153 lines
5.2 KiB
Plaintext
153 lines
5.2 KiB
Plaintext
Preliminary Notes on Porting GNU ld
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-----------------------------------
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Before porting ld itself, you will need to port the BFD library;
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see ../bfd/PORTING.
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The 'host' is the system a tool runs *on*.
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The 'target' is the system a tool runs *for*, i.e.
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a tool can read/write the binaries of the target.
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Most often, host==target, but ld supports cross-linking
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(and to some extent the same ld binary can be used a linker
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for multiple target rachitectures).
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Porting to a new host
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---------------------
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Pick a name for your host. Call that <host>.
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You need to create the file config/mh-<host>.
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Porting to a new target
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-----------------------
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Pick a name for your target. Call that <target>.
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You need to create at least config/mt-<target>.
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It should contain
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EMUL=<emulation>
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An <emulation> controls the "personality" of ld,
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such as the default linker script. Usually, the
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<emulation> will have teh same name as the <target>,
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and you will need to create a new <emulation> (see below).
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You will also need to edit Makefile.in and possible configure.in.
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To see how to do that, search for existing examples (e.g. sun3,
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sun4, hp300bsd).
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Porting to a new emulation target
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---------------------------------
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Pick a name for your target. Call that <emulation>.
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Usually, <emulation> and <target> are the same.
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You need to create at least <emulation>.sh.
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You will also need to edit Makefile.in,
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To see how to do that, search for existing examples.
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The file <emulation>.sh defines a set of parameter that
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are used to generate the emulation. Its syntax is that
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of a (Bourne) shell script, and it is "sourced" by genscripts.sh.
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Writing <emulation.sh>
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----------------------
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Usually, <emulation>.sh contains:
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EMULATION_NAME=<emulation>
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SCRIPT_NAME=<script>
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OUTPUT_FORMAT="<target-name>"
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TEXT_START_ADDR=<text_start_addr>
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PAGE_SIZE=<page_size>
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SEGMENT_SIZE=<segment_size> # If different from PAGE_SIZE.
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ARCH=<arch>
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<target-name>
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Matches the 'filename' field of the bfd_target you want
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to use. (This is a string, and currently the first field.)
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For an a.out target, <target-name> matches the TARGETNAME
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defined in ../bfd/<target>.c.
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<arch>
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The architecture: e.g. m68k, sparc, ...
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<script>
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The file <script>.sc-sh is a shell script which when
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eveluated (by genscripts.sh) writes a linker script
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file to standard output. You may need to write a new
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script. If you use the a.out format or something
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similar, you can probably set
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SCRIPT_NAME=aout
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<text_start_addr>
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<page_size>
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<segment_size>
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These set the shell variables TEXT_START_ADDR, PAGE_SIZE,
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and SEGEMNT_SIZE for use by <script>.sc-sh.
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If your script doesn't use these variables, you
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don't have to define the variables,
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For emulations using a.out files, you can get these
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values from ../bfd/<target>c.
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In some cases, you may need more more definitions.
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For example, if you can't use generic.em,
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you may need to add:
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TEMPLATE_NAME=<emulation>
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and write your own <emulation>.em file.
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Writing a new <script>.sc-sh
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----------------------------
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You may need to write a new script file for your emulation.
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Your script can use the shell variable LD_FLAG, which has the value:
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LD_FLAG= when building a script to be used by default
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LD_FLAG=n when building a script to be used for ld -n
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LD_FLAG=N when building a script to be used for ld -N
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LD_FLAG=r when building a script to be used for ld -r
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LD_FLAG=u when building a script to be used for ld -Ur
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The variable RELOCATING is only set if relocation is happening
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(i.e. unless the linker is invoked with -r).
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Thus your script should has an action ACTION
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that should only be done when relocating,
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express that as:
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${RELOCATING+ ACTION}
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In general, this is the case for most assignments, which should look like:
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${RELOCATING+ _end = .}
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Also, you should assign absolute addresses to sections only
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when relocating, so:
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.text ${RELOCATING+ ${TEXT_START_ADDR}}:
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The forms:
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.section { ... } > section
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should be:
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.section { ... } > ${RELOCATING+ section}
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RELOCATING is set except when LD_FLAG=r or LD_FLAG=u.
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CONSTRUCTING is set except when LD_FLAG=u.
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Alignment of the data segments is controlled by the variables
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DATA_ALIGNMENT_ (note trailing underscore), DATA_ALIGNMENT_n,
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DATA_ALIGNMENT_N, DATA_ALIGNMENT_r, or DTA_ALIGNMENT_u
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depending on LD_FLAGS's value.
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Normally, the default value works (this is "ALIGN(${SEGMENT_SIZE})"
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for the _n, and __ (default) variants; "." for the _N, variant;
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and "" for the _r and _u variants).
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Handling -n and -N style binaries in your linker script
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-------------------------------------------------------
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The -n linker flag requests the linker to create a binary
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with a write-protected text segment, but not demand-pagable (NMAGIC).
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Sunos starts the text segment for demand-paged binaries at 0x2020
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and other binaries at 0x2000, since the exec header (0x20 bytes)
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is paged in with the text. Some other Unix variants do the same.
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In that case, the <emulation.sh> should define:
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NONPAGED_TEXT_START_ADDR
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The text start address to use when linking with -n or -N flags.
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For example, on a sun4:
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TEXT_START_ADDR=0x2020
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NONPAGED_TEXT_START_ADDR=0x2000
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The -N linker flag requests the linker to create a binary
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without a write-protected text segment (NMAGIC).
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This is like -n, except that the data segment needs not be page-aligned.
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