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[PAuthABIELF64] Clarify the interaction between Memtag and PAuthAbi #366

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smithp35 merged 1 commit into from
Feb 10, 2026
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[PAuthABIELF64] Clarify the interaction between Memtag and PAuthAbi #366

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48 changes: 47 additions & 1 deletion pauthabielf64/pauthabielf64.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,7 @@
.. _ARM64E: https://github.com/apple/llvm-project/blob/a63a81bd9911f87a0b5dcd5bdd7ccdda7124af87/clang/docs/PointerAuthentication.rst
.. _CPPABI64: https://github.com/ARM-software/abi-aa/releases
.. _LSB: https://refspecs.linuxfoundation.org/LSB_1.2.0/gLSB/noteabitag.html
.. _MEMTAGABI: https://github.com/ARM-software/abi-aa/releases
.. _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
Expand Down Expand Up @@ -248,6 +249,8 @@ changes to the content of the document for that release.
| | | - Move AUTH GOT generating relocations out of private experiment |
| | | Range. |
+------------+-----------------------------+------------------------------------------------------------------+
| 2026Q2 | 6\ :sup:`th` January 2026 | - Clarify combination of MemtagABI and PAuthABI relocations |
+------------+-----------------------------+------------------------------------------------------------------+

References
----------
Expand All @@ -269,6 +272,8 @@ This document refers to, or is referred to by, the following documents.
+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+
| LSB_ | | Linux Standards Base |
+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+
| MEMTAGABI_ | memtagabi64 | Memtag ABI Extension for the Arm 64-bit Architecture |
+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+
| 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 |
Expand Down Expand Up @@ -667,7 +672,7 @@ this should be sufficient.
* ``reserved for addend`` is used in ``SHT_AUTH_RELR`` or ``SHT_REL``
relocation implementations where the relocation addend is written to
the contents of the place. It must be set to 0 if not used for an
addend.
addend, or not used for the (MEMTAGABI_) addend correction.

* ``reserved`` are bits reserved for future expansion. These bits must
be set to 0 by a producer. A consumer must not assume that reserved
Expand Down Expand Up @@ -1229,6 +1234,47 @@ The ``R_AARCH64_AUTH_GOT_ADR_PREL_LO21`` relocation is used with the
| 0x414 (1044) | R\_AARCH64\_AUTH\_IRELATIVE | SIGN(Indirect(S + A), SCHEMA(\*P)) |
+--------------------+------------------------------+------------------------------------+

Combination of PAuthABI with the Memtag ABI Extension
-----------------------------------------------------

The Memtag ABI Extension (MEMTAGABI_) extends the semantics of some
dynamic relocations in (AAELF64_) to support memory tagging. The
extended semantics described in (MEMTAGABI_) can be extended to the
AUTH variant dynamic relocations described in this document.

For ``R_AARCH64_AUTH_ABS64`` and ``R_AARCH64_AUTH_GLOB_DAT`` the
extension is for the dynamic linker to materialize the tag for the
pointer, then to sign it.

For ``R_AARCH64_AUTH_RELATIVE``, both the (MEMTAGABI_) and the
PAuthABI require metadata to be stored in the place of the
relocation. The (MEMTAGABI_) stores an optional addend correction, and
the PAuthABI stores the signing schema with an addend field for use
with RELR compression. As the (MEMTAGABI_) excludes relocations that
require addend correction from RELR compression, and the addend
correction is expected to be small, the 32-bit addend field in the
signing schema can be used for the (MEMTAGABI_) addend correction.

Relocation Operation
====================

* ``LDG(pointer)`` is defined in (MEMTAGABI_).

* ``ADDEND(\*P)`` represents the dynamic linker reading the addend
field from the contents of the place ``P``.

.. table:: Relocations with extended semantics with MemtagABI

+--------------+-----------------------------+---------------------------------+-------------------------------------------------------------------+
| ELF64 Code | Name | PAuthABI Base Operation | MemtagABI Extended Operation |
+==============+=============================+=================================+===================================================================+
| 0x244 (580) | R\_AARCH64\_AUTH\_ABS64 | SIGN((S + A), SCHEMA(\*P)) | SIGN((LDG(S) + A), SCHEMA(\*P)) |
+--------------+-----------------------------+---------------------------------+-------------------------------------------------------------------+
| 0x413 (1043) | R\_AARCH64\_AUTH\_RELATIVE | SIGN(DELTA(S) + A, SCHEMA(\*P)) | SIGN((LDG(Delta(S) + A + ADDEND(\*P)) - ADDEND(\*P), SCHEMA(\*P)) |
+--------------+-----------------------------+---------------------------------+-------------------------------------------------------------------+
| 0x414 (1044) | R\_AARCH64\_AUTH\_GLOB\_DAT | SIGN((S + A), SCHEMA(\*P)) | SIGN((LDG(S) + A), SCHEMA(\*P)) |
+--------------+-----------------------------+---------------------------------+-------------------------------------------------------------------+

Compatibility between relocatable object files
----------------------------------------------

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