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[AAELF64][SYSVABI64] Move dynamic linking contents to sysvabi64
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The Elf for the Arm 64-bit Architecture (aaelf64) describes the
processor specific parts of the ELF specification common to all
ELF environments from a statically linked freestanding environment
with no OS to a hosted environment that supports dynamic linking.

The dynamic linking parts that are specific to SystemV style
operating environments such as Linux, and in particular the
GNU properties which are based on a Linux ABI extension are
better documented in the SystemV ABI for Arm 64-bit
Architecture.
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smithp35 committed Nov 21, 2023
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159 changes: 39 additions & 120 deletions aaelf64/aaelf64.rst
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.. _SYM-VER: http://www.akkadia.org/drepper/symbol-versioning
.. _TLSDESC: http://www.fsfla.org/~lxoliva/writeups/TLS/paper-lk2006.pdf
.. _MTEEXTENSIONS: https://www.kernel.org/doc/html/latest/arm64/memory-tagging-extension.html#core-dump-support
.. _SYSVABI64: https://github.com/ARM-software/abi-aa/releases
.. _VFABI64: https://github.com/ARM-software/abi-aa/releases

ELF for the Arm® 64-bit Architecture (AArch64)
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| | | - In `Program Loading`_, Relax BTI PLT |
| | | requirement |
+---------------+--------------------+-----------------------------------------+
| 2023Q4 | 24\ :sup:`th` | - In `Program Property`_, |
| | October 2023 | `Program Loading`_ and |
| | | `Dynamic Linking`. Move description to|
| | | `SYSVABI64`_ |
+---------------+--------------------+-----------------------------------------+

References
----------
Expand All @@ -279,29 +285,31 @@ This document refers to, or is referred to by, the following documents.

.. table::

+------------------------------+----------------------------------------------------------------------------------------------+-------------------------------------------------------------+
| Ref | External reference or URL | Title |
+==============================+==============================================================================================+=============================================================+
| AAELF64 | Source for this document | ELF for the Arm 64-bit Architecture (AArch64). |
+------------------------------+----------------------------------------------------------------------------------------------+-------------------------------------------------------------+
| AAPCS64_ | IHI 0055 | Procedure Call Standard for the Arm 64-bit Architecture |
+------------------------------+----------------------------------------------------------------------------------------------+-------------------------------------------------------------+
| Addenda32_ | IHI 0045 | Addenda to, and Errata in, the ABI for the Arm Architecture |
+------------------------------+----------------------------------------------------------------------------------------------+-------------------------------------------------------------+
| PAuthABIELF64_ | pauthabielf64 | PAuth Extension to ELF for the Arm 64-bit Architecture |
+------------------------------+----------------------------------------------------------------------------------------------+-------------------------------------------------------------+
| LSB_ | http://www.linuxbase.org/ | Linux Standards Base |
+------------------------------+----------------------------------------------------------------------------------------------+-------------------------------------------------------------+
| SCO-ELF_ | http://www.sco.com/developers/gabi/ | System V Application Binary Interface – DRAFT |
+------------------------------+----------------------------------------------------------------------------------------------+-------------------------------------------------------------+
| LINUX_ABI_ | https://github.com/hjl-tools/linux-abi/wiki | Linux Extensions to gABI |
+------------------------------+----------------------------------------------------------------------------------------------+-------------------------------------------------------------+
| SYM-VER_ | http://people.redhat.com/drepper/symbol-versioning | GNU Symbol Versioning |
+------------------------------+----------------------------------------------------------------------------------------------+-------------------------------------------------------------+
| TLSDESC_ | http://www.fsfla.org/~lxoliva/writeups/TLS/paper-lk2006.pdf | TLS Descriptors for Arm. Original proposal document |
+------------------------------+----------------------------------------------------------------------------------------------+-------------------------------------------------------------+
| MTEEXTENSIONS_ | https://www.kernel.org/doc/html/latest/arm64/memory-tagging-extension.html#core-dump-support | Linux Kernel MTE core dump format |
+------------------------------+----------------------------------------------------------------------------------------------+-------------------------------------------------------------+
+------------------------------+----------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------+
| Ref | External reference or URL | Title |
+==============================+==============================================================================================+=============================================================================+
| AAELF64 | Source for this document | ELF for the Arm 64-bit Architecture (AArch64). |
+------------------------------+----------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------+
| AAPCS64_ | IHI 0055 | Procedure Call Standard for the Arm 64-bit Architecture |
+------------------------------+----------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------+
| Addenda32_ | IHI 0045 | Addenda to, and Errata in, the ABI for the Arm Architecture |
+------------------------------+----------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------+
| PAuthABIELF64_ | pauthabielf64 | PAuth Extension to ELF for the Arm 64-bit Architecture |
+------------------------------+----------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------+
| LSB_ | http://www.linuxbase.org/ | Linux Standards Base |
+------------------------------+----------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------+
| SCO-ELF_ | http://www.sco.com/developers/gabi/ | System V Application Binary Interface – DRAFT |
+------------------------------+----------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------+
| LINUX_ABI_ | https://github.com/hjl-tools/linux-abi/wiki | Linux Extensions to gABI |
+------------------------------+----------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------+
| SYM-VER_ | http://people.redhat.com/drepper/symbol-versioning | GNU Symbol Versioning |
+------------------------------+----------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------+
| TLSDESC_ | http://www.fsfla.org/~lxoliva/writeups/TLS/paper-lk2006.pdf | TLS Descriptors for Arm. Original proposal document |
+------------------------------+----------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------+
| MTEEXTENSIONS_ | https://www.kernel.org/doc/html/latest/arm64/memory-tagging-extension.html#core-dump-support | Linux Kernel MTE core dump format |
+------------------------------+----------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------+
| SYSVABI64_ | sysvabi64 | System V Application Binary Interface (ABI) for the Arm 64-bit Architecture |
+------------------------------+----------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------+

Terms and abbreviations
-----------------------
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Program Property
----------------

The following processor-specific program property types [LINUX_ABI_] are
defined:

.. table:: Program Property Type

+-----------------------------------------+------------+
| Name | Value |
+=========================================+============+
| GNU\_PROPERTY\_AARCH64\_FEATURE\_1\_AND | 0xc0000000 |
+-----------------------------------------+------------+

``GNU_PROPERTY_AARCH64_FEATURE_1_AND`` describes a set of processor features
with which an ELF object or executable image is compatible, but does not
require in order to execute correctly. It has a single 32-bit value for the
``pr_data`` field. Each bit represents a separate feature.

Static linkers processing ELF relocatable objects must set the feature bit in
the output object or image only if all the input objects have the corresponding
feature bit set. For each feature bit set in an ELF executable or shared library,
a loader may enable the corresponding processor feature for that ELF file.

The following bits are defined for GNU_PROPERTY_AARCH64_FEATURE_1_AND:

.. table:: GNU_PROPERTY_AARCH64_FEATURE_1_AND Bit Flags

+-----------------------------------------+------------+
| Name | Value |
+=========================================+============+
| GNU\_PROPERTY\_AARCH64\_FEATURE\_1\_BTI | 1U << 0 |
+-----------------------------------------+------------+
| GNU\_PROPERTY\_AARCH64\_FEATURE\_1\_PAC | 1U << 1 |
+-----------------------------------------+------------+

``GNU_PROPERTY_AARCH64_FEATURE_1_BTI`` This indicates that all executable
sections are compatible with Branch Target Identification mechanism. An
executable or shared object with this bit set is required to generate
`Custom PLTs`_ with BTI instruction.

``GNU_PROPERTY_AARCH64_FEATURE_1_PAC`` This indicates that all
executable sections have been protected with Return Address Signing.
Its use is optional, meaning that an ELF file where this feature bit
is unset can still have Return Address signing enabled in some or all of
its executable sections.
The information on Program Property has been moved to [SYSVABI64_].

Program Loading
---------------

Process ``GNU_PROPERTY_AARCH64_FEATURE_1_BTI``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If Branch Target Identification mechanism is enabled on a processor then
the Guard Page (GP) bit must be disabled on the memory image of loaded
executable segments of executables and shared objects that do not have
``GNU_PROPERTY_AARCH64_FEATURE_1_BTI`` set, before execution is transferred
to them.
The information on program loading has been moved to [SYSVABI64_].

Dynamic Linking
---------------

Custom PLTs
^^^^^^^^^^^^

- To support Branch Target Identification mechanism, in the presence
of a ``GNU_PROPERTY_AARCH64_FEATURE_1_BTI`` all PLT entries
generated by the linker that can be called indirectly must have a
BTI instruction as the first instruction. The linker must add the
``DT_AARCH64_BTI_PLT`` (`AArch64 specific dynamic array tags`_) tag
to the dynamic section.

- To support Pointer Authentication, PLT entries generated
by the linker can have an authenticating instruction as the final
instruction before branching back. The linker must add the
``DT_AARCH64_PAC_PLT`` (`AArch64 specific dynamic array tags`_) tag to the dynamic section.

- If the linker generates custom PLT entries with both BTI and PAC
instructions, it must add both ``DT_AARCH64_BTI_PLT`` and
``DT_AARCH64_PAC_PLT`` tags to the dynamic section.

The information on Dynamic Linking has been moved to [SYSVABI64_].

Dynamic Section
^^^^^^^^^^^^^^^

AArch64 specifies the following processor-specific dynamic array tags.

.. _`AArch64 specific dynamic array tags`:

.. table:: AArch64 specific dynamic array tags

+---------------------------+------------+--------+-------------------+-------------------+
| Name | Value | d\_un | Executable | Shared Object |
+===========================+============+========+===================+===================+
| DT\_AARCH64\_BTI\_PLT | 0x70000001 | d\_val | Platform specific | Platform Specific |
+---------------------------+------------+--------+-------------------+-------------------+
| DT\_AARCH64\_PAC\_PLT | 0x70000003 | d\_val | Platform specific | Platform Specific |
+---------------------------+------------+--------+-------------------+-------------------+
| DT\_AARCH64\_VARIANT\_PCS | 0x70000005 | d\_val | Platform specific | Platform Specific |
+---------------------------+------------+--------+-------------------+-------------------+

``DT_AARCH64_BTI_PLT`` indicates PLTs enabled with Branch Target Identification
mechanism.
---------------

``DT_AARCH64_PAC_PLT`` indicates PLTs enabled with Pointer Authentication.
The information on the Dynamic Section has been moved to [SYSVABI64_].

The presence of both ``DT_AARCH64_BTI_PLT`` and ``DT_AARCH64_PAC_PLT``
indicates PLTs enabled with both Branch Target Identification mechanism and
Pointer Authentication.
Custom PLTs
^^^^^^^^^^^^

``DT_AARCH64_VARIANT_PCS`` must be present if there are ``R_<CLS>_JUMP_SLOT``
relocations that reference symbols marked with the ``STO_AARCH64_VARIANT_PCS``
flag set in their ``st_other`` field.
The information on custom PLTs has been moved to [SYSVABI64_].

.. raw:: pdf
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