Ubuntu Manpage: dpkg-buildflags - returns build flags to use during package build

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

       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 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.

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)