Skip to content

Commit

Permalink
Distinguish between function pointers and code pointers through addit…
Browse files Browse the repository at this point in the history 
…ional relocation types (#306)

First cut of a proposal to distinguish between function pointers and
code pointers through additional relocation types.

Previously, function pointers and code pointers (e.g., those used for
C++ exception landing pads) are relocated in the same way. This PR
introduces new relocation types to distinguish between them to help
library-based compartmentalisation.

Function pointers continue to use `R_MORELLO_CAPINIT` and code pointers
use the new `R_MORELLO_CODE_CAPINIT` relocation. Use
`R_{AARCH64,MORELLO}_FUNC_RELATIVE` for function pointers that need to
be relatively relocated, whilst code pointers and integer values
continue to use `R_{AARCH64,MORELLO}_RELATIVE`.
  • Loading branch information
@dpgao
dpgao authored Feb 13, 2025
1 parent 11f628a commit 92cd879
Showing 1 changed file with 54 additions and 25 deletions.
79 changes: 54 additions & 25 deletions aaelf64-morello/aaelf64-morello.rst
View file
Original file line number Diff line number Diff line change
Expand Up @@ -476,6 +476,11 @@ The following nomenclature is used in the descriptions of relocation operations:
the second entry holds a platform-specific offset or pointer. The pair of
pointer-sized entries will be relocated with ``R_MORELLO_TLSDESC(S+A)``.

- ``Delta(S)`` if ``S`` is a normal symbol, resolves to the difference between the
static link address of ``S`` and the execution address of ``S``. If ``S`` is the
null symbol (ELF symbol index 0), resolves to the difference between the static
link address of ``P`` and the execution address of ``P``.

- ``TPREL(S)`` resolves to a pair of two 64-bit values. The first value
contains the offset in the static TLS block of the thread-local symbol ``S``.
The second value contains the size of the symbol ``S``
Expand Down Expand Up @@ -688,31 +693,40 @@ Dynamic Morello relocations

.. table:: Dynamic relocations

+-------+-------------------------+-----------------------------------------+------------------------------------------+
| ELF64 | Name | Operation | Comment |
| Code | | | |
+=======+=========================+=========================================+==========================================+
| 59392 | ``R_MORELLO_CAPINIT`` | ``CAP_INIT(S, A, CAP_SIZE, CAP_PERM)`` | See note below. |
| | | | |
+-------+-------------------------+-----------------------------------------+------------------------------------------+
| 59393 | ``R_MORELLO_GLOB_DAT`` | ``CAP_INIT(S, A, CAP_SIZE, CAP_PERM)`` | See note below. |
| | | | |
+-------+-------------------------+-----------------------------------------+------------------------------------------+
| 59394 | ``R_MORELLO_JUMP_SLOT`` | ``CAP_INIT(S, A, CAP_SIZE, CAP_PERM)`` | See note below. |
| | | | |
+-------+-------------------------+-----------------------------------------+------------------------------------------+
| 59395 | ``R_MORELLO_RELATIVE`` | ``CAP_INIT(S, A, CAP_SIZE, CAP_PERM)`` | See note below. |
| | | | |
+-------+-------------------------+-----------------------------------------+------------------------------------------+
| 59396 | ``R_MORELLO_IRELATIVE`` | ``CAP_INIT(S, A, CAP_SIZE, CAP_PERM)`` | See note below. |
| | | | |
+-------+-------------------------+-----------------------------------------+------------------------------------------+
| 59397 | ``R_MORELLO_TLSDESC`` | ``TLSDESC(S+A)`` | Identifies a TLS descriptor to be filled.|
| | | | |
+-------+-------------------------+-----------------------------------------+------------------------------------------+
| 59398 | ``R_MORELLO_TPREL128`` | ``TPREL(S)`` | See note below. |
| | | | |
+-------+-------------------------+-----------------------------------------+------------------------------------------+
+-------+-----------------------------+-----------------------------------------+------------------------------------------+
| ELF64 | Name | Operation | Comment |
| Code | | | |
+=======+=============================+=========================================+==========================================+
| 59392 | ``R_MORELLO_CAPINIT`` | ``CAP_INIT(S, A, CAP_SIZE, CAP_PERM)`` | See note below. |
| | | | |
+-------+-----------------------------+-----------------------------------------+------------------------------------------+
| 59393 | ``R_MORELLO_GLOB_DAT`` | ``CAP_INIT(S, A, CAP_SIZE, CAP_PERM)`` | See note below. |
| | | | |
+-------+-----------------------------+-----------------------------------------+------------------------------------------+
| 59394 | ``R_MORELLO_JUMP_SLOT`` | ``CAP_INIT(S, A, CAP_SIZE, CAP_PERM)`` | See note below. |
| | | | |
+-------+-----------------------------+-----------------------------------------+------------------------------------------+
| 59395 | ``R_MORELLO_RELATIVE`` | ``CAP_INIT(S, A, CAP_SIZE, CAP_PERM)`` | See note below. |
| | | | |
+-------+-----------------------------+-----------------------------------------+------------------------------------------+
| 59396 | ``R_MORELLO_IRELATIVE`` | ``CAP_INIT(S, A, CAP_SIZE, CAP_PERM)`` | See note below. |
| | | | |
+-------+-----------------------------+-----------------------------------------+------------------------------------------+
| 59397 | ``R_MORELLO_TLSDESC`` | ``TLSDESC(S+A)`` | Identifies a TLS descriptor to be filled.|
| | | | |
+-------+-----------------------------+-----------------------------------------+------------------------------------------+
| 59398 | ``R_MORELLO_TPREL128`` | ``TPREL(S)`` | See note below. |
| | | | |
+-------+-----------------------------+-----------------------------------------+------------------------------------------+
| 59399 | ``R_MORELLO_CODE_CAPINIT`` | ``CAP_INIT(S, A, CAP_SIZE, CAP_PERM)`` | See note below. |
| | | | |
+-------+-----------------------------+-----------------------------------------+------------------------------------------+
| 59400 | ``R_MORELLO_FUNC_RELATIVE`` | ``CAP_INIT(S, A, CAP_SIZE, CAP_PERM)`` | See note below. |
| | | | |
+-------+-----------------------------+-----------------------------------------+------------------------------------------+
| 59401 | ``R_AARCH64_FUNC_RELATIVE`` | ``Delta(S) + A`` | See note below. |
| | | | |
+-------+-----------------------------+-----------------------------------------+------------------------------------------+

.. note::

Expand Down Expand Up @@ -759,6 +773,21 @@ Dynamic Morello relocations

``| 64-bits offset | 64-bits size |``

``R_MORELLO_CODE_CAPINIT`` is similar to ``R_MORELLO_CAPINIT`` but signifies
that the capability being relocated is a code pointer as opposed to a data
pointer or function pointer. As such, it must only be used against
``STT_FUNC`` symbols. This relocation type is needed for library-based
compartmentalization (c18n). Currently for security reasons this relocation
may only be used against non-preemptible symbols, and thus will be converted
to ``R_MORELLO_RELATIVE`` at link time, making it in effect a static
relocation. However, this restriction may be removed in future if needed, and
it is therefore classed as a dynamic relocation.

``R_MORELLO_FUNC_RELATIVE`` and ``R_AARCH64_FUNC_RELATIVE`` : similar to
their non-``_FUNC`` counterparts but signify that the capability being
relocated is a function pointer as opposed to a data pointer or code pointer.
This distinction is needed for library-based compartmentalization (c18n).

Static linking with Morello
^^^^^^^^^^^^^^^^^^^^^^^^^^^

Expand Down

0 comments on commit 92cd879

Please sign in to comment.