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[aaelf64][pauthabi64] Remove addend in GDAT relocation operation
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The GDAT(S + A) relocation operation requires a static linker to
create a GOT entry for (S + A). Requiring at least one GOT entry
for each unique tuple (S, A). Unfortunately no known static linker
has implemented this correctly, with one of two forms being
implemented instead:
* GDAT(S) with the addend ignored.
* GDAT(S) + A with a single GOT entry per S, and A added to the
  value of GDAT(S).
These implementations are correct and consistent only for an
addend (A) of zero.

No known compiler uses non-zero addends in relocations that use
the GDAT(S+A) operation, although it is possible to generate
them using assembly language.

This change synchronizes the ABI with the behavior of existing
static linker implementations. The benefit of permitting code
generators [*] to use a non zero addend in GDAT(S + A) is judged
to be lower than implementing GDAT(S + A) correctly in existing
static linkers, many of which assume that there is a single
GOT entry per unique symbol S.

It is QoI whether a static linker gives an error if a non zero
addend is used for a relocation that uses the GDAT(S) operation.

Fixes #217
Also resolves #247

[*] The most common use case for a non-zero addend is in
constructing a C++ object with a vtable. The first two entries
in the vtable are the offset to top and a pointer to RTTI, the
vtable pointer in the object starts at offset 0x10. This offset
can be encoded in the relocation addend. We would save an add
instruction for each construction of a C++ object with a vtable
if addends were permitted.
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@smithp35
smithp35 committed Aug 20, 2024
1 parent 4b25a26 commit 201a7cb
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31 changes: 16 additions & 15 deletions aaelf64/aaelf64.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -1036,6 +1036,7 @@ In ELF32 **(Beta)** relocations additional care must be taken when relocating an
R_<CLS>_TLSIE_ADR_GOTTPREL_PAGE21,
R_<CLS>_TLSDESC_ADR_PAGE21

Relocations using the ``GDAT(S)`` operation must have a zero addend. Previous versions of this document included the addend ``A`` in ``GDAT(S + A)`` resulting in a GOT entry for ``S + A``. With a zero addend ``GDAT(S + 0)`` is equivalent to ``GDAT(S)`` and ``GDAT(S) + 0``.

Static miscellaneous relocations
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Expand Down Expand Up @@ -1240,19 +1241,19 @@ The following tables record single instruction relocations and relocations that
+------------+------------+--------------------------------+-------------------+----------------------------------------------------------------------+
| ELF64 Code | ELF32 Code | Name | Operation | Comment |
+============+============+================================+===================+======================================================================+
| 300 | \- | R\_<CLS>\_MOVW\_GOTOFF\_G0 | G(GDAT(S+A)) -GOT | Set a MOV[NZ] immediate field to bits [15:0] of X (see notes below) |
| 300 | \- | R\_<CLS>\_MOVW\_GOTOFF\_G0 | G(GDAT(S)) -GOT | Set a MOV[NZ] immediate field to bits [15:0] of X (see notes below) |
+------------+------------+--------------------------------+-------------------+----------------------------------------------------------------------+
| 301 | \- | R\_<CLS>\_MOVW\_GOTOFF\_G0\_NC | G(GDAT(S+A)) -GOT | Set a MOVK immediate field to bits [15:0] of X. No overflow check |
| 301 | \- | R\_<CLS>\_MOVW\_GOTOFF\_G0\_NC | G(GDAT(S)) -GOT | Set a MOVK immediate field to bits [15:0] of X. No overflow check |
+------------+------------+--------------------------------+-------------------+----------------------------------------------------------------------+
| 302 | \- | R\_<CLS>\_MOVW\_GOTOFF\_G1 | G(GDAT(S+A)) -GOT | Set a MOV[NZ] immediate value to bits [31:16] of X (see notes below) |
| 302 | \- | R\_<CLS>\_MOVW\_GOTOFF\_G1 | G(GDAT(S)) -GOT | Set a MOV[NZ] immediate value to bits [31:16] of X (see notes below) |
+------------+------------+--------------------------------+-------------------+----------------------------------------------------------------------+
| 303 | \- | R\_<CLS>\_MOVW\_GOTOFF\_G1\_NC | G(GDAT(S+A)) -GOT | Set a MOVK immediate value to bits [31:16] of X. No overflow check |
| 303 | \- | R\_<CLS>\_MOVW\_GOTOFF\_G1\_NC | G(GDAT(S)) -GOT | Set a MOVK immediate value to bits [31:16] of X. No overflow check |
+------------+------------+--------------------------------+-------------------+----------------------------------------------------------------------+
| 304 | \- | R\_<CLS>\_MOVW\_GOTOFF\_G2 | G(GDAT(S+A)) -GOT | Set a MOV[NZ] immediate value to bits [47:32] of X (see notes below) |
| 304 | \- | R\_<CLS>\_MOVW\_GOTOFF\_G2 | G(GDAT(S)) -GOT | Set a MOV[NZ] immediate value to bits [47:32] of X (see notes below) |
+------------+------------+--------------------------------+-------------------+----------------------------------------------------------------------+
| 305 | \- | R\_<CLS>\_MOVW\_GOTOFF\_G2\_NC | G(GDAT(S+A)) -GOT | Set a MOVK immediate value to bits [47:32] of X. No overflow check |
| 305 | \- | R\_<CLS>\_MOVW\_GOTOFF\_G2\_NC | G(GDAT(S)) -GOT | Set a MOVK immediate value to bits [47:32] of X. No overflow check |
+------------+------------+--------------------------------+-------------------+----------------------------------------------------------------------+
| 306 | \- | R\_<CLS>\_MOVW\_GOTOFF\_G3 | G(GDAT(S+A)) -GOT | Set a MOV[NZ] immediate value to bits [63:48] of X (see notes below) |
| 306 | \- | R\_<CLS>\_MOVW\_GOTOFF\_G3 | G(GDAT(S)) -GOT | Set a MOV[NZ] immediate value to bits [63:48] of X (see notes below) |
+------------+------------+--------------------------------+-------------------+----------------------------------------------------------------------+

.. note::
Expand All @@ -1274,7 +1275,7 @@ The following tables record single instruction relocations and relocations that
| 308 | \- | R\_<CLS>\_GOTREL32 | S+A-GOT | Write bits [31:0] of X at byte-aligned place P. This represents a 32-bit offset relative to GOT, treated as signed; |
| | | | | Check that -2\ :sup:`31` <= X < 2\ :sup:`31`. |
+------------+------------+----------------------+------------------+-------------------------------------------------------------------------------------------------------------------------+
| 315 | \- | R\_<CLS>\_GOTPCREL32 | G(GDAT(S+A))- P | Write bits [31:0] of X at byte-aligned place P. This represents a 32-bit offset relative to GOT entry for an address, |
| 315 | \- | R\_<CLS>\_GOTPCREL32 | G(GDAT(S))- P | Write bits [31:0] of X at byte-aligned place P. This represents a 32-bit offset relative to GOT entry for an address, |
| | | | | treated as signed; Check that -2\ :sup:`31` <= X < 2\ :sup:`31`. |
+------------+------------+----------------------+------------------+-------------------------------------------------------------------------------------------------------------------------+

Expand All @@ -1287,19 +1288,19 @@ The following tables record single instruction relocations and relocations that
+-------------+------------+-------------------------------+----------------------------+------------------------------------------------------------------------------------------------------+
| ELF64 Code | ELF32 Code | Name | Operation | Comment |
+=============+============+===============================+============================+======================================================================================================+
| 309 | 25 | R\_<CLS>\_GOT\_LD\_PREL19 | G(GDAT(S+A))- P | Set a load-literal immediate field to bits [20:2] of X; check –2\ :sup:`20` <= X < 2\ :sup:`20` |
| 309 | 25 | R\_<CLS>\_GOT\_LD\_PREL19 | G(GDAT(S))- P | Set a load-literal immediate field to bits [20:2] of X; check –2\ :sup:`20` <= X < 2\ :sup:`20` |
+-------------+------------+-------------------------------+----------------------------+------------------------------------------------------------------------------------------------------+
| 310 | \- | R\_<CLS>\_LD64\_GOTOFF\_LO15 | G(GDAT(S+A))- GOT | Set a LD/ST immediate field to bits [14:3] of X; check that 0 <= X < 2\ :sup:`15`, X&7 = 0 |
| 310 | \- | R\_<CLS>\_LD64\_GOTOFF\_LO15 | G(GDAT(S))- GOT | Set a LD/ST immediate field to bits [14:3] of X; check that 0 <= X < 2\ :sup:`15`, X&7 = 0 |
+-------------+------------+-------------------------------+----------------------------+------------------------------------------------------------------------------------------------------+
| 311 | 26 | R\_<CLS>\_ADR\_GOT\_PAGE | Page(G(GDAT(S+A)))-Page(P) | Set the immediate value of an ADRP to bits [32:12] of X; check that –2\ :sup:`32` <= X < 2\ :sup:`32`|
| 311 | 26 | R\_<CLS>\_ADR\_GOT\_PAGE | Page(G(GDAT(S)))-Page(P) | Set the immediate value of an ADRP to bits [32:12] of X; check that –2\ :sup:`32` <= X < 2\ :sup:`32`|
+-------------+------------+-------------------------------+----------------------------+------------------------------------------------------------------------------------------------------+
| 312 | \- | R\_<CLS>\_LD64\_GOT\_LO12\_NC | G(GDAT(S+A)) | Set the LD/ST immediate field to bits [11:3] of X. No overflow check; check that X&7 = 0 |
| 312 | \- | R\_<CLS>\_LD64\_GOT\_LO12\_NC | G(GDAT(S)) | Set the LD/ST immediate field to bits [11:3] of X. No overflow check; check that X&7 = 0 |
+-------------+------------+-------------------------------+----------------------------+------------------------------------------------------------------------------------------------------+
| \- | 27 | R\_<CLS>\_LD32\_GOT\_LO12\_NC | G(GDAT(S+A)) | Set the LD/ST immediate field to bits [11:2] of X. No overflow check; check that X&3 = 0 |
| \- | 27 | R\_<CLS>\_LD32\_GOT\_LO12\_NC | G(GDAT(S)) | Set the LD/ST immediate field to bits [11:2] of X. No overflow check; check that X&3 = 0 |
+-------------+------------+-------------------------------+----------------------------+------------------------------------------------------------------------------------------------------+
| 313 | \- | R\_<CLS>\_LD64\_GOTPAGE\_LO15 | G(GDAT(S+A))-Page(GOT) | Set the LD/ST immediate field to bits [14:3] of X; check that 0 <= X < 2\ :sup:`15`, X&7 = 0 |
| 313 | \- | R\_<CLS>\_LD64\_GOTPAGE\_LO15 | G(GDAT(S))-Page(GOT) | Set the LD/ST immediate field to bits [14:3] of X; check that 0 <= X < 2\ :sup:`15`, X&7 = 0 |
+-------------+------------+-------------------------------+----------------------------+------------------------------------------------------------------------------------------------------+
| \- | 28 | R\_<CLS>\_LD32\_GOTPAGE\_LO14 | G(GDAT(S+A))-Page(GOT) | Set the LD/ST immediate field to bits [13:2] of X; check that 0 <= X < 2\ :sup:`14`, X&3 = 0 |
| \- | 28 | R\_<CLS>\_LD32\_GOTPAGE\_LO14 | G(GDAT(S))-Page(GOT) | Set the LD/ST immediate field to bits [13:2] of X; check that 0 <= X < 2\ :sup:`14`, X&3 = 0 |
+-------------+------------+-------------------------------+----------------------------+------------------------------------------------------------------------------------------------------+


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