dpkg-buildflags (1)

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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)

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