1Linker Script implementation notes and policy
2=============================================
3
4LLD implements a large subset of the GNU ld linker script notation. The LLD
5implementation policy is to implement linker script features as they are
6documented in the ld `manual <https://sourceware.org/binutils/docs/ld/Scripts.html>`_
7We consider it a bug if the lld implementation does not agree with the manual
8and it is not mentioned in the exceptions below.
9
10The ld manual is not a complete specification, and is not sufficient to build
11an implementation. In particular some features are only defined by the
12implementation and have changed over time.
13
14The lld implementation policy for properties of linker scripts that are not
15defined by the documentation is to follow the GNU ld implementation wherever
16possible. We reserve the right to make different implementation choices where
17it is appropriate for LLD. Intentional deviations will be documented in this
18file.
19
20Symbol assignment
21~~~~~~~~~~~~~~~~~
22
23A symbol assignment looks like:
24
25::
26
27  symbol = expression;
28  symbol += expression;
29
30The first form defines ``symbol``. If ``symbol`` is already defined, it will be
31overridden. The other form requires ``symbol`` to be already defined.
32
33For a simple assignment like ``alias = aliasee;``, the ``st_type`` field is
34copied from the original symbol. Any arithmetic operation (e.g. ``+ 0`` will
35reset ``st_type`` to ``STT_NOTYPE``.
36
37The ``st_size`` field is set to 0.
38
39SECTIONS command
40~~~~~~~~~~~~~~~~
41
42A ``SECTIONS`` command looks like:
43
44::
45
46  SECTIONS {
47    section-command
48    section-command
49    ...
50  } [INSERT [AFTER|BEFORE] anchor_section;]
51
52Each section-command can be a symbol assignment, an output section description,
53or an overlay description.
54
55When the ``INSERT`` keyword is present, the ``SECTIONS`` command describes some
56output sections which should be inserted after or before the specified anchor
57section. The insertion occurs after input sections have been mapped to output
58sections but before orphan sections have been processed.
59
60In the case where no linker script has been provided or every ``SECTIONS``
61command is followed by ``INSERT``, LLD applies built-in rules which are similar
62to GNU ld's internal linker scripts.
63
64- Align the first section in a ``PT_LOAD`` segment according to
65  ``-z noseparate-code``, ``-z separate-code``, or
66  ``-z separate-loadable-segments``
67- Define ``__bss_start``, ``end``, ``_end``, ``etext``, ``_etext``, ``edata``,
68  ``_edata``
69- Sort ``.ctors.*``/``.dtors.*``/``.init_array.*``/``.fini_array.*`` and
70  PowerPC64 specific ``.toc``
71- Place input ``.text.*`` into output ``.text``, and handle certain variants
72  (``.text.hot.``, ``.text.unknown.``, ``.text.unlikely.``, etc) in the
73  presence of ``-z keep-text-section-prefix``.
74
75Output section description
76~~~~~~~~~~~~~~~~~~~~~~~~~~
77
78The description of an output section looks like:
79
80::
81
82  section [address] [(type)] : [AT(lma)] [ALIGN(section_align)] [SUBALIGN](subsection_align)] {
83    output-section-command
84    ...
85  } [>region] [AT>lma_region] [:phdr ...] [=fillexp] [,]
86
87Output section address
88----------------------
89
90When an *OutputSection* *S* has ``address``, LLD will set sh_addr to ``address``.
91
92The ELF specification says:
93
94> The value of sh_addr must be congruent to 0, modulo the value of sh_addralign.
95
96The presence of ``address`` can cause the condition unsatisfied. LLD will warn.
97GNU ld from Binutils 2.35 onwards will reduce sh_addralign so that
98sh_addr=0 (modulo sh_addralign).
99
100Output section type
101-------------------
102
103When an *OutputSection* *S* has ``(type)``, LLD will set ``sh_type`` or
104``sh_flags`` of *S*. ``type`` is one of:
105
106- ``NOLOAD``: set ``sh_type`` to ``SHT_NOBITS``.
107- ``COPY``, ``INFO``, ``OVERLAY``: clear the ``SHF_ALLOC`` bit in ``sh_flags``.
108- ``TYPE=<value>``: set ``sh_type`` to the specified value. ``<value>`` must be
109  an integer or one of ``SHT_PROGBITS, SHT_NOTE, SHT_NOBITS, SHT_INIT_ARRAY,
110  SHT_FINI_ARRAY, SHT_PREINIT_ARRAY``.
111
112When ``sh_type`` is specified, it is an error if an input section in *S* has a
113different type.
114
115Output section alignment
116------------------------
117
118sh_addralign of an *OutputSection* *S* is the maximum of
119``ALIGN(section_align)`` and the maximum alignment of the input sections in
120*S*.
121
122When an *OutputSection* *S* has both ``address`` and ``ALIGN(section_align)``,
123GNU ld will set sh_addralign to ``ALIGN(section_align)``.
124
125Output section LMA
126------------------
127
128A load address (LMA) can be specified by ``AT(lma)`` or ``AT>lma_region``.
129
130- ``AT(lma)`` specifies the exact load address. If the linker script does not
131  have a PHDRS command, then a new loadable segment will be generated.
132- ``AT>lma_region`` specifies the LMA region. The lack of ``AT>lma_region``
133  means the default region is used. Note, GNU ld propagates the previous LMA
134  memory region when ``address`` is not specified. The LMA is set to the
135  current location of the memory region aligned to the section alignment.
136  If the linker script does not have a PHDRS command, then if
137  ``lma_region`` is different from the ``lma_region`` for
138  the previous OutputSection a new loadable segment will be generated.
139
140The two keywords cannot be specified at the same time.
141
142If neither ``AT(lma)`` nor ``AT>lma_region`` is specified:
143
144- If the previous section is also in the default LMA region, and the two
145  section have the same memory regions, the difference between the LMA and the
146  VMA is computed to be the same as the previous difference.
147- Otherwise, the LMA is set to the VMA.
148
149Overwrite sections
150~~~~~~~~~~~~~~~~~~
151
152An ``OVERWRITE_SECTIONS`` command looks like:
153
154::
155
156  OVERWRITE_SECTIONS {
157    output-section-description
158    output-section-description
159    ...
160  }
161
162Unlike a ``SECTIONS`` command, ``OVERWRITE_SECTIONS``  does not specify a
163section order or suppress the built-in rules.
164
165If a described output section description also appears in a ``SECTIONS``
166command, the ``OVERWRITE_SECTIONS`` command wins; otherwise, the output section
167will be added somewhere following the usual orphan section placement rules.
168
169If a described output section description also appears in an ``INSERT
170[AFTER|BEFORE]`` command, the description will be provided by the
171description in the ``OVERWRITE_SECTIONS`` command while the insert command
172still applies (possibly after orphan section placement). It is recommended to
173leave the brace empty (i.e. ``section : {}``) for the insert command, because
174its description will be ignored anyway.
175