redhat-rpm-config/buildflags.md

This document contains documentation of the individual compiler flags and how to use them.

[TOC]

Using RPM build flags

For packages which use autoconf to set up the build environment, 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, LDFLAGS and LT_SYS_LIBRARY_PATH, so they can be used by makefiles and other build tools. (However, existing values for these variables are not overwritten.)

If your package does not use autoconf, you can still set the same environment variables using

%set_build_flags

early in the %build section. (Again, existing environment variables are not overwritten.)

Individual build flags are also available through RPM macros:

• %{build_cc} for the command name of the C compiler.
• %{build_cxx} for the command name of the C++ compiler.
• %{build_cpp} for the command name of the C-compatible preprocessor.
• %{build_cflags} for the C compiler flags (also known as the CFLAGS variable). Also historically available as %{optflags}. Furthermore, at the start of the %build section, the environment variable RPM_OPT_FLAGS is set to this value.
• %{build_cxxflags} for the C++ compiler flags (usually assigned to the CXXFLAGS shell variable).
• %{build_fflags} for FFLAGS(the Fortran compiler flags, also known as theFCFLAGS` variable).
• %{build_ldflags} for the link editor (ld) flags, usually known as LDFLAGS. Note that the contents quotes linker arguments using -Wl, so this variable is intended for use with the gcc compiler driver. At the start of the %build section, the environment variable RPM_LD_FLAGS is set to this value.

The variable LT_SYS_LIBRARY_PATH is defined here to prevent the libtool script (v2.4.6+) from hardcoding %_libdir into the binaries' RPATH.

These RPM macros do not alter shell environment variables.

For some other build tools 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

Customizing compiler and other build 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 workarounds are not available.

Toolchain selection

The default toolchain uses GCC, and the %toolchain macro is defined as gcc.

It is enough to override toolchain macro and all relevant macro for C/C++ compilers will be switched. Either in the spec or in the command-line.

%global toolchain clang

or:

rpmbuild -D "toolchain clang" …

Inside a spec file it is also possible to determine which toolchain is in use by testing the same macro. For example:

%if "%{toolchain}" == "gcc"
BuildRequires: gcc
%endif

or:

%if "%{toolchain}" == "clang"
BuildRequires: clang compiler-rt
%endif

Disabling Link-Time Optimization

By default, builds use link-time optimization. In this build mode, object code is generated at the time of the final link, by combining information from all available translation units, and taking into account which symbols are exported.

To disable this optimization, include this in the spec file:

%define _lto_cflags %{nil}

Lazy binding

If your package depends on the semantics of lazy binding (e.g., it has plugins which load additional plugins to complete their dependencies, before which some referenced functions are undefined), you should put -Wl,-z,lazy at the end of the LDFLAGS setting when linking objects which have such requirements. Under these circumstances, it is unnecessary to disable hardened builds (and thus lose full ASLR for executables), or link everything without -Wl,z,now (non-lazy binding).

Hardened builds

By default, the build flags enable fully hardened builds. To change this, include this in the RPM spec file:

%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.

Annotated builds/watermarking

By default, the build flags cause a special output section to be included in ELF files which describes certain aspects of the build. To change this for all compiler invocations, include this in the RPM spec file:

%undefine _annotated_build

Be warned that this turns off watermarking, making it impossible to do full hardening coverage analysis for any binaries produced.

It is possible to disable annotations for individual compiler invocations, using the -fplugin-arg-annobin-disable flag. However, the annobin plugin must still be loaded for this flag to be recognized, so it has to come after the hardening flags on the command line (it has to be added at the end of CFLAGS, or specified after the CFLAGS variable contents).

Keeping dependencies on unused shared objects

By default, ELF shared objects which are listed on the linker command line, but which have no referencing symbols in the preceding objects, are not added to the output file during the final link.

In order to keep dependencies on shared objects even if none of their symbols are used, include this in the RPM spec file:

%undefine _ld_as_needed

For example, this can be required if shared objects are used for their side effects in ELF constructors, or for making them available to dynamically loaded plugins.

Strict symbol checks in the link editor (ld)

Optionally, the link editor will refuse to link shared objects which contain undefined symbols. Such symbols lack symbol versioning information and can be bound to the wrong (compatibility) symbol version at run time, and not the actual (default) symbol version which would have been used if the symbol definition had been available at static link time. Furthermore, at run time, the dynamic linker will not have complete dependency information (in the form of DT_NEEDED entries), which can lead to errors (crashes) if IFUNC resolvers are executed before the shared object containing them is fully relocated.

To switch on these checks, define this macro in the RPM spec file:

%define _strict_symbol_defs_build 1

If this RPM spec option is active, link failures will occur if the linker command line does not list all shared objects which are needed. In this case, you need to add the missing DSOs (with linker arguments such as -lm). As a result, the link editor will also generated the necessary DT_NEEDED entries.

In some cases (such as when a DSO is loaded as a plugin and is expected to bind to symbols in the main executable), undefined symbols are expected. In this case, you can add

%undefine _strict_symbol_defs_build

to the RPM spec file to disable these strict checks. Alternatively, you can pass -z undefs to ld (written as -Wl,-z,undefs on the gcc command line). The latter needs binutils 2.29.1-12.fc28 or later.

Legacy -fcommon

Since version 10, gcc defaults to -fno-common. Builds may fail with multiple definition of ... errors.

As a short term workaround for such failure, it is possible to add -fcommon to the flags by defining %_legacy_common_support.

%define _legacy_common_support 1

Properly fixing the failure is always preferred!

Post-build ELF object processing

By default, DWARF debugging information is separated from installed ELF objects and put into -debuginfo subpackages. To disable most debuginfo processing (and thus the generation of these subpackages), define _enable_debug_packages as 0.

Processing of debugging information is controlled using the find-debuginfo tool from the debugedit package. Several aspects of its operation can be controlled at the RPM level.

• Creation of -debuginfo subpackages is enabled by default. To disable, undefine _debuginfo_subpackages.
• Likewise, -debugsource subpackages are automatically created. To disable, undefine _debugsource_subpackages. See Separate Subpackage and Source Debuginfo for background information.
• _build_id_links, _unique_build_ids, _unique_debug_names, _unique_debug_srcs control how debugging information and corresponding source files are represented on disk. See /usr/lib/rpm/macros for details. The defaults enable parallel installation of -debuginfo packages for different package versions, as described in Parallel Installable Debuginfo.
• By default, a compressed symbol table is preserved in the .gnu_debugdata section. To disable that, undefine _include_minidebuginfo.
• To speed up debuggers, a .gdb_index section is created. It can be disabled by undefining _include_gdb_index.
• Missing build IDs result in a build failure. To ignore such problems, undefine _missing_build_ids_terminate_build.
• During processing, build IDs are recomputed to match the binary content. To skip this step, define _no_recompute_build_ids as 1.
• By default, the options in _find_debuginfo_dwz_opts turn on dwz (DWARF compression) processing. Undefine this macro to disable this step.
• Additional options can be passed by defining the _find_debuginfo_opts macro.

After separation of debugging information, additional transformations are applied, most of them also related to debugging information. These steps can be skipped by undefining the corresponding macros:

• __brp_strip: Removal of leftover debugging information. The tool specified by the __strip macro is invoked with the -g option on ELF object (.o) files.
• __brp_strip_static_archive: This is similar to __brp_strip, but processes static .a archives instead.
• __brp_strip_comment_note: This step removes unallocated .note sections, and .comment sections from ELF files.
• __brp_strip_lto: This step removes GCC LTO intermediate representation in ELF sections starting with .gnu.lto_ and .gnu.debuglto_. Skipping this step is strongly discouraged because the tight coupling of LTO data with the GCC version. The underlying tool is again determined by the __strip macro.
• __brp_llvm_compile_lto_elf: This step replaces LLVM bitcode files with object files, thereby removing LLVM bitcode from the installed files. This transformation is applied to object files in static .a archives, too.
• __brp_ldconfig: For each shared object on the library search path whose soname does not match its file name, a symbolic link from the soname to the file name is created. This way, these shared objects are loadable immediately after installation, even if they are not yet listed in the /etc/ld.so.cache file (because ldconfig has not been invoked yet).

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. 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.
• -Werror=format-security: Turn on format string warnings and treat them as errors. See the GCC manual. 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 and the 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).
• -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 ienformation. (Not enabled on armhfp due to architectural differences in stack management.)
• -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.)
• -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. This flag is currently not available on armhfp (both gcc and clang toolchains aond on aarch64 with the clang toolchain.
• -flto=auto: Enable link-time optimization (LTO), using make job server integration for parallel processing. (gcc toolchain only)
• -ffat-lto-objects: Generate EFL object files which contain both object code and LTO intermediate representation. (gcc toolchain only)
• -flto: Enable link-time optimization. (clang toolchain only)
• -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.
• -fcommon: This optional flag is used to build legacy software which relies on C tentative definitions. It is disabled by default.

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 using annobin, the -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 flag is added by default (with the gcc toolchain). This can be switched off by undefining the %_annotated_build RPM macro (see above). Binary watermarks are currently disabled on armhpf, and with the clang toolchain.

Architecture-specific compiler flags

These compiler flags are enabled for all builds (hardened/annotated or not), but their selection depends on the architecture:

• -fcf-protection: Instrument binaries to guard against ROP/JOP attacks. Used on i686 and x86_64.
• -mbranch-protection=standard: Instrument binaries to guard against ROP/JOP attacks. Used on aarch64.
• -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.

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. -mtune=cortex-8a selects tuning for the Cortex-A8 implementation (while preserving compatibility with other ARMv7-A implementations). -mabi=aapcs-linux switches to the AAPCS ABI for GNU/Linux.
• i686: -march=i686 is used to select a minmum support CPU level of i686 (corresponding to the Pentium Pro). SSE2 support is enabled with -msse2 (so only CPUs with SSE2 support can run the compiled code; SSE2 was introduced first with the Pentium 4). -mtune=generic activates tuning for a current blend of CPUs (under the assumption that most users of i686 packages obtain them through an x86_64 installation on current hardware). -mfpmath=sse instructs GCC to use the SSE2 unit for floating point math to avoid excess precision issues. -mstackrealign avoids relying on the stack alignment guaranteed by the current version of the i386 ABI.
• 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.
• aarch64 does 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).
• --as-needed: In the final link, only generate ELF dependencies for shared objects that actually provide symbols required by the link. Shared objects which are not needed to fulfill symbol dependencies are essentially ignored due to this flag.
• -z defs: Refuse to link shared objects (DSOs) with undefined symbols (optional, see above).

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.

Support for extension builders

Some packages include extension builders that allow users to build extension modules (which are usually written in C and C++) under the control of a special-purpose build system. This is a common functionality provided by scripting languages such as Python and Perl. Traditionally, such extension builders captured the Fedora build flags when these extension were built. However, these compiler flags are adjusted for a specific Fedora release and toolchain version and therefore do not work with a custom toolchain (e.g., different C/C++ compilers), and users might want to build their own extension modules with such toolchains.

The macros %{extension_cflags}, %{extension_cxxflags}, %{extension_fflags}, %{extension_ldflags} contain a subset of flags that have been adjusted for compatibility with alternative toolchains, while still preserving some of the compile-time security hardening that the standard Fedora build flags provide.

The current set of differences are:

• No GCC plugins (such as annobin) are activated.
• No GCC spec files (-specs= arguments) are used.

Additional flags may be removed in the future if they prove to be incompatible with alternative toolchains.

Extension builders should detect whether they are performing a regular RPM build (e.g., by looking for an RPM_OPT_FLAGS variable). In this case, they should use the current set of Fedora build flags (that is, the output from rpm --eval '%{build_cflags}' and related commands). Otherwise, when not performing an RPM build, they can either use hard-coded extension builder flags (thus avoiding a run-time dependency on redhat-rpm-config), or use the current extension builder flags (with a run-time dependency on redhat-rpm-config).

As a result, extension modules built for Fedora will use the official Fedora build flags, while users will still be able to build their own extension modules with custom toolchains.