bionic (1) dpkg-buildflags.1.gz
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, 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
Area: reproducible
Features:
timeless=no
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). 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
Feature: stackprotector
Enabled: yes
--help Show the usage message and exit.
--version
Show the version and exit.
SUPPORTED FLAGS
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.
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.
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 default since dpkg 1.18.23, and injected by default by gcc on the amd64,
arm64, armel, armhf, i386, kfreebsd-amd64, kfreebsd-i386, mips, mipsel, mips64el, ppc64el, s390x,
sparc and sparc64 Debian architectures) adds the required options via gcc specs files if needed to
enable or disable PIE. When enabled and injected by gcc, adds nothing. When enabled and not
injected by gcc, adds -fPIE to CFLAGS, CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS and
FCFLAGS, and -fPIE -pie to LDFLAGS. When disabled and injected by gcc, adds -fno-PIE to CFLAGS,
CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS and FCFLAGS, and -fno-PIE -no-pie to LDFLAGS.
Position Independent Executable are needed to take advantage of Address Space Layout
Randomization, 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 build 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.
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.
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)