899fdf548e
This implements https://sourceware.org/bugzilla/show_bug.cgi?id=26404 An `OVERWRITE_SECTIONS` command is a `SECTIONS` variant which contains several output section descriptions. The output sections do not have specify an order. Similar to `INSERT [BEFORE|AFTER]`, `LinkerScript::hasSectionsCommand` is not set, so the built-in rules (see `docs/ELF/linker_script.rst`) still apply. `OVERWRITE_SECTIONS` can be more convenient than `INSERT` because it does not need an anchor section. The initial syntax is intentionally narrow to facilitate backward compatible extensions in the future. Symbol assignments cannot be used. This feature is versatile. To list a few usage: * Use `section : { KEEP(...) }` to retain input sections under GC * Define encapsulation symbols (start/end) for an output section * Use `section : ALIGN(...) : { ... }` to overalign an output section (similar to ld64 `-sectalign`) When an output section is specified by both `OVERWRITE_SECTIONS` and `INSERT`, `INSERT` is processed after overwrite sections. To make this work, this patch changes `InsertCommand` to use name based matching instead of pointer based matching. (This may cause a difference when `INSERT` moves one output section more than once. Such duplicate commands should not be used in practice (seems that in GNU ld the output sections may just disappear).) A linker script can be used without -T/--script. The traditional `SECTIONS` commands are concatenated, so a wrong rule can be more noticeable from the section order. This feature if misused can be less noticeable, just like `INSERT`. Differential Revision: https://reviews.llvm.org/D103303
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5.8 KiB
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157 lines
5.8 KiB
ReStructuredText
Linker Script implementation notes and policy
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=============================================
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LLD implements a large subset of the GNU ld linker script notation. The LLD
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implementation policy is to implement linker script features as they are
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documented in the ld `manual <https://sourceware.org/binutils/docs/ld/Scripts.html>`_
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We consider it a bug if the lld implementation does not agree with the manual
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and it is not mentioned in the exceptions below.
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The ld manual is not a complete specification, and is not sufficient to build
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an implementation. In particular some features are only defined by the
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implementation and have changed over time.
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The lld implementation policy for properties of linker scripts that are not
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defined by the documentation is to follow the GNU ld implementation wherever
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possible. We reserve the right to make different implementation choices where
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it is appropriate for LLD. Intentional deviations will be documented in this
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file.
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Symbol assignment
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~~~~~~~~~~~~~~~~~
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A symbol assignment looks like:
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::
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symbol = expression;
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symbol += expression;
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The first form defines ``symbol``. If ``symbol`` is already defined, it will be
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overridden. The other form requires ``symbol`` to be already defined.
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For a simple assignment like ``alias = aliasee;``, the ``st_type`` field is
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copied from the original symbol. Any arithmetic operation (e.g. ``+ 0`` will
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reset ``st_type`` to ``STT_NOTYPE``.
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The ``st_size`` field is set to 0.
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SECTIONS command
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~~~~~~~~~~~~~~~~
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A ``SECTIONS`` command looks like:
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::
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SECTIONS {
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section-command
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section-command
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...
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} [INSERT [AFTER|BEFORE] anchor_section;]
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Each section-command can be a symbol assignment, an output section description,
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or an overlay description.
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When the ``INSERT`` keyword is present, the ``SECTIONS`` command describes some
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output sections which should be inserted after or before the specified anchor
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section. The insertion occurs after input sections have been mapped to output
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sections but before orphan sections have been processed.
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In the case where no linker script has been provided or every ``SECTIONS``
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command is followed by ``INSERT``, LLD applies built-in rules which are similar
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to GNU ld's internal linker scripts.
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- Align the first section in a ``PT_LOAD`` segment according to ``-z noseparate-code``,
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``-z separate-code``, or ``-z separate-loadable-segments``
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- Define ``__bss_start``, ``end``, ``_end``, ``etext``, ``_etext``, ``edata``, ``_edata``
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- Sort ``.ctors.*``/``.dtors.*``/``.init_array.*``/``.fini_array.*`` and PowerPC64 specific ``.toc``
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- Place input ``.text.*`` into output ``.text``, and handle certain variants
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(``.text.hot.``, ``.text.unknown.``, ``.text.unlikely.``, etc) in the precense of
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``-z keep-text-section-prefix``.
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Output section description
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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The description of an output section looks like:
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::
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section [address] [(type)] : [AT(lma)] [ALIGN(section_align)] [SUBALIGN](subsection_align)] {
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output-section-command
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...
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} [>region] [AT>lma_region] [:phdr ...] [=fillexp] [,]
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Output section address
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----------------------
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When an *OutputSection* *S* has ``address``, LLD will set sh_addr to ``address``.
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The ELF specification says:
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> The value of sh_addr must be congruent to 0, modulo the value of sh_addralign.
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The presence of ``address`` can cause the condition unsatisfied. LLD will warn.
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GNU ld from Binutils 2.35 onwards will reduce sh_addralign so that
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sh_addr=0 (modulo sh_addralign).
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Output section alignment
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------------------------
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sh_addralign of an *OutputSection* *S* is the maximum of
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``ALIGN(section_align)`` and the maximum alignment of the input sections in
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*S*.
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When an *OutputSection* *S* has both ``address`` and ``ALIGN(section_align)``,
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GNU ld will set sh_addralign to ``ALIGN(section_align)``.
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Output section LMA
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------------------
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A load address (LMA) can be specified by ``AT(lma)`` or ``AT>lma_region``.
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- ``AT(lma)`` specifies the exact load address. If the linker script does not
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have a PHDRS command, then a new loadable segment will be generated.
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- ``AT>lma_region`` specifies the LMA region. The lack of ``AT>lma_region``
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means the default region is used. Note, GNU ld propagates the previous LMA
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memory region when ``address`` is not specified. The LMA is set to the
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current location of the memory region aligned to the section alignment.
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If the linker script does not have a PHDRS command, then if
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``lma_region`` is different from the ``lma_region`` for
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the previous OutputSection a new loadable segment will be generated.
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The two keywords cannot be specified at the same time.
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If neither ``AT(lma)`` nor ``AT>lma_region`` is specified:
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- If the previous section is also in the default LMA region, and the two
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section have the same memory regions, the difference between the LMA and the
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VMA is computed to be the same as the previous difference.
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- Otherwise, the LMA is set to the VMA.
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Overwrite sections
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~~~~~~~~~~~~~~~~~~
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An ``OVERWRITE_SECTIONS`` command looks like:
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::
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OVERWRITE_SECTIONS {
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output-section-description
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output-section-description
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...
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}
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Unlike a ``SECTIONS`` command, ``OVERWRITE_SECTIONS`` does not specify a
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section order or suppress the built-in rules.
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If a described output section description also appears in a ``SECTIONS``
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command, the ``OVERWRITE_SECTIONS`` command wins; otherwise, the output section
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will be added somewhere following the usual orphan section placement rules.
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If a described output section description also appears in an ``INSERT
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[AFTER|BEFORE]`` command, the description will be provided by the
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description in the ``OVERWRITE_SECTIONS`` command while the insert command
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still applies (possibly after orphan section placement). It is recommended to
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leave the brace empty (i.e. ``section : {}``) for the insert command, because
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its description will be ignored anyway.
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