dpkg-buildflags(1) dpkg suite dpkg-buildflags(1)NAMEdpkg-buildflags - returns build flags to use during package build
SYNOPSISdpkg-buildflags [option...] [command]
DESCRIPTIONdpkg-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 behav‐
iour of dpkg-buildflags: 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 start‐
ing with an upper case character are included, others are
assumed to not be suitable for the environment. Supported for‐
mats:
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 direc‐
tive.
--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 configura‐
tion;
env the flag is set/modified by an environment-specific con‐
figuration.
--query-features area
Print the features enabled for a given area. The only currently
recognized area is 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.
GCJFLAGS
Options for the GNU Java compiler (gcj). A subset of CFLAGS.
FFLAGS Options for the Fortran compiler. A subset of CFLAGS.
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
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.
Each hardening feature can be enabled and disabled in the
DEB_BUILD_MAINT_OPTIONS environment variable's hardening value with the
"+" and "-" modifier. For example, to enable the "pie" feature and dis‐
able the "fortify" feature you can do this in debian/rules:
export DEB_BUILD_MAINT_OPTIONS=hardening=+pie,-fortify
The special feature all can be used to enable or disable all hardening
features at the same time. Thus disabling everything and enabling only
"format" and "fortify" can be achieved with:
export DEB_BUILD_MAINT_OPTIONS=hardening=-all,+format,+fortify
format This setting (enabled by default) adds -Wformat -Werror=for‐
mat-security to CFLAGS and CXXFLAGS. 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 con‐
tain '%n' are blocked. If an application depends on such a for‐
mat 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) adds -fstack-protector
--param=ssp-buffer-size=4 to CFLAGS and CXXFLAGS. This adds
safety checks against stack overwrites. This renders many poten‐
tial 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.
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 and
CXXFLAGS, and -fPIE -pie to LDFLAGS. Position Independent Exe‐
cutable 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-indepen‐
dent. 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 Pro‐
gramming) 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.
Additionally, since PIE is implemented via a general register,
some architectures (most notably i386) can see performance
losses of up to 15% in very text-segment-heavy application work‐
loads; most workloads see less than 1%. Architectures with more
general registers (e.g. amd64) do not see as high a worst-case
penalty.
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_MAINT_OPTIONS
This variable can be used to disable/enable various hardening
build flags through the hardening option. See the FEATURE AREAS
section for details.
FILES
Configuration files
/etc/dpkg/buildflags.conf
System wide configuration file.
$XDG_CONFIG_HOME/dpkg/buildflags.conf or $HOME/.config/dpkg/build‐
flags.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 $(shell dpkg-buildflags --export=cmdline)
To set build flags in a shell script or shell fragment, "eval" can be
used to interpret the output and to export the flags in the environ‐
ment:
eval "$(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 rele‐
vant 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-build‐
flags 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)
Debian Project 2013-10-20 dpkg-buildflags(1)
