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[aaelf64-morello] ELF markers to identify Morello purecap binaries #101

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Jun 23, 2021
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[aaelf64-morello] ELF markers to identify Morello purecap binaries #101

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f980118
[aaelf64-morello] ELF markers to identify Morello purecap binaries
amilendra May 21, 2021
9d49c95
Use more generic name EF_AARCH64_CHERI_PURECAP
amilendra May 25, 2021
b3c0e2e
Fix grammar error
amilendra Jun 22, 2021
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35 changes: 35 additions & 0 deletions aaelf64-morello/aaelf64-morello.rst
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Expand Up @@ -13,6 +13,8 @@
.. _MORELLO_ARM: https://developer.arm.com/documentation/ddi0606/latest
.. |tlsdesc-url| replace:: http://www.fsfla.org/~lxoliva/writeups/TLS/paper-lk2006.pdf
.. _TLSDESC: http://www.fsfla.org/~lxoliva/writeups/TLS/paper-lk2006.pdf
.. |cherielf-url| replace:: https://github.com/CTSRD-CHERI/cheri-elf-gabi/blob/main/gabi.md
.. _CHERI_ELF: https://github.com/CTSRD-CHERI/cheri-elf-gabi/blob/main/gabi.md

Morello extensions to ELF for the Arm\ :sup:`®` 64-bit Architecture (AArch64)
*****************************************************************************
Expand Down Expand Up @@ -220,6 +222,8 @@ This document refers to, or is referred to by, the following documents.
+------------------+----------------------------+-----------------------------------------------------------------------------------+
| TLSDESC_ | |tlsdesc-url| | TLS Descriptors for Arm. Original proposal document. |
+------------------+----------------------------+-----------------------------------------------------------------------------------+
| CHERI_ELF_ | |cherielf-url| | CHERI ELF gABI Extensions |
+------------------+----------------------------+-----------------------------------------------------------------------------------+

Terms and abbreviations
-----------------------
Expand Down Expand Up @@ -252,6 +256,37 @@ the same name in `AAELF64`_:
Object Files
============

ELF Header
----------

The ELF header provides a number of fields that assist in interpretation of the
file. Most of these are specified in the base standard. The following fields
have Morello-specific meanings.

``e_flags``
The processor-specific flags are shown in the following table.

.. _Morello-specific e_flags:

.. class:: aaelf64-morello-elf-flags

.. table:: Morello-specific e_flags

+---------------------------------+--------------------------------------------------------------+
| Value | Meaning |
+---------------------------------+--------------------------------------------------------------+
| :code:`EF_AARCH64_MORELLO_EXT` | The ELF file contains uses an ABI where all pointers are |
| (:code:`0x00010000`) | implemented using capabilities (Pure-capability ABI) |
+---------------------------------+--------------------------------------------------------------+

Sections
--------

Special Sections
^^^^^^^^^^^^^^^^

A Morello toolchain can emit ELF Note sections in accordance to [CHERI_ELF_].
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Should Morello prove successful, do you want to reserve the ability to reuse this flag to be its real non-prototype successor? As it stands this bit will forever be wasted once the Morello program comes to an end.

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i.e. should this really be EF_AARCH64_CHERI_PURECAP?

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That is my only concern as well and the reason I had originally suggested an ELF note with a name of "Morello" as it could be easily jettisoned in the future. Checking e_flags bits is of course vastly simpler to deal with than looking for an ELF note, so if the bit is potentially forward looking I'm happy with having a flag in e_flags.

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It is difficult to make predictions at this stage. We can either reuse or rename the flag for a future architecture.
However we don't have any strong opinions on the name so will go with the more generic EF_AARCH64_CHERI_PURECAP.

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@jrtc27, @bsdjhb
Are you happy with these changes?


Symbol Table
------------

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4 changes: 4 additions & 0 deletions tools/rst2pdf/rst2pdf-abi.style
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Expand Up @@ -246,6 +246,10 @@ styles:
parent : table
colWidths: [ 25%, 30%, 45% ]

aaelf64-morello-elf-flags:
parent : table
colWidths: [ 33%, 67% ]

aaelf64-morello-mapping-symbols:
parent : table
colWidths: [ 25%, 75% ]
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