path: root/man/dpkg-buildflags.1

.\" dpkg manual page - dpkg-buildflags(1)
.\"
.\" Copyright © 2010-2011 Raphaël Hertzog <hertzog@debian.org>
.\" Copyright © 2011 Kees Cook <kees@debian.org>
.\"
.\" This is free software; you can redistribute it and/or modify
.\" it under the terms of the GNU General Public License as published by
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.
.TH dpkg\-buildflags 1 "2012-04-03" "Debian Project" "dpkg suite"
.SH NAME
dpkg\-buildflags \- returns build flags to use during package build
.
.SH SYNOPSIS
.B dpkg\-buildflags
.RI [ option "...] [" command ]
.
.SH DESCRIPTION
\fBdpkg\-buildflags\fP 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:
.IP 1.
system-wide with \fB/etc/dpkg/buildflags.conf\fP;
.IP 2.
for the current user with \fB$XDG_CONFIG_HOME/dpkg/buildflags.conf\fP
where \fB$XDG_CONFIG_HOME\fP defaults to \fB$HOME/.config\fP;
.IP 3.
temporarily by the user with environment variables (see section
\fBENVIRONMENT\fP);
.IP 4.
dynamically by the package maintainer with environment variables set via
\fBdebian/rules\fP (see section \fBENVIRONMENT\fP).
.P
The configuration files can contain two types of directives:
.TP
.BI SET " flag value"
Override the flag named \fIflag\fP to have the value \fIvalue\fP.
.TP
.BI STRIP " flag value"
Strip from the flag named \fIflag\fP all the build flags listed in \fIvalue\fP.
.TP
.BI APPEND " flag value"
Extend the flag named \fIflag\fP by appending the options given in \fIvalue\fP.
A space is prepended to the appended value if the flag's current value is non-empty.
.TP
.BI PREPEND " flag value"
Extend the flag named \fIflag\fP by prepending the options given in \fIvalue\fP.
A space is appended to the prepended value if the flag's current value is non-empty.
.P
The configuration files can contain comments on lines starting with a hash
(#). Empty lines are also ignored.
.SH COMMANDS
.TP
.BI \-\-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
("\fIflag\fP=\fIvalue\fP"). This is the default action.
.TP
.BI \-\-list
Print the list of flags supported by the current vendor
(one per line). See the \fBSUPPORTED FLAGS\fP section for more
information about them.
.TP
.BI \-\-status
Display any information that can be useful to explain the behaviour of
\fBdpkg\-buildflags\fP: 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 \fBdebian/rules\fP, so that the build log
keeps a clear trace of the build flags used. This can be useful to diagnose
problems related to them.
.TP
.BI \-\-export= format
Print to standard output shell (if \fIformat\fP is \fBsh\fP) or make
(if \fIformat\fP is \fBmake\fP) commands that can be used to export
all the compilation flags in the environment. If \fIformat\fP is
\fBconfigure\fP then the output can be used on a \fB./configure\fP
command-line. If the \fIformat\fP value is not
given, \fBsh\fP is assumed. Only compilation flags starting with an
upper case character are included, others are assumed to not be suitable
for the environment.
.TP
.BI \-\-get " flag"
Print the value of the flag on standard output. Exits with 0
if the flag is known otherwise exits with 1.
.TP
.BI \-\-origin " flag"
Print the origin of the value that is returned by \fB\-\-get\fP. Exits
with 0 if the flag is known otherwise exits with 1. The origin can be one
of the following values:
.RS
.TP
.B vendor
the original flag set by the vendor is returned;
.TP
.B system
the flag is set/modified by a system-wide configuration;
.TP
.B user
the flag is set/modified by a user-specific configuration;
.TP
.B env
the flag is set/modified by an environment-specific configuration.
.RE
.TP
.BI \-\-query\-features " area"
Print the features enabled for a given area. The only currently recognized
area is \fBhardening\fP. Exits with 0 if the area is known otherwise exits
with 1.
.IP
The output format is RFC822 header-style, with one section per feature.
For example:
.IP
.nf
  Feature: pie
  Enabled: no

  Feature: stackprotector
  Enabled: yes
.fi
.TP
.B \-\-help
Show the usage message and exit.
.TP
.B \-\-version
Show the version and exit.
.
.SH SUPPORTED FLAGS
.TP
.B CFLAGS
Options for the C compiler. The default value set by the vendor
includes \fI\-g\fP and the default optimization level (\fI\-O2\fP usually,
or \fI\-O0\fP if the \fBDEB_BUILD_OPTIONS\fP environment variable defines
\fInoopt\fP).
.TP
.B CPPFLAGS
Options for the C preprocessor. Default value: empty.
.TP
.B CXXFLAGS
Options for the C++ compiler. Same as \fBCFLAGS\fP.
.TP
.B FFLAGS
Options for the Fortran compiler. Same as \fBCFLAGS\fP.
.TP
.B LDFLAGS
Options passed to the compiler when linking executables or shared
objects (if the linker is called directly, then
.B \-Wl
and
.B ,
have to be stripped from these options). Default value: empty.
.
.SH FILES
.TP
.B /etc/dpkg/buildflags.conf
System wide configuration file.
.TP
.BR $XDG_CONFIG_HOME/dpkg/buildflags.conf " or " $HOME/.config/dpkg/buildflags.conf
User configuration file.
.SH ENVIRONMENT
There are 2 sets of environment variables doing the same operations, the
first one (DEB_\fIflag\fP_\fIop\fP) should never be used within
\fBdebian/rules\fP. It's meant for any user that wants to rebuild the
source package with different build flags. The second set
(DEB_\fIflag\fP_MAINT_\fIop\fP) should only be used in \fBdebian/rules\fP
by package maintainers to change the resulting build flags.
.TP
.BI DEB_ flag _SET
.TQ
.BI DEB_ flag _MAINT_SET
This variable can be used to force the value returned for the given
\fIflag\fP.
.TP
.BI DEB_ flag _STRIP
.TQ
.BI 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
\fIflag\fP.
.TP
.BI DEB_ flag _APPEND
.TQ
.BI DEB_ flag _MAINT_APPEND
This variable can be used to append supplementary options to the value
returned for the given \fIflag\fP.
.TP
.BI DEB_ flag _PREPEND
.TQ
.BI DEB_ flag _MAINT_PREPEND
This variable can be used to prepend supplementary options to the value
returned for the given \fIflag\fP.
.TP
.B DEB_BUILD_MAINT_OPTIONS
This variable can be used to disable/enable various hardening build
flags through the \fBhardening\fP option. See the \fBHARDENING\fP section
for details.
.
.SH 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.
.P
Each hardening feature can be enabled and disabled in the
\fBDEB_BUILD_MAINT_OPTIONS\fP environment variable's \fBhardening\fP
value with the "+" and "\-" modifier. For example, to enable the
"pie" feature and disable the "fortify" feature you can do this
in \fBdebian/rules\fP:
.P
  export DEB_BUILD_MAINT_OPTIONS=hardening=+pie,\-fortify
.P
The special feature \fBall\fP 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:
.P
  export DEB_BUILD_MAINT_OPTIONS=hardening=\-all,+format,+fortify
.
.TP
.B format
This setting (enabled by default) adds
.B \-Wformat \-Werror=format\-security
to \fBCFLAGS\fP and \fBCXXFLAGS\fP. 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 \fBprintf\fP and \fBscanf\fP functions where the format string is
not a string literal and there are no format arguments, as in
\fBprintf(foo);\fP instead of \fPprintf("%s", foo);\fP
This may be a security hole if the format string came from untrusted
input and contains "%n".
.
.TP
.B fortify
This setting (enabled by default) adds
.B \-D_FORTIFY_SOURCE=2
to \fBCPPFLAGS\fP. 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 \fB\-O1\fP or higher.
.TP
.B stackprotector
This setting (enabled by default) adds
.B \-fstack-protector \-\-param=ssp\-buffer\-size=4
to \fBCFLAGS\fP and \fBCXXFLAGS\fP. 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
\fB__stack_chk_fail\fP), so needs to be disabled when building with
\fB\-nostdlib\fP or \fB\-ffreestanding\fP or similar.
.
.TP
.B relro
This setting (enabled by default) adds
.B \-Wl,\-z,relro
to \fBLDFLAGS\fP.  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,
\fBbindnow\fP will become disabled as well.
.
.TP
.B bindnow
This setting (disabled by default) adds
.B \-Wl,\-z,now
to \fBLDFLAGS\fP. During program load, all dynamic symbols are resolved,
allowing for the entire PLT to be marked read-only (due to \fBrelro\fP
above). The option cannot become enabled if \fBrelro\fP is not enabled.
.
.TP
.B pie
This setting (disabled by default) adds \fB\-fPIE\fP to \fBCFLAGS\fP and
\fBCXXFLAGS\fP, and \fB\-fPIE \-pie\fP to \fBLDFLAGS\fP. 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 \fB\-fPIC\fP 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.