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.

--export=format
    Print to standard output shell (if format is sh) or make (if format is make) commands that can be used to export all the compilation flags in the environment. If format is configure then the output can be used on a ./configure command-line. 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.

--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.

--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.

FFLAGS
    Options for the Fortran compiler. Same as 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.

FILES

/etc/dpkg/buildflags.conf
    System wide configuration file.

$XDG_CONFIG_HOME/dpkg/buildflags.conf or $HOME/.config/dpkg/buildflags.conf
    User configuration file.

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 HARDENING section for details.

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 -Wformat-security -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 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.

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.

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).

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.

AUTHOR
Copyright © 2010-2011 Raphaël Hertzog

Copyright © 2011 Kees Cook

This is free software; see the GNU General Public Licence version 2 or later for copying conditions. There is NO WARRANTY.