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