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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. Prior to that, some common problems in autotools scripts are automatically patched across the source tree. 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.) `%set_build_flags` does not perform autotools script rewriting, unlike `%configure`. Individual build flags are also available through RPM macros: * `%{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 the `FCFLAGS` 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](https://docs.fedoraproject.org/en-US/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 ### Disable autotools compatibility patching By default, the invocation of the `%configure` macro replaces `config.guess` files in the source tree with the system version. To disable that, define this macro: %global _configure_gnuconfig_hack 0 `%configure` also patches `ltmain.sh` scripts, so that linker flags are set as well during libtool-. This can be switched off using: %global _configure_libtool_hardening_hack 0 Further patching happens in LTO mode, see below. ### 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} If LTO is enabled, `%configure` applies some common required fixes to `configure` scripts. To disable that, define the RPM macro `_fix_broken_configure_for_lto` as `true` (sic; it has to be a shell command). ### 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`](https://gcc.gnu.org/gcc-10/porting_to.html#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](https://fedoraproject.org/wiki/Changes/SubpackageAndSourceDebuginfo) 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](https://fedoraproject.org/wiki/Changes/ParallelInstallableDebuginfo). * 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](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`). * `-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. * `-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 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](https://fedoraproject.org/wiki/Toolchain/Watermark) 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 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. * `-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: * **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=power9 -mtune=power9` selects a minimum supported CPU level of POWER9. * **s390x**: `-march=z14 -mtune=z15` specifies a minimum supported CPU level of z14, while optimizing for a subsequent CPU generation (z15). * **x86_64**: `-march=x86-64-v2 -mtune=generic` builds for the [x86-64-v2 micro-architecture level](https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/low-level-sys-info.tex) and 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.