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 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.

SECTIONS command
~~~~~~~~~~~~~~~~

A ``SECTIONS`` command looks like:

::

  SECTIONS {
    section-command
    section-command
    ...
  } [INSERT [AFTER|BEFORE] anchor_section;]

Each section-command can be a symbol assignment, an output section description,
or an overlay description.

When the ``INSERT`` keyword is present, the ``SECTIONS`` command describes some
output sections which should be inserted after or before the specified anchor
section. The insertion occurs after input sections have been mapped to output
sections but before orphan sections have been processed.

In the case where no linker script has been provided or every ``SECTIONS``
command is followed by ``INSERT``, LLD applies built-in rules which are similar
to GNU ld's internal linker scripts.

- Align the first section in a ``PT_LOAD`` segment according to
  ``-z noseparate-code``, ``-z separate-code``, or
  ``-z separate-loadable-segments``
- Define ``__bss_start``, ``end``, ``_end``, ``etext``, ``_etext``, ``edata``,
  ``_edata``
- Sort ``.ctors.*``/``.dtors.*``/``.init_array.*``/``.fini_array.*`` and
  PowerPC64 specific ``.toc``
- Place input ``.text.*`` into output ``.text``, and handle certain variants
  (``.text.hot.``, ``.text.unknown.``, ``.text.unlikely.``, etc) in the
  presence of ``-z keep-text-section-prefix``.

Output section description

The description of an output section looks like:

::

section [address] [(type)] : [AT(lma)] [ALIGN(section_align)] SUBALIGN] { 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 type

When an OutputSection S has (type), LLD will set sh_type or sh_flags of S. type is one of:

• NOLOAD: set sh_type to SHT_NOBITS.
• COPY, INFO, OVERLAY: clear the SHF_ALLOC bit in sh_flags.
• TYPE=<value>: set sh_type to the specified value. <value> must be an integer or one of SHT_PROGBITS, SHT_NOTE, SHT_NOBITS, SHT_INIT_ARRAY, SHT_FINI_ARRAY, SHT_PREINIT_ARRAY.

When sh_type is specified, it is an error if an input section in S has a different type.

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.

Overwrite sections

An ``OVERWRITE_SECTIONS`` command looks like:

::

  OVERWRITE_SECTIONS {
    output-section-description
    output-section-description
    ...
  }

Unlike a ``SECTIONS`` command, ``OVERWRITE_SECTIONS``  does not specify a
section order or suppress the built-in rules.

If a described output section description also appears in a ``SECTIONS``
command, the ``OVERWRITE_SECTIONS`` command wins; otherwise, the output section
will be added somewhere following the usual orphan section placement rules.

If a described output section description also appears in an ``INSERT
[AFTER|BEFORE]`` command, the description will be provided by the
description in the ``OVERWRITE_SECTIONS`` command while the insert command
still applies (possibly after orphan section placement). It is recommended to
leave the brace empty (i.e. ``section : {}``) for the insert command, because
its description will be ignored anyway.