| Linker Script implementation notes and policy |
| ============================================= |
| |
| LLD implements a large subset of the GNU ld linker script notation. The LLD |
| implementation policy is to implement linker script features as they are |
| documented in the ld `manual <https://sourceware.org/binutils/docs/ld/Scripts.html>`_ |
| We consider it a bug if the lld implementation does not agree with the manual |
| and it is not mentioned in the exceptions below. |
| |
| The ld manual is not a complete specification, and is not sufficient to build |
| an implementation. In particular some features are only defined by the |
| implementation and have changed over time. |
| |
| The lld implementation policy for properties of linker scripts that are not |
| defined by the documentation is to follow the GNU ld implementation wherever |
| possible. We reserve the right to make different implementation choices where |
| it is appropriate for LLD. Intentional deviations will be documented in this |
| file. |
| |
| Symbol assignment |
| ~~~~~~~~~~~~~~~~~ |
| |
| A symbol assignment looks like: |
| |
| :: |
| |
| symbol = expression; |
| symbol += expression; |
| |
| The first form defines ``symbol``. If ``symbol`` is already defined, it will be |
| overridden. The other form requires ``symbol`` to be already defined. |
| |
| For a simple assignment like ``alias = aliasee;``, the ``st_type`` field is |
| copied from the original symbol. Any arithmetic operation (e.g. ``+ 0`` will |
| reset ``st_type`` to ``STT_NOTYPE``. |
| |
| The ``st_size`` field is set to 0. |
| |
| Output section description |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| The description of an output section looks like: |
| |
| :: |
| |
| section [address] [(type)] : [AT(lma)] [ALIGN(section_align)] [SUBALIGN](subsection_align)] { |
| output-section-command |
| ... |
| } [>region] [AT>lma_region] [:phdr ...] [=fillexp] [,] |
| |
| Output section address |
| ---------------------- |
| |
| When an *OutputSection* *S* has ``address``, LLD will set sh_addr to ``address``. |
| |
| The ELF specification says: |
| |
| > The value of sh_addr must be congruent to 0, modulo the value of sh_addralign. |
| |
| The presence of ``address`` can cause the condition unsatisfied. LLD will warn. |
| GNU ld from Binutils 2.35 onwards will reduce sh_addralign so that |
| sh_addr=0 (modulo sh_addralign). |
| |
| Output section alignment |
| ------------------------ |
| |
| sh_addralign of an *OutputSection* *S* is the maximum of |
| ``ALIGN(section_align)`` and the maximum alignment of the input sections in |
| *S*. |
| |
| When an *OutputSection* *S* has both ``address`` and ``ALIGN(section_align)``, |
| GNU ld will set sh_addralign to ``ALIGN(section_align)``. |
| |
| Output section LMA |
| ------------------ |
| |
| A load address (LMA) can be specified by ``AT(lma)`` or ``AT>lma_region``. |
| |
| - ``AT(lma)`` specifies the exact load address. If the linker script does not |
| have a PHDRS command, then a new loadable segment will be generated. |
| - ``AT>lma_region`` specifies the LMA region. The lack of ``AT>lma_region`` |
| means the default region is used. Note, GNU ld propagates the previous LMA |
| memory region when ``address`` is not specified. The LMA is set to the |
| current location of the memory region aligned to the section alignment. |
| If the linker script does not have a PHDRS command, then if |
| ``lma_region`` is different from the ``lma_region`` for |
| the previous OutputSection a new loadable segment will be generated. |
| |
| The two keywords cannot be specified at the same time. |
| |
| If neither ``AT(lma)`` nor ``AT>lma_region`` is specified: |
| |
| - If the previous section is also in the default LMA region, and the two |
| section have the same memory regions, the difference between the LMA and the |
| VMA is computed to be the same as the previous difference. |
| - Otherwise, the LMA is set to the VMA. |