Provided by: dpkg-dev_1.17.5ubuntu5.8_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/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: 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. 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 disable 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=format-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 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) adds -fstack-protector --param=ssp-buffer-size=4
to CFLAGS and CXXFLAGS. 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.
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 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.
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 workloads; 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/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)