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[PAuthABIELF64] Clarify the interaction between Memtag and PAuthAbi (#366)
The PAuthABI introduces some new dynamic relocations and the MemtagABI adds additional semantics to existing core dynamic relocations. The additional semantics for MemtagABI can be applied to the new PAuthABI dynamic relocations. In most cases the extension is natural as the LDG and SIGN operations operate on different bits in the pointer and can apply in either order. Both the MemtagABI and the PAuthABI require additional metadata for the R_AARCH64_AUTH_RELATIVE relocation which is stored in the place of the relocation. The MemtagABI stores an additional addend compensator so that the dynamic linker can derive the start address of the datatype so that it can materialize the correct tag. The PAuthABI stores the signing schema, including an addend field which contains the relocation addend when RELR compression is used. As the MemtagABI excludes any relocations with an addend compensator from RELR compression, and PAuthABI requires the addend field in the signing schema to be 0 when RELR compression is not used; we can use the signing schema addend field for the Memtag addend compensator. Document the relocation operators and how the MemtagABI applies to the PAuthABI relocations. This behaviour was implemented in LLD PR llvm/llvm-project#173291
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‎pauthabielf64/pauthabielf64.rst‎

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.. _ARM64E: https://github.com/apple/llvm-project/blob/a63a81bd9911f87a0b5dcd5bdd7ccdda7124af87/clang/docs/PointerAuthentication.rst
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.. _CPPABI64: https://github.com/ARM-software/abi-aa/releases
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.. _LSB: https://refspecs.linuxfoundation.org/LSB_1.2.0/gLSB/noteabitag.html
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.. _MEMTAGABI: https://github.com/ARM-software/abi-aa/releases
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.. _SCO-ELF: http://www.sco.com/developers/gabi/
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.. _SYSVABI64: https://github.com/ARM-software/abi-aa/releases
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.. _TLSDESC: http://www.fsfla.org/~lxoliva/writeups/TLS/paper-lk2006.pdf
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| | | - Move AUTH GOT generating relocations out of private experiment |
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| | | Range. |
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+------------+-----------------------------+------------------------------------------------------------------+
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| 2026Q2 | 6\ :sup:`th` January 2026 | - Clarify combination of MemtagABI and PAuthABI relocations |
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+------------+-----------------------------+------------------------------------------------------------------+
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References
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----------
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+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+
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| LSB_ | | Linux Standards Base |
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+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+
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| MEMTAGABI_ | memtagabi64 | Memtag ABI Extension for the Arm 64-bit Architecture |
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+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+
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| SCO-ELF_ | http://www.sco.com/developers/gabi/ | System V Application Binary Interface – DRAFT |
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+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+
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| SYSVABI64_ | sysvabi64 | System V Application Binary Interface (ABI) for the Arm 64-bit |
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* ``reserved for addend`` is used in ``SHT_AUTH_RELR`` or ``SHT_REL``
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relocation implementations where the relocation addend is written to
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the contents of the place. It must be set to 0 if not used for an
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addend.
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addend, or not used for the (MEMTAGABI_) addend correction.
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* ``reserved`` are bits reserved for future expansion. These bits must
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be set to 0 by a producer. A consumer must not assume that reserved
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| 0x414 (1044) | R\_AARCH64\_AUTH\_IRELATIVE | SIGN(Indirect(S + A), SCHEMA(\*P)) |
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+--------------------+------------------------------+------------------------------------+
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Combination of PAuthABI with the Memtag ABI Extension
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-----------------------------------------------------
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The Memtag ABI Extension (MEMTAGABI_) extends the semantics of some
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dynamic relocations in (AAELF64_) to support memory tagging. The
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extended semantics described in (MEMTAGABI_) can be extended to the
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AUTH variant dynamic relocations described in this document.
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For ``R_AARCH64_AUTH_ABS64`` and ``R_AARCH64_AUTH_GLOB_DAT`` the
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extension is for the dynamic linker to materialize the tag for the
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pointer, then to sign it.
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For ``R_AARCH64_AUTH_RELATIVE``, both the (MEMTAGABI_) and the
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PAuthABI require metadata to be stored in the place of the
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relocation. The (MEMTAGABI_) stores an optional addend correction, and
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the PAuthABI stores the signing schema with an addend field for use
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with RELR compression. As the (MEMTAGABI_) excludes relocations that
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require addend correction from RELR compression, and the addend
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correction is expected to be small, the 32-bit addend field in the
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signing schema can be used for the (MEMTAGABI_) addend correction.
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Relocation Operation
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====================
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* ``LDG(pointer)`` is defined in (MEMTAGABI_).
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* ``ADDEND(\*P)`` represents the dynamic linker reading the addend
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field from the contents of the place ``P``.
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.. table:: Relocations with extended semantics with MemtagABI
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+--------------+-----------------------------+---------------------------------+-------------------------------------------------------------------+
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| ELF64 Code | Name | PAuthABI Base Operation | MemtagABI Extended Operation |
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+==============+=============================+=================================+===================================================================+
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| 0x244 (580) | R\_AARCH64\_AUTH\_ABS64 | SIGN((S + A), SCHEMA(\*P)) | SIGN((LDG(S) + A), SCHEMA(\*P)) |
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+--------------+-----------------------------+---------------------------------+-------------------------------------------------------------------+
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| 0x413 (1043) | R\_AARCH64\_AUTH\_RELATIVE | SIGN(DELTA(S) + A, SCHEMA(\*P)) | SIGN((LDG(Delta(S) + A + ADDEND(\*P)) - ADDEND(\*P), SCHEMA(\*P)) |
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+--------------+-----------------------------+---------------------------------+-------------------------------------------------------------------+
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| 0x414 (1044) | R\_AARCH64\_AUTH\_GLOB\_DAT | SIGN((S + A), SCHEMA(\*P)) | SIGN((LDG(S) + A), SCHEMA(\*P)) |
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+--------------+-----------------------------+---------------------------------+-------------------------------------------------------------------+
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Compatibility between relocatable object files
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----------------------------------------------
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