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/overriden 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 two 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-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 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: no
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
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.
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 (disabled by default) adds -fPIE to CFLAGS,
CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS and FCFLAGS,
and -fPIE -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.
This is not compatible with -fPIC so care must be taken when
building shared objects.
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.
-fPIC Can be linked into any program and shared library.
Unconditionally 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.
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)
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