mold
[option...] file...
mold
is a faster drop-in replacement for the default GNU ld(1).
See https://github.com/rui314/mold#how-to-use.
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 contrary to 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 additional linker script features.
The linker script is an ad-hoc, over-designed, complex language which we
believe needs to be replaced by a simpler mechanism. We have a plan to add a
replacement for the linker script to mold
instead.
Traditionally, Unix linkers are sensitive to the order in which input files
appear on the command line. They process input files from the first (leftmost)
file to the last (rightmost) 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 move 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 behavior, 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 remain as undefined.
If you put archive files at the beginning of a command line, a linker doesn't
have any undefined symbols, and thus no object files will be pulled out from
archives. You can change the processing order by using the --start-group
and
--end-group
options, though they make a linker slower.
mold
, as well as the LLVM lld(1) linker, takes a different approach. They
remember which symbols can be resolved from archive files instead of
forgetting them 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.
Some Unix linker features are difficult to understand without comprehending
the semantics of dynamic symbol resolution. Therefore, even though it's not
specific to mold
, we'll explain it here.
We use "ELF module" or just "module" as a collective term to refer to 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 constructs 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 dependent 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 a
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 it in the symbol search list provides another definition of the same symbol.
Let's take malloc
as an example. Assume that you define your version of
malloc
in your main executable file. Then, all malloc
calls from any
module are resolved to your function instead of the one in libc, because the
executable is always at the beginning of the dynamic symbol search list. Note
that even malloc
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
in libc becomes dead
code.
These Unix semantics are tricky and sometimes considered harmful. For example,
assume that you accidentally define atoi
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 will
very likely cause a problem.
That is a somewhat surprising consequence for an accidental name conflict. On the other hand, this semantic is sometimes useful because it allows users to override library functions without rebuilding modules containing them.
Whether good or bad, you should keep these semantics in mind to understand Unix linkers' behaviors.
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 that mold
produces the bit-by-bit identical 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, for which some configurable values, such as system-dependent
library search paths, are hard-coded. mold
depends only on its command-line
arguments.
Multi-letter long options may precede either a single dash or double dashes, except for those starting with the letter "o". For historical reasons, long options beginning with "o" must precede double dashes.
For example, you can spell --as-needed
as -as-needed
, but --omagic
must
not be spelled as -omagic
. -omagic
will be interpreted not as --omagic
but as -o magic
.
-
--chroot
=dir: Set dir as the root directory. -
--color-diagnostics
=[ auto | always | never ]: Show diagnostic messages in color using ANSI escape sequences.auto
means thatmold
prints out messages in color only if the standard output is connected to a TTY. Default isauto
. -
--color-diagnostics
: Synonym for--color-diagnostics=auto
. -
--no-color-diagnostics
: Synonym for--color-diagnostics=never
. -
--detach
, `--no-detach: Permit or do not permit mold to create a debug info file in the background. -
--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 amold
process.--fork
hides that latency. By default, it does fork. -
--perf
: Print performance statistics. -
--print-dependencies
: Print out dependency information for input files.Each line of the output for this option shows which file depends on which file to use a specific symbol. This option is useful for debugging why some object file in a static archive got linked or why some shared library is kept in an output file's dependency list even with
--as-needed
. -
--relocatable-merge-sections
: By default,mold
doesn't merge input sections by name when merging input object files into a single output object file for-r
. For example,.text.foo
and.text.bar
aren't merged for-r
even though they are merged into.text
based on the default section merging rules.This option changes the behavior so that
mold
merges input sections by name by the default section merging rules. -
--repro
: Archive input files, as well as a text file containing command line options, in a tar file so that you can runmold
with the exact same inputs again. This is useful for reporting a bug with a reproducer. The output filename ispath/to/output.tar
, wherepath/to/output
is an output filename specified by-o
. -
--reverse-sections
: Reverse the order of input sections before assigning them the offsets in the output file.This option is useful for finding bugs that depend on the initialization order of global objects. In C++, constructors of global objects in a single source file are guaranteed to be executed in the source order, but there's no such guarantee across compilation units. Usually, constructors are executed in the order given to the linker, but depending on it is a mistake.
By reversing the order of input sections using
--reverse-sections
, you can easily test that your program works in the reversed initialization order. -
--run
command arg...: Run command withmold
as/usr/bin/ld
. Specifically,mold
runs a given command with theLD_PRELOAD
environment set to intercept exec(3) family functions and replacesargv[0]
with itself if it isld
,ld.gold
, orld.lld
. -
--separate-debug-file
,--separate-debug-file
=file: Bundle debug info sections into a separate file instead of embedding them in an output executable or a shared library. mold creates a debug info file in the background by default, so that you can start running your executable as soon as possible.By default, the debug info file is created in the same directory as is the output file, with the
.dbg
file extension. That filename is embedded into the output file so thatgdb
can automatically find the debug info file for the output file. For more info about gdb features related to separate debug files, see https://sourceware.org/gdb/current/onlinedocs/gdb.html/Separate-Debug-Files.html.mold holds a file lock with flock(2) while creating a debug info file in the background.
If you don't want to create a debug info file in the background, pass the
--no-detach
option. -
--shuffle-sections
,--shuffle-sections
=number: Randomize the output by shuffling the order of input sections before assigning them the offsets in the output file. If a number is given, it's used as a seed for the random number generator, so that the linker produces the same output for the same seed. If no seed is given, a random number is used as a seed.This option is useful for benchmarking. Modern CPUs are sensitive to a program's memory layout. A seemingly benign change in program layout, such as a small size increase of a function in the middle of a program, can affect the program's performance. Therefore, even if you write new code and get a good benchmark result, it is hard to say whether the new code improves the program's performance; it is possible that the new memory layout happens to perform better.
By running a benchmark multiple times with randomized memory layouts using
--shuffle-sections
, you can isolate your program's real performance number from the randomness caused by memory layout changes. -
--spare-program-headers
=number: Append the given number ofPT_NULL
entries to the end of the program header, so that post-link processing tools can easily add new segments by overwriting the null entries.Note that ELF requires all
PT_LOAD
segments to be sorted byp_vaddr
. Therefore, if you add a new LOAD segment, you may need to sort the entire program header. -
--stats
: Print input statistics. -
--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 smaller. The reason it is capped at 32 is becausemold
doesn't scale well beyond that point. To use only one thread, pass--no-threads
or--thread-count=1
. -
--quick-exit
,--no-quick-exit
: Use or do not usequick_exit
to exit. -
-z rewrite-endbr
,-z norewrite-endbr
: As a security measure, some CPU instruction sets have recently gained a feature to protect control flow integrity by disallowing indirect branches by default. If the feature is enabled, the instruction that is executed immediately after an indirect branch must be an branch target marker instruction, or a CPU-level fault will raise. The marker instruction is also known as "landing pad" instruction, to which indirect branches can land. This feature makes ROP attacks harder to conduct.To use the feature, a function whose pointer is taken needs to begin with a landing pad because a function call via a function pointer is compiled to an indirect branch. On the other hand, if a function is called only directly (i.e. referred to only by direct branch instructions), it doesn't have to begin with it.
By default, the compiler always emits a landing pad at the beginning of each global function because it doesn't know whether or not the function's pointer is taken in another translation unit. As a result, the resulting binary has more attack surface than necessary.
If
--rewrite-endbr
is given, mold conducts a whole program analysis to identify functions whose addresses are actually taken and rewrites landing pads with no-ops for non-address-taken functions, reducing the attack surface.This feature is currently available only on x86-64.
-
--help
: Report usage information to stdout and exit. -
-v
,--version
: Report version information to stdout. -
-V
: Report version and target information to stdout. -
-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 theDT_FILTER
dynamic section field to libname. -
-I
file,--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 set a default dynamic linker path in that case. This difference doesn't usually make any difference because the compiler driver always passes-I
to the linker. -
-L
dir,--library-path
=dir: Add dir to the list of library search paths from whichmold
searches libraries for the-l
option.Unlike the GNU linkers,
mold
does not have default search paths. This difference doesn't usually make any difference because the compiler driver always passes all necessary search paths to the linker. -
-M
,--print-map
: Write a map file to stdout. -
-N
,--omagic
,--no-omagic
: Forcemold
to emit an output file with an old-fashioned memory layout. First, it makes the first data segment not 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 theDT_AUXILIARY
dynamic section field to shlib. -
-h
libname,--soname
=libname: Set theDT_SONAME
dynamic section field to libname. This option is used when creating a shared object file. Typically, when you createlibfoo.so
, you want to pass--soname=foo
to a linker. -
-l
libname: Search forlib
libname.so
orlib
libname.a
from library search paths. -
-m
target: Choose a target. -
-o
file,--output
=file: Use file as the output file name instead of the default namea.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 a 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. -
-y
symbol,--trace-symbol
=symbol: Trace references to 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 are always resolved locally, negating symbol override at runtime. See "Dynamic symbol resolution" for more information about symbol imports and exports. -
--Bsymbolic-functions
: This option has the same effect as--Bsymbolic
but works only for function symbols. Data symbols remain being both imported and exported. -
--Bsymbolic-non-weak
: This option has the same effect as--Bsymbolic
but works only for non-weak symbols. Weak symbols remain being both imported and exported. -
--Bsymbolic-non-weak-functions
: This option has the same effect as--Bsymbolic
but works only for non-weak function symbols. Data symbols and weak function symbols remain being both imported and exported. -
--Bno-symbolic
: Cancel--Bsymbolic
,--Bsymbolic-functions
,--Bsymbolic-non-weak
and--Bsymbolic-non-weak-functions
. -
--Map
=file: Write map file to file. -
--Tbss
=address: Alias for--section-start=.bss=
address. -
--Tdata
=address: Alias for--section-start=.data=
address. -
--Ttext
=address: Alias for--section-start=.text=
address. -
--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. -
--allow-shlib-undefined
,--no-allow-shlib-undefined
: Even if mold succeeds in linking a main executable without undefined symbol errors, you may still encounter symbol lookup errors at runtime because the dynamic linker cannot find some symbols in shared libraries in any ELF module. This occurs because mold ignores undefined symbols in shared libraries by default.If you pass
--no-allow-shlib-undefined
, mold verifies that undefined symbols in shared libraries given to the linker can be resolved at link-time. In other words, this converts the runtime error to a link-time error.Note that you need to pass all shared libraries, including indirectly dependent ones, to the linker as arguments for
-l
. If a shared library depends on a library that's not passed to the linker, the verification will be skipped for that file. -
--as-needed
,--no-as-needed
: By default, shared libraries given to the 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 the output 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
=[md5
|sha1
|sha256
|fast
|uuid
|0x
hexstring |none
]: Create a.note.gnu.build-id
section containing a byte string to uniquely identify an output file.sha256
compute a 256-bit cryptographic hash of an output file and set it to build-id.md5
andsha1
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.0x
hexstring sets hexstring.fast
is a synonym forsha256
. -
--build-id
: Synonym for--build-id=sha256
. -
--no-build-id
: Synonym for--build-id=none
. -
--compress-debug-sections
=[zlib
|zlib-gabi
|zstd
|none
]: Compress DWARF debug info (.debug_*
sections) using the zlib or zstd compression algorithm.zlib-gabi
is an alias forzlib
. -
--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++ and Rust symbols in log messages. -
--dependency-file
=file: Write a dependency file to file. The contents of the written file is readable by make(1), which defines only one rule with the linker's output file as a target and all input files as its prerequisites. 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. Same as--export-dynamic-symbol-list
, except that it implies--Bsymbolic
. If file does not exist in the current directory, it is searched from library search paths for the sake of compatibility with GNU ld. -
--eh-frame-hdr
,--no-eh-frame-hdr
: Create.eh_frame_hdr
section. -
--emit-relocs
: The linker usually "consumes" relocation sections. That is, the 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. -
--enable-new-dtags
,--disable-new-dtags
: By default,mold
emitsDT_RUNPATH
for--rpath
. If you pass--disable-new-dtags
,mold
emitsDT_RPATH
for--rpath
instead. -
--execute-only
:Traditionally, setting the executable bit to 1 for a memory page implies that the page also become readable, which allows machine code to be read as data at runtime. That is actually what an attacker often does after gaining a limited control of a process to find pieces of machine code they can use to gain the full control of the process. As a mitigation, recent processors including some ARM64 ones allows "execute-only" pages. If a page is execute-only, you can call a function there as long as you know its address but can't read it as data.
This option marks text segments as execute-only by setting just the "X" bit instead of "RX". Note that on most systems, the absence of the "R" bit in the text segment serves just as a hint. If you run a program linked with
--execute-only
on a processor that doesn't support execute-only pages, your executable will likely still function normally, but the text segment will remain readable. -
--exclude-libs
=libraries ...: Mark all symbols in the given libraries hidden. -
--export-dynamic-symbol
=symbol: Put symbols matching symbol in the dynamic symbol table. symbol may be a glob pattern in the same syntax as for the--export-dynamic-symbol-list
or--version-script
options. -
--export-dynamic-symbol-list
=file: Read a list of dynamic symbols from file. -
--fatal-warnings
,--no-fatal-warnings
: Treat warnings as errors. -
--fini
=symbol: Call symbol at unload-time. -
--gc-sections
,--no-gc-sections
: Remove unreferenced sections. -
--gdb-index
: Create a.gdb_index
section to speed up GNU debugger. To use this, you need to compile source files with the-ggnu-pubnames
compiler flag. -
--hash-style
=[sysv
|gnu
|both
|none
]: Set hash style. -
--icf
=[safe
|all
|none
],--no-icf
: It is not uncommon for a program to contain many identical functions that differ only in name. For example, a C++ templatestd::vector
is very likely to be instantiated to the identical code forstd::vector<int>
andstd::vector<unsigned>
because the container cares only about the size of the parameter type. Identical Code Folding (ICF) is a size optimization to identify and merge such identical functions.If
--icf=all
is given,mold
tries to merge all identical functions. This reduces the size of the output most, but it is not a "safe" optimization. It is guaranteed in C and C++ that two pointers pointing two different functions will never be equal, but--icf=all
breaks that assumption as two identical functions have the same address after merging. So a care must be taken when you use this flag that your program does not depend on the function pointer uniqueness.--icf=safe
is a flag to merge functions only when it is safe to do so. That is, if a program does not take an address of a function, it is safe to merge that function with other function, as you cannot compare a function pointer with something else without taking an address of a function.--icf=safe
needs to be used with a compiler that supports.llvm_addrsig
section which contains the information as to what symbols are address-taken. LLVM/Clang supports that section by default. Since GCC does not support it yet, you cannot use--icf=safe
with GCC (it doesn't do any harm but can't optimize at all.)--icf=none
and--no-icf
disables ICF. -
--ignore-data-address-equality
: Make ICF to merge not only functions but also data. This option should be used in combination with--icf=all
. -
--image-base
=addr: Set the base address to addr. -
--init
=symbol: Call symbol at load-time. -
--no-undefined
: Report undefined symbols (even with--shared
). -
--noinhibit-exec
: Create an output file even if errors occur. -
--pack-dyn-relocs
=[relr
|none
]: Ifrelr
is specified, allR_*_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
. As of 2022, only ChromeOS, Android and Fuchsia support it. -
--pie
,--pic-executable
,--no-pie
,--no-pic-executable
: Create a position-independent executable. -
--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. -
--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. -
--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 symbol 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. -
--rpath
=dir: Add dir to runtime search paths. -
--section-start
=section=address: Set address to section. address is a hexadecimal number that may start with an optional0x
. -
--shared
,--Bshareable
: Create a share library. -
--spare-dynamic-tags
=number: Append the given number ofDT_NULL
entries to the end of the.dynamic
section, so that post-link processing tools can easily add new dynamic tags by overwriting the null entries. -
--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. -
--sysroot
=dir: Set target system root directory to dir. -
--trace
: Print name of each input file. -
--undefined-glob
=pattern: Synonym for--undefined
, except that--undefined-glob
takes a glob pattern instead of just a single symbol name. -
--undefined-version
,--no-undefined-version
: By default,mold
warns on a symbol specified by a version script or by--export-dynamic-symbol
if it is not defined. You can silence the warning by--undefined-version
. -
--unique
=pattern: Don't merge input sections that match the given glob pattern 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. If file does not exist in the current directory, it is searched from library search paths for the sake of compatibility with GNU ld. -
--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 the 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 the linker extracts all object files and links 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 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
=[warning
|error
|none
]: 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 the 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. Ifwarning
orerror
are given,mold
prints out a warning or an error message if an object file was not compiled with the compiler flag.mold
looks forGNU_PROPERTY_X86_FEATURE_1_IBT
bit andGNU_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 to 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 resolved 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 onGNU_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 which is the default on x86-64. This is the default. -
-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 instead of merging them as.text
. -
-z rodynamic
: Make the.dynamic
section read-only. -
-z relro
,-z norelro
: Some sections such as.dynamic
have to be writable only during a module 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 segment calledrelro
. The dynamic linker makes a relro segment read-only after it finishes its job.By default,
mold
generates a relro segment.-z norelro
disables the feature. -
-z sectionheader
,-z nosectionheader
:-z nosectionheader
tell the linker to omit the section header. By default, the linker does not omit the section header. -
-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 the needed attributes (writable or executable) are satisfied 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 turningGNU_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 start_stop_visibility
=[hidden
|protected
]: If a section name is valid as a C identifier (i.e., it matches/^[_a-zA-Z][_a-zA-Z0-9]*$/
), mold creates__start_SECNAME
and__stop_SECNAME
symbols to mark the beginning and end of the section, whereSECNAME
is the section name.You can make these marker symbols visible from other ELF modules by passing
-z start_stop_visibility=protected
. Default ishidden
. -
-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
=number: Some CPU ISAs support multiple memory page sizes. This option specifies the maximum page size that an output binary can run on. In general, binaries built for a larger page size can run on a system with a smaller page size, but not vice versa. The default value is 4 KiB for i386, x86-64, and RISC-V, and 64 KiB for ARM64. -
-z nodefaultlib
: Make the dynamic loader ignore default search paths. -
-z nodelete
: Mark DSO non-deletable at runtime. -
-z nodlopen
: Mark DSO not available to dlopen(3). This option makes it possible for the linker to optimize thread-local variable accesses by rewriting instructions for some targets. -
-z nodump
: Mark DSO not available to dldump(3). -
-z nocopyreloc
: Do not create copy relocations. -
-z initfirst
: Mark DSO to be initialized first at runtime. -
-z interpose
: Mark object to interpose all DSOs but executable. -
-(
,-)
,-EL
,-O
number,--dc
,--dp
,--end-group
,--no-add-needed
,--no-copy-dt-needed-entries
,--nostdlib
,--rpath-link=Ar dir
,--sort-common
,--sort-section
,--start-group
,--warn-constructors
,--warn-once
,--fix-cortex-a53-835769
,--fix-cortex-a53-843419
,-z combreloc
,-z common-page-size
,-z nocombreloc
: Ignored
-
MOLD_JOBS
: If this variable is set to1
, only onemold
process will run at a time. If a new mold process is initiated while another is already active, the new process will wait until the active one completes before starting.The primary reason for this environment variable is to minimize peak memory usage. Since mold is designed to operate with high parallelism, running multiple mold instances simultaneously may not be beneficial. If you execute N instances of mold concurrently, it could require N times the time and N times the memory. On the other hand, running them one after the other might still take N times longer, but the peak memory usage would be the same as running just a single instance.
If your build system invokes multiple linker processes simultaneously and some of them often get killed due to out-of-memory errors, you might consider setting this environment variable to
1
to see if it addresses the OOM issue.Currently, any value other than
1
is silently ignored. -
MOLD_DEBUG
: If this variable is set to a non-empty string,mold
embeds its command-line options in the output file's.comment
section. -
MOLD_REPRO
: Setting this variable to a non-empty string has the same effect as passing the--repro
option.
gold(1), ld(1), elf(5), ld.so(8)
Rui Ueyama ruiu@cs.stanford.edu
Report bugs to https://github.com/rui314/mold/issues.