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

Provided by: dpkg-dev_1.21.1ubuntu2.3_all

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)

1.21.1                                             2024-02-23                                 dpkg-buildflags(1)