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[SYSVABI64] Add Guarded Control Stack (GCS) Feature Bit #231

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Dec 11, 2023
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[SYSVABI64] Add Guarded Control Stack (GCS) Feature Bit #231

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[SYSVABI64] Add Guarded Control Stack (GCS) Feature Bit
smithp35 Nov 22, 2023
38336c3
"Address John Brawn's review comment on procedure push/pop operation
smithp35 Nov 27, 2023
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37 changes: 35 additions & 2 deletions sysvabi64/sysvabi64.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -200,8 +200,11 @@ Change History
| 02Alpha | 6\ :sup:`th` April 2023 | Define the processor specific interpretation for |
| | | DT_PLTGOT |
+------------+------------------------------+-------------------------------------------------------+
| 2023Q4 | 24\ :sup:`th` 2023 | Added Program Property, Program Loading and |
| 2023Q4 | 22\ :sup:`nd` November 2023 | Added Program Property, Program Loading and |
| | | Dynamic Linking contents from `AAELF64`_ |
| | | |
| | | Added Dynamic linking content for GCS including |
| | | GNU_PROPERTY_AARCH64_FEATURE_1_GCS Feature Bit |
+------------+------------------------------+-------------------------------------------------------+

References
Expand Down Expand Up @@ -1631,6 +1634,8 @@ The following bits are defined for GNU_PROPERTY_AARCH64_FEATURE_1_AND:
+-----------------------------------------+------------+
| GNU\_PROPERTY\_AARCH64\_FEATURE\_1\_PAC | 1U << 1 |
+-----------------------------------------+------------+
| GNU\_PROPERTY\_AARCH64\_FEATURE\_1\_GCS | 1U << 2 |
+-----------------------------------------+------------+

``GNU_PROPERTY_AARCH64_FEATURE_1_BTI`` This indicates that all executable
sections are compatible with Branch Target Identification mechanism. An
Expand All @@ -1643,11 +1648,27 @@ 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.

``GNU_PROPERTY_AARCH64_FEATURE_1_GCS`` This indicates that all
executable sections are compatible with the Guarded control stack
(GCS) mechanism. Minimum requirements for setting this feature bit
include:

* Each function that is called using a BL instruction (or other
instruction that is a GCS ``procedure return address push
operation``) returns using a RET instruction (or other instruction
that is a GCS ``procedure return address pop operation``). This
means that RET instructions are only used for function returns, and
never as an indirect branch.

* Any functions used by the program that manipulate the stack such as
``setjmp`` and ``longjmp``, must be aware of GCS.


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
Expand All @@ -1658,6 +1679,18 @@ to them.
Dynamic Linking
---------------

Process ``GNU_PROPERTY_AARCH64_FEATURE_1_GCS``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For the Guarded control stack mechanism to be enabled for a process.
The feature bit ``GNU_PROPERTY_AARCH64_FEATURE_1_GCS`` must be set on
the executable and all shared libraries loaded by the program.

In a program that is running with the Guarded control stack mechanism enabled,
the action taken when ``dlopen`` is used to load a shared library without
the feature bit ``GNU_PROPERTY_AARCH64_FEATURE_1_GCS`` set is defined by the
platform.

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

Expand Down
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