note.gnu.property``, this can be used
as the base for an alternative schema. The format of this section is
defined in LINUX_ABI_.
The ``descsz`` field shall be 16. See ``desc`` below.

The following processor-specific program property types are defined:
The type field shall be ``NT_ARM_TYPE_PAUTH_ABI_TAG``, defined to the
value 1.

+----------------------------------------+------------+
| Name | Value |
+========================================+============+
| GNU\_PROPERTY\_AARCH64\_FEATURE\_PAUTH | 0xc0000001 |
+----------------------------------------+------------+
The ``name`` field shall be the null-terminated string ``ARM``.

The format of the data in ``pr_data`` is at least two 64-bit words,
the first being a platform identifier, and the second being a version
number specific to the platform identified in the first word.
Consequently, the ``pr_datasz`` field must be at least 16. When
``pr_datasz`` is larger than 16, the remainder of the contents of
``pr_data`` are specific to the (platform id, version number). If
``pr_datasz`` is not a multiple of 8, ``pr_padding`` must be added so
that PR_DATASZ + PR_PADDING is a multiple of 8 and the property
remains 8-byte aligned.

The rules for the fields are the same as for the default marking
schema: this ABI does not define the exact format of the platform and
version identifiers, but reserves the combination of (platform,
version) equal to (0,0) to represent an ELF file incompatible with
this ABI.
The ``desc`` contain 2 64-bit words. With the first 64-bit word being
the ``platform identifier``, and the second 64-bit word being the
``version number``. Both of these form the information required in
`Core Information`_ above.

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