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Document build flags

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Florian Weimer 2018-01-22 14:28:15 +01:00
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This document contains documentation of the individual compiler flags
and how to use them.
[TOC]
# Using RPM build flags
At present, the only supported way is to use the `%configure` macro to
obtain the full complement of flags, like this:
%configure
This will invoke the `./configure` with arguments (such as
`--prefix=/usr`) to adjust the paths to the packaging defaults.
As a side effect, this will set the environment variables `CFLAGS`,
`CXXFLAGS`, `FFLAGS`, `FCFLAGS`, and `LDFLAGS`, so they can be used by
makefiles and other build tools.
For some other build tools besides `autoconf`, separate mechanisms exist:
* CMake builds use the the `%cmake` macro from the `cmake-rpm-macros`
package.
Care must be taking not to compile the current selection of compiler
flags into any RPM package besides `redhat-rpm-config`, so that flag
changes are picked up automatically once `redhat-rpm-config` is
updated.
# Flag selection for the build type
The default flags are suitable for building applications.
For building shared objects, you must compile with `-fPIC` in
(`CFLAGS` or `CXXFLAGS`) and link with `-shared` (in `LDFLAGS`).
For other considerations involving shared objects, see:
* [Fedora Packaging Guidelines: Shared Libraries](https://fedoraproject.org/wiki/Packaging:Guidelines#Shared_Libraries)
# Customizing compiler flags
It is possible to set RPM macros to change some aspects of the
compiler flags. Changing these flags should be used as a last
recourse if other workarunds are not available.
### Disable hardened builds
%undefine _hardened_build
This turns off certain hardening features, as described in detail
below. The main difference is that executables will be
position-dependent (no full ASLR) and use lazy binding.
### Disable annotated builds/watermarking
%undefine _annotated_build
This turns off watermarking, making it impossible to do full hardening
coverage analysis for any binaries produced.
# Individual compiler flags
Compiler flags end up in the environment variables `CFLAGS`,
`CXXFLAGS`, `FFLAGS`, and `FCFLAGS`.
The general (architecture-independent) build flags are:
* `-O2`: Turn on various GCC optimizations. See the [GCC manual](https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-O2).
Optimization improves performance, the accuracy of warnings, and the
reach of toolchain-based hardening, but it makes debugging harder.
* `-g`: Generate debugging information (DWARF). In Fedora, this data
is separated into `-debuginfo` RPM packages whose installation is
optional, so debuging information does not increase the size of
installed binaries by default.
* `-pipe`: Run compiler and assembler in parallel and do not use a
temporary file for the assembler input. This can improve
compilation performance. (This does not affect code generation.)
* `-Wall`: Turn on various GCC warnings.
See the [GCC manual](https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wall).
* `-Werror=format-security`: Turn on format string warnings and treat
them as errors.
See the [GCC manual](https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wformat-security).
This can occasionally result in compilation errors. In this case,
the best option is to rewrite the source code so that only constant
format strings (string literals) are used.
* `-Wp,-D_FORTIFY_SOURCE=2`: Source fortification activates various
hardening features in glibc:
* String functions such as `memcpy` attempt to detect buffer lengths
and terminate the process if a buffer overflow is detected.
* `printf` format strings may only contain the `%n` format specifier
if the format string resides in read-only memory.
* `open` and `openat` flags are checked for consistency with the
presence of a *mode* argument.
* Plus other minor hardening changes.
(These changes can occasionally break valid programs.)
* `-fexceptions`: Provide exception unwinding support for C programs.
See the [`-fexceptions` option in the GCC
manual](https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#index-fexceptions)
and the [`cleanup` variable
attribute](https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-cleanup-variable-attribute).
This also hardens cancellation handling in C programs because
it is not required to use an on-stack jump buffer to install
a cancellation handler with `pthread_cleanup_push`. It also makes
it possible to unwind the stack (using C++ `throw` or Rust panics)
from C callback functions if a C library supports non-local exits
from them (e.g., via `longjmp`).
* `-Wp,-D_GLIBCXX_ASSERTIONS`: Enable lightweight assertions in the
C++ standard library, such as bounds checking for the subscription
operator on vectors. (This flag is added to both `CFLAGS` and
`CXXFLAGS`; C compilations will simply ignore it.)
* `-fstack-protector-strong`: Instrument functions to detect
stack-based buffer overflows before jumping to the return address on
the stack. The *strong* variant only performs the instrumentation
for functions whose stack frame contains addressable local
variables. (If the address of a variable is never taken, it is not
possible that a buffer overflow is caused by incorrect pointer
arithmetic involving a pointer to that variable.)
* `-grecord-gcc-switches`: Include select GCC command line switches in
the DWARF debugging information. This is useful for detecting the
presence of certain build flags and general hardening coverage.
For hardened builds (which are enabled by default, see above for how
to disable them), the flag
`-specs=/usr/lib/rpm/redhat/redhat-hardened-cc1` is added to the
command line. It adds the following flag to the command line:
* `-fPIE`: Compile for a position-independent executable (PIE),
enabling full address space layout randomization (ASLR). This is
similar to `-fPIC`, but avoids run-time indirections on certain
architectures, resulting in improved performance and slightly
smaller executables. However, compared to position-dependent code
(the default generated by GCC), there is still a measurable
performance impact.
If the command line also contains `-r` (producing a relocatable
object file), `-fpic` or `-fPIC`, this flag is automatically
dropped. (`-fPIE` can only be used for code which is linked into
the main program.) Code which goes into static libraries should be
compiled with `-fPIE`, except when this code is expected to be
linked into DSOs, when `-fPIC` must be used.
To be effective, `-fPIE` must be used with the `-pie` linker flag
when producing an executable, see below.
To support [binary watermarks for ELF
objects](https://fedoraproject.org/wiki/Toolchain/Watermark) using
annobin, the `-specs=/usr/lib/rpm/redhat/redhat-annobin-cc1` flag is
added by default. This can be switched off by undefining the
`%_annotated_build` RPM macro (see above).
### Architecture-specific compiler flags
These compiler flags are enabled for all builds (hardened/annotated or
not), but their selection depends on the architecture:
* `-fstack-clash-protection`: Turn on instrumentation to avoid
skipping the guard page in large stack frames. (Without this flag,
vulnerabilities can result where the stack overlaps with the heap,
or thread stacks spill into other regions of memory.) This flag is
fully ABI-compatible and has adds very little run-time overhead, but
is only on certain architectures (currently aarch64, i386, ppc64,
ppc64le, s390x, x86-64).
* `-m64` and `-m32`: Some GCC builds support both 32-bit and 64-bit in
the same compilation. For such architectures, the RPM build process
explicitly selects the architecture variant by passing this compiler
flag.
* `-fasynchronous-unwind-tables`: Generate full unwind information
covering all program points. This is required for support of
asynchronous cancellation and proper unwinding from signal
handlers. It also makes performance and debugging tools more useful
because unwind information is available without having to install
(and load) debugging information.
This flag is enabled explictly for i686 (and derived architectures),
and implicity for x86-64.
In addition, `redhat-rpm-config` re-selects the built-in default
tuning in the `gcc` package. These settings are:
* **armhfp**: `-march=armv7-a -mfpu=vfpv3-d16 -mfloat-abi=hard`
selects an Arm subarchitecture based on the ARMv7-A architecture
with 16 64-bit floating point registers.
* **i686**: `-march=i686` is used to select a minmum support CPU level
of i686 (corresponding to the Pentium Pro).
* **ppc64le**: `-mcpu=power8 -mtune=power8` selects a minimum supported
CPU level of POWER8 (the first CPU with ppc64le support) and tunes
for POWER8.
* **s390x**: `-march=zEC12 -mtune=z13` specifies a minimum supported CPU
level of zEC12, while optimizing for a subsequent CPU generation
(z13).
* **x86-64**: `-mtune=generic` selects tuning which is expected to
beneficial for a broad range of current CPUs.
* **ppc64** and **aarch64** do not have any architecture-specific tuning.
# Individual linker flags
Linker flags end up in the environment variable `LDFLAGS`.
The linker flags listed below are injected. Note that they are
prefixed with `-Wl` because it is expected that these flags are passed
to the compiler driver `gcc`, and not directly to the link editor
`ld`.
* `-z relro`: Activate the *read-only after relocation* feature.
Constant data and relocations are placed on separate pages, and the
dynamic linker is instructed to revoke write permissions after
dynamic linking. Full protection of relocation data requires the
`-z now` flag (see below).
For hardened builds, the
`-specs=/usr/lib/rpm/redhat/redhat-hardened-ld` flag is added to the
compiler driver command line. (This can be disabled by undefining the
`%_hardened_build` macro; see above) This activates the following
linker flags:
* `-pie`: Produce a PIE binary. This is only activated for the main
executable, and only if it is dynamically linked. This requires
that all objects which are linked in the main executable have been
compiled with `-fPIE` or `-fPIC` (or `-fpie` or `-fpic`; see above).
By itself, `-pie` has only a slight performance impact because it
disables some link editor optimization, however the `-fPIE` compiler
flag has some overhead.
* `-z now`: Disable lazy binding and turn on the `BIND_NOW` dynamic
linker feature. Lazy binding involves an array of function pointers
which is writable at run time (which could be overwritten as part of
security exploits, redirecting execution). Therefore, it is
preferable to turn of lazy binding, although it increases startup
time.

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redhat-rpm-config.spec
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Summary: Red Hat specific rpm configuration files
Name: redhat-rpm-config
Version: 79
Version: 80
Release: 1%{?dist}
# No version specified.
License: GPL+
@ -64,6 +64,9 @@ Source600: kmod.attr
Source601: kmod.prov
Source602: libsymlink.attr
# Documentation
Source900: buildflags.md
BuildArch: noarch
BuildRequires: perl-generators
Requires: coreutils
@ -140,6 +143,7 @@ install -p -m 755 -t %{buildroot}%{_rpmconfigdir} kmod.prov
%{_rpmconfigdir}/macros.d/macros.dwz
%{_rpmconfigdir}/macros.d/macros.forge
%{_rpmconfigdir}/macros.d/macros.vpath
%doc buildflags.md
%files -n kernel-rpm-macros
%dir %{rrcdir}/find-provides.d
@ -153,6 +157,9 @@ install -p -m 755 -t %{buildroot}%{_rpmconfigdir} kmod.prov
%{_rpmconfigdir}/macros.d/macros.kmp
%changelog
* Mon Jan 22 2018 Florian Weimer <fweimer@redhat.com> - 80-1
- Document build flags
* Fri Jan 19 2018 Panu Matilainen <pmatilai@redhat.com> - 79-1
- Document how to disable hardened and annotated build (#1211296)