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This document contains documentation of the individual compiler flags
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and how to use them.
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[TOC]
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# Using RPM build flags
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For packages which use autoconf to set up the build environment, use
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the `%configure` macro to obtain the full complement of flags, like
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this:
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%configure
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This will invoke the `./configure` with arguments (such as
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`--prefix=/usr`) to adjust the paths to the packaging defaults.
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Prior to that, some common problems in autotools scripts are
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automatically patched across the source tree.
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As a side effect, this will set the environment variables `CFLAGS`,
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`CXXFLAGS`, `FFLAGS`, `FCFLAGS`, `LDFLAGS` and `LT_SYS_LIBRARY_PATH`,
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so they can be used by makefiles and other build tools. (However,
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existing values for these variables are not overwritten.)
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If your package does not use autoconf, you can still set the same
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environment variables using
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%set_build_flags
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early in the `%build` section. (Again, existing environment variables
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are not overwritten.) `%set_build_flags` does not perform autotools
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script rewriting, unlike `%configure`.
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Individual build flags are also available through RPM macros:
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* `%{build_cflags}` for the C compiler flags (also known as the
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`CFLAGS` variable). Also historically available as `%{optflags}`.
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Furthermore, at the start of the `%build` section, the environment
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variable `RPM_OPT_FLAGS` is set to this value.
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* `%{build_cxxflags}` for the C++ compiler flags (usually assigned to
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the `CXXFLAGS` shell variable).
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* `%{build_fflags} for `FFLAGS` (the Fortran compiler flags, also
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known as the `FCFLAGS` variable).
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* `%{build_ldflags}` for the link editor (ld) flags, usually known as
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`LDFLAGS`. Note that the contents quotes linker arguments using
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`-Wl`, so this variable is intended for use with the `gcc` compiler
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driver. At the start of the `%build` section, the environment
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variable `RPM_LD_FLAGS` is set to this value.
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The variable `LT_SYS_LIBRARY_PATH` is defined here to prevent the `libtool`
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script (v2.4.6+) from hardcoding %_libdir into the binaries' RPATH.
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These RPM macros do not alter shell environment variables.
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For some other build tools separate mechanisms exist:
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* CMake builds use the the `%cmake` macro from the `cmake-rpm-macros`
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package.
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Care must be taking not to compile the current selection of compiler
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flags into any RPM package besides `redhat-rpm-config`, so that flag
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changes are picked up automatically once `redhat-rpm-config` is
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updated.
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# Flag selection for the build type
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The default flags are suitable for building applications.
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For building shared objects, you must compile with `-fPIC` in
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(`CFLAGS` or `CXXFLAGS`) and link with `-shared` (in `LDFLAGS`).
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For other considerations involving shared objects, see:
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* [Fedora Packaging Guidelines: Shared Libraries](https://docs.fedoraproject.org/en-US/packaging-guidelines/#_shared_libraries)
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# Customizing compiler and other build flags
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It is possible to set RPM macros to change some aspects of the
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compiler flags. Changing these flags should be used as a last
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recourse if other workarounds are not available.
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### Toolchain selection
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The default toolchain uses GCC, and the `%toolchain` macro is defined
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as `gcc`.
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It is enough to override `toolchain` macro and all relevant macro for C/C++
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compilers will be switched. Either in the spec or in the command-line.
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%global toolchain clang
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or:
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rpmbuild -D "toolchain clang" …
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
Inside a spec file it is also possible to determine which toolchain is in use
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
by testing the same macro. For example:
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
%if "%{toolchain}" == "gcc"
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
BuildRequires: gcc
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
%endif
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
or:
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
%if "%{toolchain}" == "clang"
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
BuildRequires: clang compiler-rt
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
%endif
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
### Disable autotools compatibility patching
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
By default, the invocation of the `%configure` macro replaces
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
`config.guess` files in the source tree with the system version. To
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
disable that, define this macro:
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
%global _configure_gnuconfig_hack 0
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
`%configure` also patches `ltmain.sh` scripts, so that linker flags
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
are set as well during libtool-. This can be switched off using:
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
%global _configure_libtool_hardening_hack 0
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
Further patching happens in LTO mode, see below.
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
### Disabling Link-Time Optimization
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
By default, builds use link-time optimization. In this build mode,
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
object code is generated at the time of the final link, by combining
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
information from all available translation units, and taking into
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
account which symbols are exported.
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
To disable this optimization, include this in the spec file:
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
%define _lto_cflags %{nil}
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
If LTO is enabled, `%configure` applies some common required fixes to
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
`configure` scripts. To disable that, define the RPM macro
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
`_fix_broken_configure_for_lto` as `true` (sic; it has to be a shell
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
command).
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
### Lazy binding
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
If your package depends on the semantics of lazy binding (e.g., it has
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
plugins which load additional plugins to complete their dependencies,
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
before which some referenced functions are undefined), you should put
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
`-Wl,-z,lazy` at the end of the `LDFLAGS` setting when linking objects
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
which have such requirements. Under these circumstances, it is
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
unnecessary to disable hardened builds (and thus lose full ASLR for
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
executables), or link everything without `-Wl,z,now` (non-lazy
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
binding).
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
### Hardened builds
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
By default, the build flags enable fully hardened builds. To change
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
this, include this in the RPM spec file:
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
%undefine _hardened_build
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
This turns off certain hardening features, as described in detail
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
below. The main difference is that executables will be
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
position-dependent (no full ASLR) and use lazy binding.
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
### Annotated builds/watermarking
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
By default, the build flags cause a special output section to be
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
included in ELF files which describes certain aspects of the build.
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
To change this for all compiler invocations, include this in the RPM
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
spec file:
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
%undefine _annotated_build
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
Be warned that this turns off watermarking, making it impossible to do
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
full hardening coverage analysis for any binaries produced.
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
It is possible to disable annotations for individual compiler
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
invocations, using the `-fplugin-arg-annobin-disable` flag. However,
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
the annobin plugin must still be loaded for this flag to be
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
recognized, so it has to come after the hardening flags on the command
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
line (it has to be added at the end of `CFLAGS`, or specified after
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
the `CFLAGS` variable contents).
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
### Keeping dependencies on unused shared objects
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
By default, ELF shared objects which are listed on the linker command
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
line, but which have no referencing symbols in the preceding objects,
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
are not added to the output file during the final link.
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
In order to keep dependencies on shared objects even if none of
|
|
![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
their symbols are used, include this in the RPM spec file:
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
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%undefine _ld_as_needed
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For example, this can be required if shared objects are used for their
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side effects in ELF constructors, or for making them available to
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dynamically loaded plugins.
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### Strict symbol checks in the link editor (ld)
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Optionally, the link editor will refuse to link shared objects which
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contain undefined symbols. Such symbols lack symbol versioning
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information and can be bound to the wrong (compatibility) symbol
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version at run time, and not the actual (default) symbol version which
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would have been used if the symbol definition had been available at
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static link time. Furthermore, at run time, the dynamic linker will
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not have complete dependency information (in the form of DT_NEEDED
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entries), which can lead to errors (crashes) if IFUNC resolvers are
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executed before the shared object containing them is fully relocated.
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To switch on these checks, define this macro in the RPM spec file:
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%define _strict_symbol_defs_build 1
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If this RPM spec option is active, link failures will occur if the
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linker command line does not list all shared objects which are needed.
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In this case, you need to add the missing DSOs (with linker arguments
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such as `-lm`). As a result, the link editor will also generated the
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necessary DT_NEEDED entries.
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
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In some cases (such as when a DSO is loaded as a plugin and is
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expected to bind to symbols in the main executable), undefined symbols
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are expected. In this case, you can add
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%undefine _strict_symbol_defs_build
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to the RPM spec file to disable these strict checks. Alternatively,
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you can pass `-z undefs` to ld (written as `-Wl,-z,undefs` on the gcc
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
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command line). The latter needs binutils 2.29.1-12.fc28 or later.
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f9efe2 |
### Legacy -fcommon
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f9efe2 |
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f9efe2 |
Since version 10, [gcc defaults to `-fno-common`](https://gcc.gnu.org/gcc-10/porting_to.html#common).
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f9efe2 |
Builds may fail with `multiple definition of ...` errors.
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f9efe2 |
As a short term workaround for such failure,
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
it is possible to add `-fcommon` to the flags by defining `%_legacy_common_support`.
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%define _legacy_common_support 1
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Properly fixing the failure is always preferred!
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### Post-build ELF object processing
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
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f9efe2 |
By default, DWARF debugging information is separated from installed
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
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ELF objects and put into `-debuginfo` subpackages. To disable most
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f9efe2 |
debuginfo processing (and thus the generation of these subpackages),
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define `_enable_debug_packages` as `0`.
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f9efe2 |
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f9efe2 |
Processing of debugging information is controlled using the
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f9efe2 |
`find-debuginfo` tool from the `debugedit` package. Several aspects
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of its operation can be controlled at the RPM level.
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
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f9efe2 |
* Creation of `-debuginfo` subpackages is enabled by default.
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f9efe2 |
To disable, undefine `_debuginfo_subpackages`.
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* Likewise, `-debugsource` subpackages are automatically created.
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f9efe2 |
To disable, undefine `_debugsource_subpackages`.
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
See [Separate Subpackage and Source Debuginfo](https://fedoraproject.org/wiki/Changes/SubpackageAndSourceDebuginfo)
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
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for background information.
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* `_build_id_links`, `_unique_build_ids`, `_unique_debug_names`,
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f9efe2 |
`_unique_debug_srcs` control how debugging information and
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
corresponding source files are represented on disk.
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f9efe2 |
See `/usr/lib/rpm/macros` for details. The defaults
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enable parallel installation of `-debuginfo` packages for
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
different package versions, as described in
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
[Parallel Installable Debuginfo](https://fedoraproject.org/wiki/Changes/ParallelInstallableDebuginfo).
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
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* By default, a compressed symbol table is preserved in the
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
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`.gnu_debugdata` section. To disable that, undefine
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
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`_include_minidebuginfo`.
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
* To speed up debuggers, a `.gdb_index` section is created. It can be
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
disabled by undefining `_include_gdb_index`.
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
* Missing build IDs result in a build failure. To ignore such
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
problems, undefine `_missing_build_ids_terminate_build`.
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
* During processing, build IDs are recomputed to match the binary
|
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
content. To skip this step, define `_no_recompute_build_ids` as `1`.
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
* By default, the options in `_find_debuginfo_dwz_opts` turn on `dwz`
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
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(DWARF compression) processing. Undefine this macro to disable this
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
step.
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f9efe2 |
* Additional options can be passed by defining the
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
`_find_debuginfo_opts` macro.
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![](https://seccdn.libravatar.org/avatar/eca4d0658e293ac828929c05684948cb411ba891fa2ad2130aafa91b664e89ad?s=16&d=retro) |
f9efe2 |
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f9efe2 |
After separation of debugging information, additional transformations
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are applied, most of them also related to debugging information.
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These steps can be skipped by undefining the corresponding macros:
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* `__brp_strip`: Removal of leftover debugging information. The tool
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specified by the `__strip` macro is invoked with the `-g` option on
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ELF object (`.o`) files.
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* `__brp_strip_static_archive`: This is similar to `__brp_strip`, but
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processes static `.a` archives instead.
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* `__brp_strip_comment_note`: This step removes unallocated `.note`
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sections, and `.comment` sections from ELF files.
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* `__brp_strip_lto`: This step removes GCC LTO intermediate representation
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in ELF sections starting with `.gnu.lto_` and `.gnu.debuglto_`. Skipping
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this step is strongly discouraged because the tight coupling of LTO
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data with the GCC version. The underlying tool is again determined by the
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`__strip` macro.
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* `__brp_llvm_compile_lto_elf`: This step replaces LLVM bitcode files
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with object files, thereby removing LLVM bitcode from the installed
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files. This transformation is applied to object files in static `.a`
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archives, too.
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* `__brp_ldconfig`: For each shared object on the library search path
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whose soname does not match its file name, a symbolic link from the
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soname to the file name is created. This way, these shared objects
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are loadable immediately after installation, even if they are not yet
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listed in the `/etc/ld.so.cache` file (because `ldconfig` has not been
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invoked yet).
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# Individual compiler flags
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Compiler flags end up in the environment variables `CFLAGS`,
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`CXXFLAGS`, `FFLAGS`, and `FCFLAGS`.
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The general (architecture-independent) build flags are:
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* `-O2`: Turn on various GCC optimizations. See the [GCC manual](https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-O2).
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Optimization improves performance, the accuracy of warnings, and the
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reach of toolchain-based hardening, but it makes debugging harder.
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* `-g`: Generate debugging information (DWARF). In Fedora, this data
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is separated into `-debuginfo` RPM packages whose installation is
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optional, so debuging information does not increase the size of
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installed binaries by default.
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* `-pipe`: Run compiler and assembler in parallel and do not use a
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temporary file for the assembler input. This can improve
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compilation performance. (This does not affect code generation.)
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* `-Wall`: Turn on various GCC warnings.
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See the [GCC manual](https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wall).
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* `-Werror=format-security`: Turn on format string warnings and treat
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them as errors.
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See the [GCC manual](https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wformat-security).
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This can occasionally result in compilation errors. In this case,
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the best option is to rewrite the source code so that only constant
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format strings (string literals) are used.
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* `-Wp,-D_FORTIFY_SOURCE=2`: Source fortification activates various
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hardening features in glibc:
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* String functions such as `memcpy` attempt to detect buffer lengths
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and terminate the process if a buffer overflow is detected.
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* `printf` format strings may only contain the `%n` format specifier
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if the format string resides in read-only memory.
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* `open` and `openat` flags are checked for consistency with the
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presence of a *mode* argument.
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* Plus other minor hardening changes.
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(These changes can occasionally break valid programs.)
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* `-fexceptions`: Provide exception unwinding support for C programs.
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See the [`-fexceptions` option in the GCC
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manual](https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#index-fexceptions)
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and the [`cleanup` variable
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attribute](https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-cleanup-variable-attribute).
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This also hardens cancellation handling in C programs because
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it is not required to use an on-stack jump buffer to install
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a cancellation handler with `pthread_cleanup_push`. It also makes
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it possible to unwind the stack (using C++ `throw` or Rust panics)
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from C callback functions if a C library supports non-local exits
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from them (e.g., via `longjmp`).
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* `-fasynchronous-unwind-tables`: Generate full unwind information
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covering all program points. This is required for support of
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asynchronous cancellation and proper unwinding from signal
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handlers. It also makes performance and debugging tools more
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useful because unwind information is available without having to
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install (and load) debugging information.
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* `-Wp,-D_GLIBCXX_ASSERTIONS`: Enable lightweight assertions in the
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C++ standard library, such as bounds checking for the subscription
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operator on vectors. (This flag is added to both `CFLAGS` and
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`CXXFLAGS`; C compilations will simply ignore it.)
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* `-fstack-protector-strong`: Instrument functions to detect
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stack-based buffer overflows before jumping to the return address on
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the stack. The *strong* variant only performs the instrumentation
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for functions whose stack frame contains addressable local
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variables. (If the address of a variable is never taken, it is not
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possible that a buffer overflow is caused by incorrect pointer
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arithmetic involving a pointer to that variable.)
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* `-fstack-clash-protection`: Turn on instrumentation to avoid
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skipping the guard page in large stack frames. (Without this flag,
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vulnerabilities can result where the stack overlaps with the heap,
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or thread stacks spill into other regions of memory.) This flag is
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fully ABI-compatible and has adds very little run-time overhead.
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This flag is currently not available on aarch64 with the `clang` toolchain.
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* `-flto=auto`: Enable link-time optimization (LTO), using `make` job server
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integration for parallel processing. (`gcc` toolchain only)
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* `-ffat-lto-objects`: Generate EFL object files which contain both
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object code and LTO intermediate representation. (`gcc` toolchain only)
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* `-flto`: Enable link-time optimization. (`clang` toolchain only)
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* `-grecord-gcc-switches`: Include select GCC command line switches in
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the DWARF debugging information. This is useful for detecting the
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presence of certain build flags and general hardening coverage.
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* `-fcommon`: This optional flag is used to build legacy software
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which relies on C tentative definitions. It is disabled by default.
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For hardened builds (which are enabled by default, see above for how
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to disable them), the flag
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`-specs=/usr/lib/rpm/redhat/redhat-hardened-cc1` is added to the
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command line. It adds the following flag to the command line:
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* `-fPIE`: Compile for a position-independent executable (PIE),
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enabling full address space layout randomization (ASLR). This is
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similar to `-fPIC`, but avoids run-time indirections on certain
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architectures, resulting in improved performance and slightly
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smaller executables. However, compared to position-dependent code
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(the default generated by GCC), there is still a measurable
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performance impact.
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If the command line also contains `-r` (producing a relocatable
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object file), `-fpic` or `-fPIC`, this flag is automatically
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dropped. (`-fPIE` can only be used for code which is linked into
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the main program.) Code which goes into static libraries should be
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compiled with `-fPIE`, except when this code is expected to be
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linked into DSOs, when `-fPIC` must be used.
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To be effective, `-fPIE` must be used with the `-pie` linker flag
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when producing an executable, see below.
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To support [binary watermarks for ELF
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objects](https://fedoraproject.org/wiki/Toolchain/Watermark) using
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annobin, the `-specs=/usr/lib/rpm/redhat/redhat-annobin-cc1` flag is
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added by default (with the `gcc` toolchain). This can be switched off
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by undefining the `%_annotated_build` RPM macro (see above). Binary
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watermarks are currently disabled with the `clang` toolchain.
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### Architecture-specific compiler flags
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These compiler flags are enabled for all builds (hardened/annotated or
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not), but their selection depends on the architecture:
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* `-fcf-protection`: Instrument binaries to guard against
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ROP/JOP attacks. Used on i686 and x86_64.
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* `-m64` and `-m32`: Some GCC builds support both 32-bit and 64-bit in
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the same compilation. For such architectures, the RPM build process
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explicitly selects the architecture variant by passing this compiler
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flag.
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In addition, `redhat-rpm-config` re-selects the built-in default
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tuning in the `gcc` package. These settings are:
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* **i686**: `-march=i686` is used to select a minmum support CPU level
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of i686 (corresponding to the Pentium Pro). SSE2 support is
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enabled with `-msse2` (so only CPUs with SSE2 support can run the
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compiled code; SSE2 was introduced first with the Pentium 4).
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`-mtune=generic` activates tuning for a current blend of CPUs
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(under the assumption that most users of i686 packages obtain them
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through an x86_64 installation on current hardware).
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`-mfpmath=sse` instructs GCC to use the SSE2 unit for floating
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point math to avoid excess precision issues. `-mstackrealign`
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avoids relying on the stack alignment guaranteed by the current
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version of the i386 ABI.
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* **ppc64le**: `-mcpu=power9 -mtune=power9` selects a minimum supported
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CPU level of POWER9.
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* **s390x**: `-march=z14 -mtune=z15` specifies a minimum supported CPU
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level of z14, while optimizing for a subsequent CPU generation
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(z15).
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* **x86_64**: `-march=x86-64-v2 -mtune=generic` builds for the
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[x86-64-v2 micro-architecture level](https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/low-level-sys-info.tex)
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and selects tuning which is expected to beneficial for a broad range
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of current CPUs.
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* **aarch64** does not have any architecture-specific tuning.
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# Individual linker flags
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Linker flags end up in the environment variable `LDFLAGS`.
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The linker flags listed below are injected. Note that they are
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prefixed with `-Wl` because it is expected that these flags are passed
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to the compiler driver `gcc`, and not directly to the link editor
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`ld`.
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* `-z relro`: Activate the *read-only after relocation* feature.
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Constant data and relocations are placed on separate pages, and the
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dynamic linker is instructed to revoke write permissions after
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dynamic linking. Full protection of relocation data requires the
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`-z now` flag (see below).
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* `--as-needed`: In the final link, only generate ELF dependencies
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for shared objects that actually provide symbols required by the link.
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Shared objects which are not needed to fulfill symbol dependencies
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are essentially ignored due to this flag.
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* `-z defs`: Refuse to link shared objects (DSOs) with undefined symbols
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(optional, see above).
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For hardened builds, the
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`-specs=/usr/lib/rpm/redhat/redhat-hardened-ld` flag is added to the
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compiler driver command line. (This can be disabled by undefining the
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`%_hardened_build` macro; see above) This activates the following
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linker flags:
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* `-pie`: Produce a PIE binary. This is only activated for the main
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executable, and only if it is dynamically linked. This requires
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that all objects which are linked in the main executable have been
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compiled with `-fPIE` or `-fPIC` (or `-fpie` or `-fpic`; see above).
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By itself, `-pie` has only a slight performance impact because it
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disables some link editor optimization, however the `-fPIE` compiler
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flag has some overhead.
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* `-z now`: Disable lazy binding and turn on the `BIND_NOW` dynamic
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linker feature. Lazy binding involves an array of function pointers
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which is writable at run time (which could be overwritten as part of
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security exploits, redirecting execution). Therefore, it is
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preferable to turn of lazy binding, although it increases startup
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time.
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# Support for extension builders
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Some packages include extension builders that allow users to build
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extension modules (which are usually written in C and C++) under the
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control of a special-purpose build system. This is a common
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functionality provided by scripting languages such as Python and Perl.
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Traditionally, such extension builders captured the Fedora build flags
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when these extension were built. However, these compiler flags are
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adjusted for a specific Fedora release and toolchain version and
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therefore do not work with a custom toolchain (e.g., different C/C++
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compilers), and users might want to build their own extension modules
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with such toolchains.
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The macros `%{extension_cflags}`, `%{extension_cxxflags}`,
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`%{extension_fflags}`, `%{extension_ldflags}` contain a subset of
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flags that have been adjusted for compatibility with alternative
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toolchains, while still preserving some of the compile-time security
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hardening that the standard Fedora build flags provide.
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The current set of differences are:
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* No GCC plugins (such as annobin) are activated.
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* No GCC spec files (`-specs=` arguments) are used.
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Additional flags may be removed in the future if they prove to be
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incompatible with alternative toolchains.
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Extension builders should detect whether they are performing a regular
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RPM build (e.g., by looking for an `RPM_OPT_FLAGS` variable). In this
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case, they should use the *current* set of Fedora build flags (that
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is, the output from `rpm --eval '%{build_cflags}'` and related
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commands). Otherwise, when not performing an RPM build, they can
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either use hard-coded extension builder flags (thus avoiding a
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run-time dependency on `redhat-rpm-config`), or use the current
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extension builder flags (with a run-time dependency on
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`redhat-rpm-config`).
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As a result, extension modules built for Fedora will use the official
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Fedora build flags, while users will still be able to build their own
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extension modules with custom toolchains.
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