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[PAuthABIELF64] Rewrite the section on ELF marking
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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".
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smithp35 committed Jan 31, 2024
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174 changes: 89 additions & 85 deletions pauthabielf64/pauthabielf64.rst
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Expand Up @@ -847,20 +847,51 @@ 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.
This document defines the core information that any ELF marking
scheme must contain and the base compatibility model that uses that
information.

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`_.

Core information
----------------

The core information used by the base compatibility model is made up
of two values, both of which must be present.

* ``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 |
+===========+===========+
| Invalid | 0x0 |
+-----------+-----------+
| Baremetal | 0x1 |
+-----------+-----------+

* ``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.

`Appendix Alternative ELF Marking Using SHT_NOTE section`_ defines an
alternative marking schema that uses a generic SHT_NOTE for platforms
that do not support the ``.note.gnu.property`` section.
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_).
defined in (`LINUX_ABI`_).

The following processor-specific program property types are defined:

Expand All @@ -873,74 +904,69 @@ The following processor-specific program property types are defined:
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 a platform identifier, and the second
64-bit word being a version number for the ABI for the platform
identified for the first word.
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.

The following values of the platform identifier are reserved:

+-----------+-----------+
| Platform | Hex value |
+===========+===========+
| Invalid | 0x0 |
+-----------+-----------+
| Baremetal | 0x1 |
+-----------+-----------+

The version number for the platform identifier is controlled by the
object-producer based on the signing schema that has been used for
pointers.

This ABI reserves the combination of (platform identifier, version
number) equal to (0,0) to represent an ELF file incompatible with this
ABI.

.note.gnu.properties are defined for relocatable objects. Arm intends
to use build-attributes for relocatable-object ELF marking. This will
be defined so that there is a 1:1 mapping between the program
property and the build-attributes for PAuthABI.
``.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.
* 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 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 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 the file is assigned
the (``platform identifer``, ``version number``) of (0,0).

* 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 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`_.

* 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.
* 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
reservd Invalid value 0.

Platforms may replace the base compatibility model with a
platform-specific model.
Expand Down Expand Up @@ -1262,36 +1288,14 @@ be set.

The ``namesz`` field shall be 4

The ``descsz`` field must be at least 16. See ``desc`` below.
The ``descsz`` field shall be 16. See ``desc`` below.

The type field shall be ``NT_ARM_TYPE_PAUTH_ABI_TAG``, defined to the
value 1.

The ``name`` field shall be the null-terminated string ``ARM``.

The first 16 bytes of the ``desc`` 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 following values of the platform id are reserved:

+-----------+-----------+
| Platform | Hex value |
+===========+===========+
| Invalid | 0x0 |
+-----------+-----------+
| 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.

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 ``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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