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 100When an output section has no input section, GNU ld will eliminate it if it 101only contains symbol assignments (e.g. ``.foo { symbol = 42; }``). LLD will 102retain such sections unless all the symbol assignments are unreferenced 103``PROVIDED``. 104 105When an output section has no input section but advances the location counter, 106GNU ld sets the ``SHF_WRITE`` flag. LLD sets the SHF_WRITE flag only if the 107preceding output section with non-empty input sections also has the SHF_WRITE 108flag. 109 110Output section type 111------------------- 112 113When an *OutputSection* *S* has ``(type)``, LLD will set ``sh_type`` or 114``sh_flags`` of *S*. ``type`` is one of: 115 116- ``NOLOAD``: set ``sh_type`` to ``SHT_NOBITS``. 117- ``COPY``, ``INFO``, ``OVERLAY``: clear the ``SHF_ALLOC`` bit in ``sh_flags``. 118- ``TYPE=<value>``: set ``sh_type`` to the specified value. ``<value>`` must be 119 an integer or one of ``SHT_PROGBITS, SHT_NOTE, SHT_NOBITS, SHT_INIT_ARRAY, 120 SHT_FINI_ARRAY, SHT_PREINIT_ARRAY``. 121 122When ``sh_type`` is specified, it is an error if an input section in *S* has a 123different type. 124 125Output section alignment 126------------------------ 127 128sh_addralign of an *OutputSection* *S* is the maximum of 129``ALIGN(section_align)`` and the maximum alignment of the input sections in 130*S*. 131 132When an *OutputSection* *S* has both ``address`` and ``ALIGN(section_align)``, 133GNU ld will set sh_addralign to ``ALIGN(section_align)``. 134 135Output section LMA 136------------------ 137 138A load address (LMA) can be specified by ``AT(lma)`` or ``AT>lma_region``. 139 140- ``AT(lma)`` specifies the exact load address. If the linker script does not 141 have a PHDRS command, then a new loadable segment will be generated. 142- ``AT>lma_region`` specifies the LMA region. The lack of ``AT>lma_region`` 143 means the default region is used. Note, GNU ld propagates the previous LMA 144 memory region when ``address`` is not specified. The LMA is set to the 145 current location of the memory region aligned to the section alignment. 146 If the linker script does not have a PHDRS command, then if 147 ``lma_region`` is different from the ``lma_region`` for 148 the previous OutputSection a new loadable segment will be generated. 149 150The two keywords cannot be specified at the same time. 151 152If neither ``AT(lma)`` nor ``AT>lma_region`` is specified: 153 154- If the previous section is also in the default LMA region, and the two 155 section have the same memory regions, the difference between the LMA and the 156 VMA is computed to be the same as the previous difference. 157- Otherwise, the LMA is set to the VMA. 158 159Overwrite sections 160~~~~~~~~~~~~~~~~~~ 161 162An ``OVERWRITE_SECTIONS`` command looks like: 163 164:: 165 166 OVERWRITE_SECTIONS { 167 output-section-description 168 output-section-description 169 ... 170 } 171 172Unlike a ``SECTIONS`` command, ``OVERWRITE_SECTIONS`` does not specify a 173section order or suppress the built-in rules. 174 175If a described output section description also appears in a ``SECTIONS`` 176command, the ``OVERWRITE_SECTIONS`` command wins; otherwise, the output section 177will be added somewhere following the usual orphan section placement rules. 178 179If a described output section description also appears in an ``INSERT 180[AFTER|BEFORE]`` command, the description will be provided by the 181description in the ``OVERWRITE_SECTIONS`` command while the insert command 182still applies (possibly after orphan section placement). It is recommended to 183leave the brace empty (i.e. ``section : {}``) for the insert command, because 184its description will be ignored anyway. 185 186Built-in functions 187~~~~~~~~~~~~~~~~~~ 188 189``DATA_SEGMENT_RELRO_END(offset, exp)`` defines the end of the ``PT_GNU_RELRO`` 190segment when ``-z relro`` (default) is in effect. Sections between 191``DATA_SEGMENT_ALIGN`` and ``DATA_SEGMENT_RELRO_END`` are considered RELRO. 192 193The typical use case is ``. = DATA_SEGMENT_RELRO_END(0, .);`` followed by 194writable but non-RELRO sections. LLD ignores ``offset`` and ``exp`` and aligns 195the current location to a max-page-size boundary, ensuring that the next 196``PT_LOAD`` segment will not overlap with the ``PT_GNU_RELRO`` segment. 197 198LLD will insert ``.relro_padding`` immediately before the symbol assignment 199using ``DATA_SEGMENT_RELRO_END``. 200 201Non-contiguous regions 202~~~~~~~~~~~~~~~~~~~~~~ 203 204The flag ``--enable-non-contiguous-regions`` allows input sections to spill to 205later matches rather than causing the link to fail by overflowing a memory 206region. Unlike GNU ld, ``/DISCARD/`` only matches previously-unmatched sections 207(i.e., the flag does not affect it). Also, if a section fails to fit at any of 208its matches, the link fails instead of discarding the section. Accordingly, the 209GNU flag ``--enable-non-contiguous-regions-warnings`` is not implemented, as it 210exists to warn about such occurrences. 211