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
future
Several compile-time options (detailed below) can be used to
enable features that should be enabled by default, but cannot due
to backwards compatibility reasons.
lfs This setting (disabled by default) enables Large File
Support on 32-bit architectures where their ABI does not
include LFS by default, by adding -D_LARGEFILE_SOURCE
-D_FILE_OFFSET_BITS=64 to CPPFLAGS.
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. Note: these options should not be used for
production builds as they can reduce reliability for conformant
code, reduce security or even functionality.
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 (with no global default since dpkg 1.18.23,
as it is enabled by default now by gcc on the amd64,
arm64, armel, armhf, hurd-i386, i386, kfreebsd-amd64,
kfreebsd-i386, mips, mipsel, mips64el, powerpc, ppc64,
ppc64el, riscv64, s390x, sparc and sparc64 Debian
architectures) adds the required options to enable or
disable PIE via gcc specs files, if needed, depending on
whether gcc injects on that architecture the flags by
itself or not. When the setting is enabled and gcc
injects the flags, it adds nothing. When the setting is
enabled and gcc does not inject the flags, it adds -fPIE
(via /usr/local/share/dpkg/pie-compiler.specs) to CFLAGS,
CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS and
FCFLAGS, and -fPIE -pie (via /usr/local/share/dpkg/pie-
link.specs) to LDFLAGS. When the setting is disabled and
gcc injects the flags, it adds -fno-PIE (via
/usr/local/share/dpkg/no-pie-compile.specs) to CFLAGS,
CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS and
FCFLAGS, and -fno-PIE -no-pie (via
/usr/local/share/dpkg/no-pie-link.specs) 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.
PIE is not compatible with -fPIC, so in general care must
be taken when building shared objects. But because the PIE
flags emitted get injected via gcc specs files, it should
always be safe to unconditionally set them regardless of
the object type being compiled or linked.
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 (recommended).
-fPIC Can be linked into any program and shared library.
If there is a need to set these flags manually, bypassing
the gcc specs injection, there are several things to take
into account. Unconditionally and explicitly 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. Note: This should not be needed with the default
gcc specs machinery.
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.
fixfilepath
This setting (disabled by default) adds
-ffile-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 file.
If both fixdebugpath and fixfilepath are set, this option
takes precedence, because it is a superset of the former.
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.
