NAME

mold —

a modern linker

SYNOPSIS

mold [-options] objfile ...

DESCRIPTION

mold is a faster drop-in replacement for the default GNU ld(1).

How to use `mold`

See https://github.com/rui314/mold#how-to-use.

Compatibility

mold is designed to be a drop-in replacement for the GNU linkers for linking user-land programs. If your user-land program cannot be built due to missing command-line options, please file a bug at https://github.com/rui314/mold/issues.

mold supports a very limited set of linker script features, which is just sufficient to read /usr/lib/x86_64-linux-gnu/libc.so on Linux systems (on Linux, that file is despite its name not a shared library but an ASCII linker script that loads a real libc.so file.) Beyond that, we have no plan to support any linker script features. The linker script is an ad-hoc, over-designed, complex language which we believe needs to be disrupted by a simpler mechanism. We have a plan to add a replacement for the linker script to mold instead.

Archive symbol resolution

Traditionally, Unix linkers are sensitive to the order in which input files appear on command line. They process input files from the first (left-most) file to the last (right-most) file one-by-one. While reading input files, they maintain sets of defined and undefined symbols. When visiting an archive file (.a files), they pull out object files to resolve as many undefined symbols as possible and go on to the next input file. Object files that weren't pulled out will never have a chance for a second look.

Due to this semantics, you usually have to add archive files at the end of a command line, so that when a linker reaches archive files, it knows what symbols are remain undefined. If you put archive files at the beginning of a command line, a linker doesn't have any undefined symbol, and thus no object files will be pulled out from archives.

You can change the processing order by --start-group and --end-group options, though they make a linker slower.

mold as well as LLVM lld(1) linker take a different approach. They memorize what symbols can be resolved from archive files instead of forgetting it after processing each archive. Therefore, mold and lld(1) can "go back" in a command line to pull out object files from archives, if they are needed to resolve remaining undefined symbols. They are not sensitive to the input file order.

--start-group and --end-group are still accepted by mold and lld(1) for compatibility with traditional linkers, but they are silently ignored.

Dynamic symbol resolution

Some Unix linker features are unable to be understood without understanding the semantics of dynamic symbol resolution. Therefore, even though that's not specific to mold, we'll explain it here.

We use "ELF module" or just "module" as a collective term to refer an executable or a shared library file in the ELF format.

An ELF module may have lists of imported symbols and exported symbols, as well as a list of shared library names from which imported symbols should be imported. The point is that imported symbols are not bound to any specific shared library until runtime.

Here is how the Unix dynamic linker resolves dynamic symbols. Upon the start of an ELF program, the dynamic linker construct a list of ELF modules which as a whole consist of a complete program. The executable file is always at the beginning of the list followed by its depending shared libraries. An imported symbol is searched from the beginning of the list to the end. If two or more modules define the same symbol, the one that appears first in the list takes precedence over the others.

This Unix semantics are contrary to systems such as Windows that have the two-level namespace for dynamic symbols. On Windows, for example, dynamic symbols are represented as a tuple of (symbol-name, shared-library-name), so that each dynamic symbol is guaranteed to be resolved from some specific library.

Typically, an ELF module that exports a symbol also imports the same symbol. Such a symbol is usually resolved to itself, but that's not the case if a module that appears before in the symbol search list provides another definition of the same symbol.

Let me take malloc(3) as an example. Assume that you define your version of malloc(3) in your main executable file. Then, all malloc calls from any module are resolved to your function instead of that in libc, because the executable is always at the beginning of the dynamic symbol search list. Note that even malloc(3) calls within libc are resolved to your definition since libc exports and imports malloc. Therefore, by defining malloc yourself, you can overwrite a library function, and the malloc(3) in libc becomes dead code.

These Unix semantics are tricky and sometimes considered harmful. For example, assume that you accidentally define atoi(3) as a global function in your executable that behaves completely differently from the one in the C standard. Then, all atoi function calls from any modules (even function calls within libc) are redirected to your function instead of the one in libc which obviously causes a problem. That is a somewhat surprising consequence for an accidental name conflict. On the other hand, this semantic is sometimes considered useful because it allows users to overwrite library functions without recompiling modules containing them. Whether good or bad, you should keep this semantic in mind to understand the Unix linkers behaviors.

Build reproducibility

mold's output is deterministic. That is, if you pass the same object files and the same command-line options to the same version of mold, it is guaranteed to always produce the same output. The linker's internal randomness, such as the timing of thread scheduling or iteration orders of hash tables, doesn't affect the output.

mold does not have any host-specific default settings. This is contrary to the GNU linkers to which some configurable values, such as system-dependent library search paths, are hard-coded. mold depends only on its command-line arguments.

OPTIONS

--help

Report usage information to stdout and exit.

-v, --version

Report version information to stdout.

-V

Report version and target information to stdout.

-C dir, --directory dir

Change to dir before doing anything.

-E, --export-dynamic

--no-export-dynamic

When creating an executable, using the -E option causes all global symbols to be put into the dynamic symbol table, so that the symbols are visible from other ELF modules at runtime.

By default, or if --no-export-dynamic is given, only symbols that are referenced by DSOs at link-time are exported from an executable.

-F libname, --filter=libname

Set the DT_FILTER dynamic section field to libname.

-Ifile, --dynamic-linker=file

--no-dynamic-linker

Set the dynamic linker path to file. If no -I option is given, or if --no-dynamic-linker is given, no dynamic linker path is set to an output file. This is contrary to the GNU linkers which sets a default dynamic linker path in that case. However, this difference doesn't usually make any difference because the compiler driver always passes -I to a linker.

-Ldir, --library-path=dir

Add dir to the list of library search paths from which mold searches libraries for the -l option.

Unlike the GNU linkers, mold does not have the default search paths. This difference doesn't usually make any difference because the compiler driver always passes all necessary search paths to a linker.

-M, --print-map

Write a map file to stdout.

-N, --omagic

--no-omagic

Force mold to emit an output file with an old-fashioned memory layout. First, it makes the first data segment to not be aligned to a page boundary. Second, text segments are marked as writable if the option is given.

-S, --strip-debug

Omit .debug_* sections from the output file.

-T file, --script=file

Read linker script from file.

-X, --discard-locals

Discard temporary local symbols to reduce the sizes of the symbol table and the string table. Temporary local symbols are local symbols starting with .L. Compilers usually generate such symbols for unnamed program elements such as string literals or floating-point literals.

-e symbol, --entry=symbol

Use symbol as the entry point symbol instead of the default entry point symbol _start.

-f shlib, --auxiliary=shlib

Set the DT_AUXILIARY dynamic section field to shlib.

-h libname, --soname=libname

Set the DT_SONAME dynamic section field to libname. This option is used when creating a shared object file. Typically, when you create SyXXX lib foo.so, you want to pass --soname=foo to a linker.

-llibname

Search for Sylib libname.so or Sylib libname.a from library search paths.

-m [elf_x86_64 | elf_i386 | aarch64linux]

Choose a target.

-o file, --output=file

Use file as the output file name instead of the default name a.out.

-r, --relocatable

Instead of generating an executable or a shared object file, combine input object files to generate another object file that can be used as an input to a linker.

-s, --strip-all

Omit .symtab section from the output file.

-u symbol, --undefined=symbol

If symbol remains as an undefined symbol after reading all object files, and if there is an static archive that contains an object file defining symbol, pull out the object file and link it so that the output file contains a definition of symbol.

--Bdynamic

Link against shared libraries.

--Bstatic

Do not link against shared libraries.

--Bsymbolic

When creating a shared library, make global symbols export-only (i.e. do not import the same symbol). As a result, references within a shared library is always resolved locally, negating symbol override at runtime. See Dynamic symbol resolution for more information about symbol imports and exports.

--Bsymbolic-functions

Have the same effect as --Bsymbolic but works only for function symbols. Data symbols remains being both imported and exported.

--Bno-symbolic

Cancel --Bsymbolic and --Bsymbolic-functions.

--Map=file

Write map file to file.

--allow-multiple-definition

Normally, the linker reports an error if there are more than one definition of a symbol. This option changes the default behavior so that it doesn't report an error for duplicate definitions and instead use the first definition.

--as-needed

--no-as-needed

By default, shared libraries given to a linker are unconditionally added to the list of required libraries in an output file. However, shared libraries after --as-needed are added to the list only when at least one symbol is actually used by an object file. In other words, shared libraries after --as-needed are not added to the list of needed libraries if they are not needed by a program.

The --no-as-needed option restores the default behavior for subsequent files.

--build-id

--build-id=[none | md5 | sha1 | sha256 | uuid | 0xhexstring]

--no-build-id

Create a .note.gnu.build-id section containing a byte string to uniquely identify an output file. --build-id and --build-id=sha256 compute a 256-bit cryptographic hash of an output file and set it to build-id. md5 and sha1 compute the same hash but truncate it to 128 and 160 bits, respectively, before setting it to build-id. uuid sets a random 128-bit UUID. 0xhexstring sets hexstring.

--chroot=dir

Set dir to root directory.

--compress-debug-sections=[none | zlib | zlib-gabi | zlib-gnu]

Compress DWARF debug info (.debug_* sections) using the zlib compression algorithm.

--defsym=symbol=value

Define symbol as an alias for value.

value is either an integer (in decimal or hexadecimal with ‘0x’ prefix) or a symbol name. If an integer is given as a value, symbol is defined as an absolute symbol with the given value.

--default-symver

Use soname as a symbol version and append that version to all symbols.

--demangle

--no-demangle

Demangle C++ symbols in log messages.

--dependency-file=file

Write a dependency file to file. The contents of the written file is readable by make, which defines only one rule with the linker's output file as a target and all input fiels as its prerequisite. Users are expected to include the generated dependency file into a Makefile to automate the dependency management. This option is analogous to the compiler's -MM -MF options.

--dynamic-list=file

Read a list of dynamic symbols from file.

--eh-frame-hdr

--no-eh-frame-hdr

Create .eh_frame_hdr section.

--emit-relocs

A linker usually "consumes" relocation sections. That is, a linker applies relocations to other sections, and relocation sections themselves are discarded.

The --emit-relocs instructs the linker to leave relocation sections in the output file. Some post-link binary analysis or optimization tools such as LLVM Bolt need them.

mold always creates RELA-type relocation sections even if the native ELF format is REL-type so that it is easy to read addends.

--exclude-libs=libraries...

Mark all symbols in the given libraries hidden.

--fini=symbol

Call symbol at unload-time.

--fork

--no-fork

Spawn a child process and let it do the actual linking. When linking a large program, the OS kernel can take a few hundred milliseconds to terminate a mold process. --fork hides that latency.

--gc-sections

--no-gc-sections

Remove unreferenced sections.

--hash-style=[sysv | gnu | both]

Set hash style.

--icf=all

--no-icf

Fold identical code.

--image-base=addr

Set the base address to addr.

--init=symbol

Call symbol at load-time.

--no-undefined

Report undefined symbols (even with --shared).

--pack-dyn-relocs=[none | relr]

If relr is specified, all R_*_RELATIVE relocations are put into .relr.dyn section instead of .rel.dyn or .rela.dyn section. Since .relr.dyn section uses a space-efficient encoding scheme, specifying this flag can reduce the size of the output. This is typically most effective for position-independent executable.

Note that a runtime loader has to support .relr.dyn to run executables or shared libraries linked with --pack-dyn-relocs=relr, and only ChromeOS, Android and Fuchsia support it as of now in 2022.

--perf

Print performance statistics.

--pie, --pic-executable

--no-pie, --no-pic-executable

Create a position-independent executable.

--preload

Preload object files.

--print-gc-sections

--no-print-gc-sections

Print removed unreferenced sections.

--print-icf-sections

--no-print-icf-sections

Print folded identical sections.

--push-state

--pop-state

--push-state saves the current values of --as-needed, --whole-archive, --static, and --start-lib. The saved values can be restored by --pop-state.

--push-state and --pop-state pairs can nest.

These options are useful when you want to construct linker command line options programmatically. For example, if you want to link libfoo.so by as-needed basis but don't want to change the global state of --as-needed, you can append "--push-state --as-needed -lfoo --pop-state" to the linker command line options.

--quick-exit

--no-quick-exit

Use quick_exit to exit.

--relax

--no-relax

Rewrite machine instructions with more efficient ones for some relocations. The feature is enabled by default.

--require-defined=symbol

Like --undefined, except the new symbol must be defined by the end of the link.

--repro

Embed input files into .repro section.

--retain-symbols-file=file

Keep only symbols listed in file.

file is a text file containing a symbol name on each line. mold discards all local symbols as well as global sybmol that are not in file. Note that this option removes symbols only from .symtab section and does not affect .dynsym section, which is used for dynamic linking.

--reverse-sections

Reverses the order of input sections before assigning them the offsets in the output file.

--rpath=dir

Add dir to runtime search path.

--run command arg file ...

Run command with mold as /usr/bin/ld.

--shared, --Bshareable

Create a share library.

--shuffle-sections

--shuffle-sections=number

Randomizes the output by shuffleing the order of input sections before assigning them the offsets in the output file. If number is given, it's used as a seed for the random number generator, so that the linker produces the same output as for the same seed. If no seed is given, it uses a random number as a seed.

--spare-dynamic-tags=number

Reserve given number of tags in .dynamic section.

--start-lib

--end-lib

Handle object files between --start-lib and --end-lib as if they were in an archive file. That means object files between them are linked only when they are needed to resolve undefined symbols. The options are useful if you want to link object files only when they are needed but want to avoid the overhead of running ar(3).

--static

Do not link against shared libraries.

--stats

Print input statistics.

--sysroot=dir

Set target system root directory to dir.

--thread-count=count

Use count number of threads.

--threads

--no-threads

Use multiple threads. By default, mold uses as many threads as the number of cores or 32, whichever is the smallest. The reason why it is capped to 32 is because mold doesn't scale well beyond that point. To use only one thread, pass --no-threads or --thread-count=1.

--trace

Print name of each input file.

--unique=pattern

Don't merge input sections that match pattern.

--unresolved-symbols=[report-all | ignore-all | ignore-in-object-files | ignore-in-shared-libs]

How to handle undefined symbols.

--version-script=file

Read version script from file.

--warn-common

--no-warn-common

Warn about common symbols.

--warn-once

Only warn once for each undefined symbol instead of warn for each relocation referring an undefined symbol.

--warn-unresolved-symbols

--error-unresolved-symbols

Normally, the linker reports an error for unresolved symbols. --warn-unresolved-symbols option turns it into a warning. --error-unresolved-symbols option restores the default behavior.

--whole-archive

--no-whole-archive

When archive files (.a files) are given to a linker, only object files that are needed to resolve undefined symbols are extracted from them and linked to an output file. --whole-archive changes that behavior for subsequent archives so that a linker extracts all object files and link them to an output. For example, if you are creating a shared object file and you want to include all archive members to the output, you should pass --whole-archive. --no-whole-archive restores the default behavior for subsequent archives.

--wrap=symbol

Make symbol to be resolved to __wrap_symbol. The original symbol can be resolved as __real_symbol. This option is typically used for wrapping an existing function.

-z cet-report=[none | warning | error]

Intel Control-flow Enforcement Technology (CET) is a new x86 feature available since Tiger Lake which is released in 2020. It defines new instructions to harden security to protect programs from control hijacking attacks. You can tell compiler to use the feature by specifying the -fcf-protection flag.

-z cet-report flag is used to make sure that all object files were compiled with a correct -fcf-protection flag. If warning or error are given, mold prints out a warning or an error message if an object file was not compiled with the compiler flag.

mold looks for GNU_PROPERTY_X86_FEATURE_1_IBT bit and GNU_PROPERTY_X86_FEATURE_1_SHSTK bit in .note.gnu.property section to determine whether or not an object file was compiled with -fcf-protection.

-z now

-z lazy

By default, functions referring other ELF modules are resolved by the dynamic linker when they are called for the first time. -z now marks an executable or a shared library file so that all dynamic symbols are loaded when a file is loaded to memory. -z lazy restores the default behavior.

-z origin

Mark object requiring immediate $ORIGIN processing at runtime.

-z ibt

Turn on GNU_PROPERTY_X86_FEATURE_1_IBT bit in .note.gnu.property section to indicate that the output uses IBT-enabled PLT. This option implies -z ibtplt.

-z ibtplt

Generate Intel Branch Tracking (IBT)-enabled PLT.

IBT is part of Intel Control-flow Enforcement Technology (CET). IBT is a new x86 feature available since Tiger Lake which is released in 2020. If IBT is enabled, all indirect branch instructions have to branch to a so-called "landing pad" instruction. Landing pad itself is a no-op, but it works as a marker that branching to that instruction is expected. If there's no landing pad after branch, the CPU raises an exception. This mechanism makes ROP attacks difficult.

Since PLT can be used as an indirect branch target, we need a different instruction sequence for IBT-enabled PLT. If -z ibtplt is specified, mold generates PLT entries that start with a landing pad. The size of IBT-enabled PLT is 24 bytes as opposed to 16 bytes regular PLT.

-z execstack

-z noexecstack

By default, the pages for the stack area (i.e. the pages where local variables reside) are not executable for security reasons. -z execstack makes it executable. -z noexecstack restores the default behavior.

-z keep-text-section-prefix

-z nokeep-text-section-prefix

Keep .text.hot, .text.unknown, .text.unlikely, .text.startup and .text.exit as separate sections in the final binary.

-z relro

-z norelro

Some sections such as .dynamic have to be writable only during an executable or a shared library file is being loaded to memory. Once the dynamic linker finishes its job, such sections won't be mutated by anyone. As a security mitigation, it is preferred to make such segments read-only during program execution.

-z relro puts such sections into a special section called relro. The dynamic linker make a relro segment read-only after it finishes its job.

By default, mold generates a relro segment. -z norelro disables the feature.

-z separate-loadable-segments

-z separate-code

-z noseparate-code

If one memory page contains multiple segments, the page protection bits are set in such a way that needed attributes (writable or executable) are satisifed for all segments. This usually happens at a boundary of two segments with two different attributes.

separate-loadable-segments adds paddings between segments with different attributes so that they do not share the same page. This is the default.

separate-code adds paddings only between executable and non-executable segments.

noseparate-code does not add any paddings between segments.

-z defs

-z nodefs

Report undefined symbols (even with --shared).

-z shstk

Enforce shadow stack by turning GNU_PROPERTY_X86_FEATURE_1_SHSTK bit in .note.gnu.property output section. Shadow stack is part of Intel Control-flow Enforcement Technology (CET), which is available since Tiger Lake (2020).

-z text

-z notext, -z textoff

mold by default reports an error if dynamic relocations are created in read-only sections. If -z notext or -z textoff are given, mold creates such dynamic relocations without reporting an error. -z text restores the default behavior.

-z max-page-size

Some CPU ISAs support multiple different memory page sizes. This option specifies the maximum page size that an output binary can run on. If you specify a large value, the output can run on both large and small page systems, but it wastes a bit of memory at page boundaries on systems with small pages.

The default value is 4 KiB for i386, x86-64 and RISC-V, and 64 KiB for ARM64.

-z nodefaultlib