dpkg-buildflags(1) dpkg suite dpkg-buildflags(1)
NAME
dpkg-buildflags - returns build flags to use during package build
SYNOPSIS
dpkg-buildflags [option...] [command]
DESCRIPTION
dpkg-buildflags is a tool to retrieve compilation flags to use during
build of Debian packages.
The default flags are defined by the vendor but they can be
extended/overridden in several ways:
1. system-wide with /etc/dpkg/buildflags.conf;
2. for the current user with $XDG_CONFIG_HOME/dpkg/buildflags.conf
where $XDG_CONFIG_HOME defaults to $HOME/.config;
3. temporarily by the user with environment variables (see section
ENVIRONMENT);
4. dynamically by the package maintainer with environment variables
set via debian/rules (see section ENVIRONMENT).
The configuration files can contain four types of directives:
SET flag value
Override the flag named flag to have the value value.
STRIP flag value
Strip from the flag named flag all the build flags listed in value.
APPEND flag value
Extend the flag named flag by appending the options given in value.
A space is prepended to the appended value if the flag's current
value is non-empty.
PREPEND flag value
Extend the flag named flag by prepending the options given in
value. A space is appended to the prepended value if the flag's
current value is non-empty.
The configuration files can contain comments on lines starting with a
hash (#). Empty lines are also ignored.
COMMANDS
--dump
Print to standard output all compilation flags and their values. It
prints one flag per line separated from its value by an equal sign
(“flag=value”). This is the default action.
--list
Print the list of flags supported by the current vendor (one per
line). See the SUPPORTED FLAGS section for more information about
them.
--status
Display any information that can be useful to explain the behaviour
of dpkg-buildflags (since dpkg 1.16.5): relevant environment
variables, current vendor, state of all feature flags. Also print
the resulting compiler flags with their origin.
This is intended to be run from debian/rules, so that the build log
keeps a clear trace of the build flags used. This can be useful to
diagnose problems related to them.
--export=format
Print to standard output commands that can be used to export all
the compilation flags for some particular tool. If the format value
is not given, sh is assumed. Only compilation flags starting with
an upper case character are included, others are assumed to not be
suitable for the environment. Supported formats:
sh Shell commands to set and export all the compilation flags in
the environment. The flag values are quoted so the output is
ready for evaluation by a shell.
cmdline
Arguments to pass to a build program's command line to use all
the compilation flags (since dpkg 1.17.0). The flag values are
quoted in shell syntax.
configure
This is a legacy alias for cmdline.
make
Make directives to set and export all the compilation flags in
the environment. Output can be written to a Makefile fragment
and evaluated using an include directive.
--get flag
Print the value of the flag on standard output. Exits with 0 if the
flag is known otherwise exits with 1.
--origin flag
Print the origin of the value that is returned by --get. Exits with
0 if the flag is known otherwise exits with 1. The origin can be
one of the following values:
vendor
the original flag set by the vendor is returned;
system
the flag is set/modified by a system-wide configuration;
user
the flag is set/modified by a user-specific configuration;
env the flag is set/modified by an environment-specific
configuration.
--query
Print any information that can be useful to explain the behaviour
of the program: current vendor, relevant environment variables,
feature areas, state of all feature flags, whether a feature is
handled as a builtin default by the compiler (since dpkg 1.21.14),
and the compiler flags with their origin (since dpkg 1.19.0).
For example:
Vendor: Debian
Environment:
DEB_CFLAGS_SET=-O0 -Wall
Area: qa
Features:
bug=no
canary=no
Builtins:
Area: hardening
Features:
pie=no
Builtins:
pie=yes
Area: reproducible
Features:
timeless=no
Builtins:
Flag: CFLAGS
Value: -O0 -Wall
Origin: env
Flag: CPPFLAGS
Value: -D_FORTIFY_SOURCE=2
Origin: vendor
--query-features area
Print the features enabled for a given area (since dpkg 1.16.2).
If the feature is handled (even if only on some architectures) as a
builtin default by the compiler, then a Builtin field is printed
(since dpkg 1.21.14). The only currently recognized areas on
Debian and derivatives are future, qa, reproducible, sanitize and
hardening, see the FEATURE AREAS section for more details. Exits
with 0 if the area is known otherwise exits with 1.
The output is in RFC822 format, with one section per feature. For
example:
Feature: pie
Enabled: yes
Builtin: yes
Feature: stackprotector
Enabled: yes
--help
Show the usage message and exit.
--version
Show the version and exit.
SUPPORTED FLAGS
ASFLAGS
Options for the assembler. Default value: empty. Since dpkg 1.21.0.
CFLAGS
Options for the C compiler. The default value set by the vendor
includes -g and the default optimization level (-O2 usually, or -O0
if the DEB_BUILD_OPTIONS environment variable defines noopt).
CPPFLAGS
Options for the C preprocessor. Default value: empty.
CXXFLAGS
Options for the C++ compiler. Same as CFLAGS.
OBJCFLAGS
Options for the Objective C compiler. Same as CFLAGS.
OBJCXXFLAGS
Options for the Objective C++ compiler. Same as CXXFLAGS.
GCJFLAGS
Options for the GNU Java compiler (gcj). A subset of CFLAGS.
DFLAGS
Options for the D compiler (ldc or gdc). Since dpkg 1.20.6.
FFLAGS
Options for the Fortran 77 compiler. A subset of CFLAGS.
FCFLAGS
Options for the Fortran 9x compiler. Same as FFLAGS.
LDFLAGS
Options passed to the compiler when linking executables or shared
objects (if the linker is called directly, then -Wl and , have to
be stripped from these options). Default value: empty.
New flags might be added in the future if the need arises (for example
to support other languages).
FEATURE AREAS
Each area feature can be enabled and disabled in the DEB_BUILD_OPTIONS
and DEB_BUILD_MAINT_OPTIONS environment variable's area value with the
‘+’ and ‘-’ modifier. For example, to enable the hardening “pie”
feature and disable the “fortify” feature you can do this in
debian/rules:
export DEB_BUILD_MAINT_OPTIONS=hardening=+pie,-fortify
The special feature all (valid in any area) can be used to enable or
disable all area features at the same time. Thus disabling everything
in the hardening area and enabling only “format” and “fortify” can be
achieved with:
export DEB_BUILD_MAINT_OPTIONS=hardening=-all,+format,+fortify
future
Several compile-time options (detailed below) can be used to enable
features that should be enabled by default, but cannot due to backwards
compatibility reasons.
lfs This setting (disabled by default) enables Large File Support on
32-bit architectures where their ABI does not include LFS by
default, by adding -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 to
CPPFLAGS.
qa
Several compile-time options (detailed below) can be used to help
detect problems in the source code or build system.
bug This setting (disabled by default) adds any warning option that
reliably detects problematic source code. The warnings are fatal.
The only currently supported flags are CFLAGS and CXXFLAGS with
flags set to -Werror=array-bounds, -Werror=clobbered,
-Werror=implicit-function-declaration and
-Werror=volatile-register-var.
canary
This setting (disabled by default) adds dummy canary options to the
build flags, so that the build logs can be checked for how the
build flags propagate and to allow finding any omission of normal
build flag settings. The only currently supported flags are
CPPFLAGS, CFLAGS, OBJCFLAGS, CXXFLAGS and OBJCXXFLAGS with flags
set to -D__DEB_CANARY_flag_random-id__, and LDFLAGS set to
-Wl,-z,deb-canary-random-id.
optimize
Several compile-time options (detailed below) can be used to help
optimize a resulting binary (since dpkg 1.21.0). Note: enabling all
these options can result in unreproducible binary artifacts.
lto This setting (since dpkg 1.21.0; disabled by default) enables Link
Time Optimization by adding -flto=auto -ffat-lto-objects to CFLAGS,
CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS, FCFLAGS and
LDFLAGS.
sanitize
Several compile-time options (detailed below) can be used to help
sanitize a resulting binary against memory corruptions, memory leaks,
use after free, threading data races and undefined behavior bugs.
Note: these options should not be used for production builds as they
can reduce reliability for conformant code, reduce security or even
functionality.
address
This setting (disabled by default) adds -fsanitize=address to
LDFLAGS and -fsanitize=address -fno-omit-frame-pointer to CFLAGS
and CXXFLAGS.
thread
This setting (disabled by default) adds -fsanitize=thread to
CFLAGS, CXXFLAGS and LDFLAGS.
leak
This setting (disabled by default) adds -fsanitize=leak to LDFLAGS.
It gets automatically disabled if either the address or the thread
features are enabled, as they imply it.
undefined
This setting (disabled by default) adds -fsanitize=undefined to
CFLAGS, CXXFLAGS and LDFLAGS.
hardening
Several compile-time options (detailed below) can be used to help
harden a resulting binary against memory corruption attacks, or provide
additional warning messages during compilation. Except as noted below,
these are enabled by default for architectures that support them.
format
This setting (enabled by default) adds -Wformat
-Werror=format-security to CFLAGS, CXXFLAGS, OBJCFLAGS and
OBJCXXFLAGS. This will warn about improper format string uses, and
will fail when format functions are used in a way that represent
possible security problems. At present, this warns about calls to
printf and scanf functions where the format string is not a string
literal and there are no format arguments, as in printf(foo);
instead of printf("%s", foo); This may be a security hole if the
format string came from untrusted input and contains ‘%n’.
fortify
This setting (enabled by default) adds -D_FORTIFY_SOURCE=2 to
CPPFLAGS. During code generation the compiler knows a great deal of
information about buffer sizes (where possible), and attempts to
replace insecure unlimited length buffer function calls with
length-limited ones. This is especially useful for old, crufty
code. Additionally, format strings in writable memory that contain
‘%n’ are blocked. If an application depends on such a format
string, it will need to be worked around.
Note that for this option to have any effect, the source must also
be compiled with -O1 or higher. If the environment variable
DEB_BUILD_OPTIONS contains noopt, then fortify support will be
disabled, due to new warnings being issued by glibc 2.16 and later.
stackprotector
This setting (enabled by default if stackprotectorstrong is not in
use) adds -fstack-protector --param=ssp-buffer-size=4 to CFLAGS,
CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS and FCFLAGS.
This adds safety checks against stack overwrites. This renders many
potential code injection attacks into aborting situations. In the
best case this turns code injection vulnerabilities into denial of
service or into non-issues (depending on the application).
This feature requires linking against glibc (or another provider of
__stack_chk_fail), so needs to be disabled when building with
-nostdlib or -ffreestanding or similar.
stackprotectorstrong
This setting (enabled by default) adds -fstack-protector-strong to
CFLAGS, CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS and
FCFLAGS. This is a stronger variant of stackprotector, but without
significant performance penalties.
Disabling stackprotector will also disable this setting.
This feature has the same requirements as stackprotector, and in
addition also requires gcc 4.9 and later.
relro
This setting (enabled by default) adds -Wl,-z,relro to LDFLAGS.
During program load, several ELF memory sections need to be written
to by the linker. This flags the loader to turn these sections
read-only before turning over control to the program. Most notably
this prevents GOT overwrite attacks. If this option is disabled,
bindnow will become disabled as well.
bindnow
This setting (disabled by default) adds -Wl,-z,now to LDFLAGS.
During program load, all dynamic symbols are resolved, allowing for
the entire PLT to be marked read-only (due to relro above). The
option cannot become enabled if relro is not enabled.
pie This setting (with no global default since dpkg 1.18.23, as it is
enabled by default now by gcc on the amd64, arm64, armel, armhf,
hurd-i386, i386, kfreebsd-amd64, kfreebsd-i386, mips, mipsel,
mips64el, powerpc, ppc64, ppc64el, riscv64, s390x, sparc and
sparc64 Debian architectures) adds the required options to enable
or disable PIE via gcc specs files, if needed, depending on whether
gcc injects on that architecture the flags by itself or not. When
the setting is enabled and gcc injects the flags, it adds nothing.
When the setting is enabled and gcc does not inject the flags, it
adds -fPIE (via /usr/share/dpkg/pie-compiler.specs) to CFLAGS,
CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS and FCFLAGS, and
-fPIE -pie (via /usr/share/dpkg/pie-link.specs) to LDFLAGS. When
the setting is disabled and gcc injects the flags, it adds -fno-PIE
(via /usr/share/dpkg/no-pie-compile.specs) to CFLAGS, CXXFLAGS,
OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS and FCFLAGS, and -fno-PIE
-no-pie (via /usr/share/dpkg/no-pie-link.specs) to LDFLAGS.
Position Independent Executable (PIE) is needed to take advantage
of Address Space Layout Randomization (ASLR), supported by some
kernel versions. While ASLR can already be enforced for data areas
in the stack and heap (brk and mmap), the code areas must be
compiled as position-independent. Shared libraries already do this
(-fPIC), so they gain ASLR automatically, but binary .text regions
need to be built as PIE to gain ASLR. When this happens, ROP
(Return Oriented Programming) attacks are much harder since there
are no static locations to bounce off of during a memory corruption
attack.
PIE is not compatible with -fPIC, so in general care must be taken
when building shared objects. But because the PIE flags emitted get
injected via gcc specs files, it should always be safe to
unconditionally set them regardless of the object type being
compiled or linked.
Static libraries can be used by programs or other shared libraries.
Depending on the flags used to compile all the objects within a
static library, these libraries will be usable by different sets of
objects:
none
Cannot be linked into a PIE program, nor a shared library.
-fPIE
Can be linked into any program, but not a shared library
(recommended).
-fPIC
Can be linked into any program and shared library.
If there is a need to set these flags manually, bypassing the gcc
specs injection, there are several things to take into account.
Unconditionally and explicitly passing -fPIE, -fpie or -pie to a
build-system using libtool is safe as these flags will get stripped
when building shared libraries. Otherwise on projects that build
both programs and shared libraries you might need to make sure that
when building the shared libraries -fPIC is always passed last (so
that it overrides any previous -PIE) to compilation flags such as
CFLAGS, and -shared is passed last (so that it overrides any
previous -pie) to linking flags such as LDFLAGS. Note: This should
not be needed with the default gcc specs machinery.
Additionally, since PIE is implemented via a general register, some
register starved architectures (but not including i386 anymore
since optimizations implemented in gcc >= 5) can see performance
losses of up to 15% in very text-segment-heavy application
workloads; most workloads see less than 1%. Architectures with more
general registers (e.g. amd64) do not see as high a worst-case
penalty.
reproducible
The compile-time options detailed below can be used to help improve
build reproducibility or provide additional warning messages during
compilation. Except as noted below, these are enabled by default for
architectures that support them.
timeless
This setting (enabled by default) adds -Wdate-time to CPPFLAGS.
This will cause warnings when the __TIME__, __DATE__ and
__TIMESTAMP__ macros are used.
fixfilepath
This setting (enabled by default) adds
-ffile-prefix-map=BUILDPATH=. to CFLAGS, CXXFLAGS, OBJCFLAGS,
OBJCXXFLAGS, GCJFLAGS, FFLAGS and FCFLAGS where BUILDPATH is set to
the top-level directory of the package being built. This has the
effect of removing the build path from any generated file.
If both fixdebugpath and fixfilepath are set, this option takes
precedence, because it is a superset of the former.
fixdebugpath
This setting (enabled by default) adds
-fdebug-prefix-map=BUILDPATH=. to CFLAGS, CXXFLAGS, OBJCFLAGS,
OBJCXXFLAGS, GCJFLAGS, FFLAGS and FCFLAGS where BUILDPATH is set to
the top-level directory of the package being built. This has the
effect of removing the build path from any generated debug symbols.
ENVIRONMENT
There are 2 sets of environment variables doing the same operations,
the first one (DEB_flag_op) should never be used within debian/rules.
It's meant for any user that wants to rebuild the source package with
different build flags. The second set (DEB_flag_MAINT_op) should only
be used in debian/rules by package maintainers to change the resulting
build flags.
DEB_flag_SET
DEB_flag_MAINT_SET
This variable can be used to force the value returned for the given
flag.
DEB_flag_STRIP
DEB_flag_MAINT_STRIP
This variable can be used to provide a space separated list of
options that will be stripped from the set of flags returned for
the given flag.
DEB_flag_APPEND
DEB_flag_MAINT_APPEND
This variable can be used to append supplementary options to the
value returned for the given flag.
DEB_flag_PREPEND
DEB_flag_MAINT_PREPEND
This variable can be used to prepend supplementary options to the
value returned for the given flag.
DEB_BUILD_OPTIONS
DEB_BUILD_MAINT_OPTIONS
These variables can be used by a user or maintainer to
disable/enable various area features that affect build flags. The
DEB_BUILD_MAINT_OPTIONS variable overrides any setting in the
DEB_BUILD_OPTIONS feature areas. See the FEATURE AREAS section for
details.
DEB_VENDOR
This setting defines the current vendor. If not set, it will
discover the current vendor by reading /etc/dpkg/origins/default.
DEB_BUILD_PATH
This variable sets the build path (since dpkg 1.18.8) to use in
features such as fixdebugpath so that they can be controlled by the
caller. This variable is currently Debian and derivatives-
specific.
DPKG_COLORS
Sets the color mode (since dpkg 1.18.5). The currently accepted
values are: auto (default), always and never.
DPKG_NLS
If set, it will be used to decide whether to activate Native
Language Support, also known as internationalization (or i18n)
support (since dpkg 1.19.0). The accepted values are: 0 and 1
(default).
FILES
Configuration files
/etc/dpkg/buildflags.conf
System wide configuration file.
$XDG_CONFIG_HOME/dpkg/buildflags.conf or
$HOME/.config/dpkg/buildflags.conf
User configuration file.
Packaging support
/usr/share/dpkg/buildflags.mk
Makefile snippet that will load (and optionally export) all flags
supported by dpkg-buildflags into variables (since dpkg 1.16.1).
EXAMPLES
To pass build flags to a build command in a Makefile:
$(MAKE) $(shell dpkg-buildflags --export=cmdline)
./configure $(shell dpkg-buildflags --export=cmdline)
To set build flags in a shell script or shell fragment, eval can be
used to interpret the output and to export the flags in the
environment:
eval "$(dpkg-buildflags --export=sh)" && make
or to set the positional parameters to pass to a command:
eval "set -- $(dpkg-buildflags --export=cmdline)"
for dir in a b c; do (cd $dir && ./configure "$@" && make); done
Usage in debian/rules
You should call dpkg-buildflags or include buildflags.mk from the
debian/rules file to obtain the needed build flags to pass to the build
system. Note that older versions of dpkg-buildpackage (before dpkg
1.16.1) exported these flags automatically. However, you should not
rely on this, since this breaks manual invocation of debian/rules.
For packages with autoconf-like build systems, you can pass the
relevant options to configure or make(1) directly, as shown above.
For other build systems, or when you need more fine-grained control
about which flags are passed where, you can use --get. Or you can
include buildflags.mk instead, which takes care of calling dpkg-
buildflags and storing the build flags in make variables.
If you want to export all buildflags into the environment (where they
can be picked up by your build system):
DPKG_EXPORT_BUILDFLAGS = 1
include /usr/share/dpkg/buildflags.mk
For some extra control over what is exported, you can manually export
the variables (as none are exported by default):
include /usr/share/dpkg/buildflags.mk
export CPPFLAGS CFLAGS LDFLAGS
And you can of course pass the flags to commands manually:
include /usr/share/dpkg/buildflags.mk
build-arch:
$(CC) -o hello hello.c $(CPPFLAGS) $(CFLAGS) $(LDFLAGS)
1.21.22 2023-05-11 dpkg-buildflags(1)
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