[docs][llvm-objcopy] Write documentation for llvm-objcopy

[jhenderson]

Summary

This patch addresses https://bugs.llvm.org/show_bug.cgi?id=42183 by replacing the stub markdown doc for llvm-objcopy with a full one describing the current options available in llvm-objcopy.

[MaskRay]

docs/CommandGuide/llvm-objcopy.rst

419

SUPPORTED FORMATS might be a bit more appropriate as the section header.

It may be worth mentioning elf* below are bfdnames.

[jhenderson]

• Change SUPPORTED TARGETS to SUPPORTED FORMATS.
• Add a note about the supported formats being bfdnames for compatibility.
• Normalized some wording for the --.*target options.
• Added a .. program directive to ensure that option links remain pointing to the right document.

[jakehehrlich]

This is *awesome*. This is much more clear and precise that the GNU objcopy documentation!

I hadn't realized just how far the tool has come! Great job!

docs/CommandGuide/llvm-objcopy.rst

82–83

We keep .gnu.warning sections as well right?

131

This is half fair. Technically it meets the requirements for ELF it just doesn't work optimally. I think adding a caveat section under ELF-specific section might be warranted.

227

Might add "with local binding"

380

Say "with no other arguments" to clarify that the --extract-dwo file has no flags applied to it.

395

I'd note that many non-llvm tools are incompatible with this option and that it should only be used when a) you need the absolute smallest file possible and b) should never be used on ET_REL binaries.

[MaskRay]

docs/CommandGuide/llvm-objcopy.rst

10

It may be more common to place options before positional arguments, i.e.

:program:`llvm-objcopy` [*options*] *input* [*output*]

[jhenderson]

Address review comments.

docs/CommandGuide/llvm-objcopy.rst

131

Config.OnlyKeepDebug is not used outside the COFF code, so this really is ignored for ELF as things stand. Given that the output of using this on ELF is not what GNU produces (i.e. it's a no-op), I'm not convinced we should advertise this more widely. We could make similar arguments about various options that are currently ELF-specific, and aren't checked by the COFF code for an error.

I've added a blanket comment to the top of the section saying that llvm-objcopy will either ignore or error for format-specific options (same for ELF).

227

Isn't that what "local symbols" means?

395

I've added a more general note that "many tools will not be able to" handle such objects.

I don't see a reason to mention when you should or shouldn't do this beyond that statement because ultimately if a user is using this switch, they will know why they are using it, whether it be for size, obfuscation, or whatever. Telling people that it's always about size is like telling them what their motivation is.

If it's run on an ET_REL binary, then the entire section contents of the file will be removed. I don't think this needs explicitly stating, since that can be inferred from the reference to "not within segments".

[jhenderson]

Improve text of --dump-section to clarify that the regular copying still takes place.

[jakehehrlich]

LGTM (sorry a chrome bug is preventing me from using drop down menus...sigh)

docs/CommandGuide/llvm-objcopy.rst

233

Perhaps its worth mentioning that operations are not performed on the section itself prior to dumping?

[jakehehrlich]

(fixed it)

docs/CommandGuide/llvm-objcopy.md

-# llvm-objcopy - object copying tool
-
-## SYNOPSIS
-
-**llvm-objcopy** [*options*]
-
-## DESCRIPTION
-
-**llvm-objcopy** is a tool to copy and manipulate objects.
-
-The tool is still in active development, but in most scenarios it works as a
-drop-in replacement for GNU's **objcopy**.
-
-## SEE ALSO
-
-[llvm-strip](llvm-strip.html)

docs/CommandGuide/llvm-objcopy.rst

+llvm-objcopy - object copying and editing tool
+==============================================
+
+.. program:: llvm-objcopy
+
+SYNOPSIS
+--------
+
+:program:`llvm-objcopy` [*options*] *input* [*output*]
+
+DESCRIPTION
+-----------
+
+:program:`llvm-objcopy` is a tool to copy and manipulate objects. In basic
+usage, it makes a semantic copy of the input to the output. If any options are
+specified, the output may be modified along the way, e.g. by removing sections.
+
+If no output file is specified, the input file is modified in-place. If "-" is
+specified for the input file, the input is read from the program's standard
+input stream. If "-" is specified for the output file, the output is written to
+the standard output stream of the program.
+
+If the input is an archive, any requested operations will be applied to each
+archive member individually.
+
+The tool is still in active development, but in most scenarios it works as a
+drop-in replacement for GNU's :program:`objcopy`.
+
+GENERIC AND CROSS-PLATFORM OPTIONS
+----------------------------------
+
+The following options are either agnostic of the file format, or apply to
+multiple file formats.
+
+.. option:: --add-gnu-debuglink <debug-file>
+
+ Add a .gnu_debuglink section for ``<debug-file>`` to the output.
+
+.. option:: --disable-deterministic-archives, -U
+
+ Use real values for UIDs, GIDs and timestamps when updating archive member
+ headers.
+
+.. option:: --discard-all, -x
+
+ Remove most local symbols from the output. Different file formats may limit
+ this to a subset of the local symbols. For example, file and section symbols in
+ ELF objects will not be discarded.
+
+.. option:: --enable-deterministic-archives, -D
+
+ Enable deterministic mode when copying archives, i.e. use 0 for archive member
+ header UIDs, GIDs and timestamp fields. On by default.
+
+.. option:: --help, -h
+
+ Print a summary of command line options.
+
+.. option:: --only-section <section>, -j
+
+ Remove all sections from the output, except for sections named ``<section>``.
+ Can be specified multiple times to keep multiple sections.
+
+.. option:: --regex
+
+ If specified, symbol and section names specified by other switches are treated
+ as extended POSIX regular expression patterns.
+
+.. option:: --remove-section <section>, -R
+
+ Remove the specified section from the output. Can be specified multiple times
+ to remove multiple sections simultaneously.
+
+.. option:: --strip-all-gnu
+
+ Remove all symbols, debug sections and relocations from the output. This option
+ is equivalent to GNU :program:`objcopy`'s ``--strip-all`` switch.
+
+.. option:: --strip-all, -S
+
+ For ELF objects, remove from the output all symbols and non-alloc sections not
+ within segments, except for .gnu.warning sections and the section name table.
+
+ For COFF objects, remove all symbols, debug sections, and relocations from the
+ output.
+
+.. option:: --strip-debug, -g
+
+ Remove all debug sections from the output.
+
+.. option:: --strip-symbol <symbol>, -N
+
+ Remove all symbols named ``<symbol>`` from the output. Can be specified
+ multiple times to remove multiple symbols.
+
+.. option:: --strip-symbols <filename>
+
+ Remove all symbols whose names appear in the file ``<filename>``, from the
+ output. In the file, each line represents a single symbol name, with leading
+ and trailing whitespace ignored, as is anything following a '#'. Can be
+ specified multiple times to read names from multiple files.
+
+.. option:: --strip-unneeded-symbol <symbol>
+
+ Remove from the output all symbols named ``<symbol>`` that are local or
+ undefined and are not required by any relocation.
+
+.. option:: --strip-unneeded-symbols <filename>
+
+ Remove all symbols whose names appear in the file ``<filename>``, from the
+ output, if they are local or undefined and are not required by any relocation.
+ In the file, each line represents a single symbol name, with leading and
+ trailing whitespace ignored, as is anything following a '#'. Can be specified
+ multiple times to read names from multiple files.
+
+.. option:: --strip-unneeded
+
+ Remove from the output all local or undefined symbols that are not required by
+ relocations.
+
+.. option:: --version, -V
+
+  Display the version of this program.
+
+COFF-SPECIFIC OPTIONS
+---------------------
+
+The following options are implemented only for COFF objects. If used with other
+objects, :program:`llvm-objcopy` will either emit an error or silently ignore
+them.
+
+.. option:: --only-keep-debug
+
+ Remove the contents of non-debug sections from the output, but keep the section
+ headers.
+
+ELF-SPECIFIC OPTIONS
+--------------------
+
+The following options are implemented only for ELF objects. If used with other
+objects, :program:`llvm-objcopy` will either emit an error or silently ignore
+them.
+
+.. option:: --add-section <section=file>
+
+ Add a section named ``<section>`` with the contents of ``<file>`` to the
+ output. The section will be of type `SHT_NOTE`, if the name starts with
+ ".note". Otherwise, it will have type `SHT_PROGBITS`. Can be specified multiple
+ times to add multiple sections.
+
+.. option:: --add-symbol <name>=[<section>:]<value>[,<flags>]
+
+ Add to the output a new symbol called ``<name>`` to the symbol table, in the
+ section named ``<section>``, with value ``<value>``. If ``<section>`` is not
+ specified, the symbol is added as an absolute symbol. The ``<flags>`` affect
+ the symbol properties. Accepted values are:
+
+ - `global` = the symbol will have global binding.
+ - `local` = the symbol will have local binding.
+ - `weak` = the symbol will have weak binding.
+ - `default` = the symbol will have default visibility.
+ - `hidden` = the symbol will have hidden visibility.
+ - `file` = the symbol will be an `STT_FILE` symbol.
+ - `section` = the symbol will be an `STT_SECTION` symbol.
+ - `object` = the symbol will be an `STT_OBJECT` symbol.
+ - `function` = the symbol will be an `STT_FUNC` symbol.
+ - `indirect-function` = the symbol will be an `STT_GNU_IFUNC` symbol.
+
+ Additionally, the following flags are accepted but ignored: `debug`,
+ `constructor`, `warning`, `indirect`, `synthetic`, `unique-object`, `before`.
+
+ Can be specified multiple times to add multiple symbols.
+
+.. option:: --allow-broken-links
+
+ Allow llvm-objcopy to remove sections even if it would leave invalid section
+ references. Any invalid sh_link fields will be set to zero.
+
+.. option:: --binary-architecture <arch>, -B
+
+ Specify the architecture to use, when transforming an architecture-less format
+ (e.g. binary) to another format. Valid options are:
+
+ - `aarch64`
+ - `arm`
+ - `i386`
+ - `i386:x86-64`
+ - `mips`
+ - `powerpc:common64`
+ - `riscv:rv32`
+ - `riscv:rv64`
+ - `sparc`
+ - `sparcel`
+ - `x86-64`
+
+.. option:: --build-id-link-dir <dir>
+
+ Set the directory used by :option:`--build-id-link-input` and
+ :option:`--build-id-link-output`.
+
+.. option:: --build-id-link-input <suffix>
+
+ Hard-link the input to ``<dir>/xx/xxx<suffix>``, where ``<dir>`` is the directory
+ specified by :option:`--build-id-link-dir`. The path used is derived from the
+ hex build ID.
+
+.. option:: --build-id-link-output <suffix>
+
+ Hard-link the output to ``<dir>/xx/xxx<suffix>``, where ``<dir>`` is the directory
+ specified by :option:`--build-id-link-dir`. The path used is derived from the
+ hex build ID.
+
+.. option:: --change-start <incr>, --adjust-start
+
+ Add ``<incr>`` to the program's start address. Can be specified multiple
+ times, in which case the values will be applied cumulatively.
+
+.. option:: --compress-debug-sections [<style>]
+
+ Compress DWARF debug sections in the output, using the specified style.
+ Supported styles are `zlib-gnu` and `zlib`. Defaults to `zlib` if no style is
+ specified.
+
+.. option:: --decompress-debug-sections
+
+ Decompress any compressed DWARF debug sections in the output.
+
+.. option:: --discard-locals, -X
+
+ Remove local symbols starting with ".L" from the output.
+
+.. option:: --dump-section <section>=<file>
+
+ Dump the contents of section ``<section>`` into the file ``<file>``. Can be
+ specified multiple times to dump multiple sections.
+
+.. option:: --extract-dwo
+
+ Remove all sections that are not DWARF .dwo sections from the output.
+
+.. option:: --extract-main-partition
+
+ Extract the main partition from the output.
+
+.. option:: --extract-partition <name>
+
+ Extract the named partition from the output.
+
+.. option:: --globalize-symbol <symbol>
+
+ Mark any defined symbols named ``<symbol>`` as global symbols in the output.
+ Can be specified multiple times to mark multiple symbols.
+
+.. option:: --globalize-symbols <filename>
+
+ Read a list of names from the file ``<filename>`` and mark defined symbols with
+ those names as global in the output. In the file, each line represents a single
+ symbol, with leading and trailing whitespace ignored, as is anything following
+ a '#'. Can be specified multiple times to read names from multiple files.
+
+.. option:: --input-target <format>, -I
+
+ Read the input as the specified format. See `SUPPORTED FORMATS`_ for a list of
+ valid ``<format>`` values. If unspecified, :program:`llvm-objcopy` will attempt
+ to determine the format automatically.
+
+.. option:: --keep-file-symbols
+
+ Keep symbols of type `STT_FILE`, even if they would otherwise be stripped.
+
+.. option:: --keep-global-symbol <symbol>
+
+ Make all symbols local in the output, except for symbols with the name
+ ``<symbol>``. Can be specified multiple times to ignore multiple symbols.
+
+.. option:: --keep-global-symbols <filename>
+
+ Make all symbols local in the output, except for symbols named in the file
+ ``<filename>``. In the file, each line represents a single symbol, with leading
+ and trailing whitespace ignored, as is anything following a '#'. Can be
+ specified multiple times to read names from multiple files.
+
+.. option:: --keep-section <section>
+
+ When removing sections from the output, do not remove sections named
+ ``<section>``. Can be specified multiple times to keep multiple sections.
+
+.. option:: --keep-symbol <symbol>, -K
+
+ When removing symbols from the output, do not remove symbols named
+ ``<symbol>``. Can be specified multiple times to keep multiple symbols.
+
+.. option:: --keep-symbols <filename>
+
+ When removing symbols from the output do not remove symbols named in the file
+ ``<filename>``. In the file, each line represents a single symbol, with leading
+ and trailing whitespace ignored, as is anything following a '#'. Can be
+ specified multiple times to read names from multiple files.
+
+.. option:: --localize-hidden
+
+ Make all symbols with hidden or internal visibility local in the output.
+
+.. option:: --localize-symbol <symbol>, -L
+
+ Mark any defined non-common symbol named ``<symbol>`` as a local symbol in the
+ output. Can be specified multiple times to mark multiple symbols as local.
+
+.. option:: --localize-symbols <filename>
+
+ Read a list of names from the file ``<filename>`` and mark defined non-common
+ symbols with those names as local in the output. In the file, each line
+ represents a single symbol, with leading and trailing whitespace ignored, as is
+ anything following a '#'. Can be specified multiple times to read names from
+ multiple files.
+
+.. option:: --output-target <format>, -O
+
+ Write the output as the specified format. See `SUPPORTED FORMATS`_ for a list
+ of valid ``<format>`` values. If unspecified, the output format is assumed to
+ be the same as the input file's format.
+
+.. option:: --prefix-alloc-sections <prefix>
+
+ Add ``<prefix>`` to the front of the names of all allocatable sections in the
+ output.
+
+.. option:: --prefix-symbols <prefix>
+
+ Add ``<prefix>`` to the front of every symbol name in the output.
+
+.. option:: --preserve-dates, -p
+
+ Preserve access and modification timestamps in the output.
+
+.. option:: --redefine-sym <old>=<new>
+
+ Rename symbols called ``<old>`` to ``<new>`` in the output. Can be specified
+ multiple times to rename multiple symbols.
+
+.. option:: --redefine-syms <filename>
+
+ Rename symbols in the output as described in the file ``<filename>``. In the
+ file, each line represents a single symbol to rename, with the old name and new
+ name separated by an equals sign. Leading and trailing whitespace is ignored,
+ as is anything following a '#'. Can be specified multiple times to read names
+ from multiple files.
+
+.. option:: --rename-section <old>=<new>[,<flag>,...]
+
+ Rename sections called ``<old>`` to ``<new>`` in the output, and apply any
+ specified ``<flag>`` values. See :option:`--set-section-flags` for a list of
+ supported flags. Can be specified multiple times to rename multiple sections.
+
+.. option:: --set-section-flags <section>=<flag>[,<flag>,...]
+
+ Set section properties in the output of section ``<section>`` based on the
+ specified ``<flag>`` values. Can be specified multiple times to update multiple
+ sections.
+
+ Following is a list of supported flags and their effects:
+
+ - `alloc` = add the `SHF_ALLOC` flag.
+ - `load` = if the section has `SHT_NOBITS` type, mark it as a `SHT_PROGBITS`
+   section.
+ - `readonly` = if this flag is not specified, add the `SHF_WRITE` flag.
+ - `code` = add the `SHF_EXECINSTR` flag.
+ - `merge` = add the `SHF_MERGE` flag.
+ - `strings` = add the `SHF_STRINGS` flag.
+ - `contents` = if the section has `SHT_NOBITS` type, mark it as a `SHT_PROGBITS`
+   section.
+
+ The following flags are also accepted, but are ignored for GNU compatibility:
+ `noload`, `debug`, `data`, `rom`, `share`.
+
+.. option:: --set-start-addr <addr>
+
+ Set the start address of the output to ``<addr>``. Overrides any previously
+ specified :option:`--change-start` or :option:`--adjust-start` options.
+
+.. option:: --split-dwo <dwo-file>
+
+ Equivalent to running :program:`llvm-objcopy` with :option:`--extract-dwo` and
+ ``<dwo-file>`` as the output file and no other options, and then with
+ :option:`--strip-dwo` on the input file.
+
+.. option:: --strip-dwo
+
+ Remove all DWARF .dwo sections from the output.
+
+.. option:: --strip-non-alloc
+
+ Remove from the output all non-allocatable sections that are not within
+ segments.
+
+.. option:: --strip-sections
+
+ Remove from the output all section headers and all section data not within
+ segments. Note that many tools will not be able to use an object without
+ section headers.
+
+.. option:: --target <format>, -F
+
+ Equivalent to :option:`--input-target` and :option:`--output-target` for the
+ specified format. See `SUPPORTED FORMATS`_ for a list of valid ``<format>``
+ values.
+
+.. option:: --weaken-symbol <symbol>, -W
+
+ Mark any global symbol named ``<symbol>`` as a weak symbol in the output. Can
+ be specified multiple times to mark multiple symbols as weak.
+
+.. option:: --weaken-symbols <filename>
+
+ Read a list of names from the file ``<filename>`` and mark global symbols with
+ those names as weak in the output. In the file, each line represents a single
+ symbol, with leading and trailing whitespace ignored, as is anything following
+ a '#'. Can be specified multiple times to read names from multiple files.
+
+.. option:: --weaken
+
+ Mark all defined global symbols as weak in the output.
+
+SUPPORTED FORMATS
+-----------------
+
+The following values are currently supported by :program:`llvm-objcopy` for the
+:option:`--input-target`, :option:`--output-target`, and :option:`--target`
+options. For GNU :program:`objcopy` compatibility, the values are all bfdnames.
+
+- `binary`
+- `ihex`
+- `elf32-i386`
+- `elf32-x86-64`
+- `elf64-x86-64`
+- `elf32-iamcu`
+- `elf32-littlearm`
+- `elf64-aarch64`
+- `elf64-littleaarch64`
+- `elf32-littleriscv`
+- `elf64-littleriscv`
+- `elf32-powerpc`
+- `elf32-powerpcle`
+- `elf64-powerpc`
+- `elf64-powerpcle`
+- `elf32-bigmips`
+- `elf32-ntradbigmips`
+- `elf32-ntradlittlemips`
+- `elf32-tradbigmips`
+- `elf32-tradlittlemips`
+- `elf64-tradbigmips`
+- `elf64-tradlittlemips`
+- `elf32-sparc`
+- `elf32-sparcel`
+
+Additionally, all targets except ``binary`` and ``ihex`` can have ``-freebsd``
+as a suffix.
+
+EXIT STATUS
+-----------
+
+:program:`llvm-objcopy` exits with a non-zero exit code if there is an error.
+Otherwise, it exits with code 0.
+
+BUGS
+----
+
+To report bugs, please visit <http://llvm.org/bugs/>.
+
+There is a known issue with :option:`--input-target` and :option:`--target`
+causing only ``binary`` and ``ihex`` formats to have any effect. Other values
+will be ignored and :program:`llvm-objcopy` will attempt to guess the input
+format.
+
+SEE ALSO
+--------
+
+:manpage:`llvm-strip(1)`