commit: 6bb5978549aaac5802f704d4aa1a53ad4fcdbcd7 Author: Sam James <sam <AT> gentoo <DOT> org> AuthorDate: Mon Jan 23 06:21:41 2023 +0000 Commit: Sam James <sam <AT> gentoo <DOT> org> CommitDate: Mon Jan 23 06:22:23 2023 +0000 URL: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=6bb59785
sys-devel/gcc: provide detailed explanations of USE flags Prompted by a question on the forums, but been meaning to do this for a while: https://forums.gentoo.org/viewtopic-t-1160941.html. Folks may also be interested in https://wiki.gentoo.org/wiki/Hardened/Toolchain#Changes. Signed-off-by: Sam James <sam <AT> gentoo.org> sys-devel/gcc/metadata.xml | 81 +++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 73 insertions(+), 8 deletions(-) diff --git a/sys-devel/gcc/metadata.xml b/sys-devel/gcc/metadata.xml index 4d9f63a02b27..ccedf9b38e3f 100644 --- a/sys-devel/gcc/metadata.xml +++ b/sys-devel/gcc/metadata.xml @@ -7,29 +7,94 @@ </maintainer> <use> <flag name="ada">Build the ADA language (GNAT) frontend</flag> - <flag name="cet" restrict=">=sys-devel/gcc-10">Enable support for Intel Control Flow Enforcement Technology (CET)</flag> + <flag name="cet" restrict=">=sys-devel/gcc-10"> + Enable support for Intel Control Flow Enforcement Technology (CET). + + Only provides benefits on newer CPUs. For Intel, the CPU + must be at least as new as Tiger Lake. For AMD, it must be + at least as new as Zen 3. This is harmless on older CPUs, + but provides no benefit either. + + When combined with USE=hardened, GCC will set -fcf-protection + by default when building software. The effect is minimal + on systems which do not support it, other than a possible + small increase in codesize for the NOPs. + </flag> <flag name="d">Enable support for the D programming language</flag> - <flag name="default-stack-clash-protection">Build packages with stack clash protection on by default</flag> - <flag name="default-znow">Request full relocation on start from ld.so by default</flag> + <flag name="default-stack-clash-protection"> + Build packages with stack clash protection on by default as + a hardening measure. + + This enables -fstack-clash-protection by default which protects against + large memory allocations allowing stack smashing. + + May cause slightly increased codesize, but modern compilers + have been adapted to optimize well for this case, as + this mitigation is now quite common. + + See https://developers.redhat.com/blog/2020/05/22/stack-clash-mitigation-in-gcc-part-3 + and https://www.qualys.com/2017/06/19/stack-clash/stack-clash.txt. + </flag> + <flag name="default-znow"> + Request full relocation on start from ld.so by default. + + This sets the -z,now (BIND_NOW) flag by default on all linker invocations. By + resolving all dynamic symbols at application startup, parts of the program + can be made read-only as a hardening measure. + + This is closely related to RELRO which is also separately + enabled by default. + + In some applications with many unresolved symbols (heavily plugin based, + for example), startup time may be impacted. + </flag> <flag name="fixed-point">Enable fixed-point arithmetic support for MIPS targets in gcc (Warning: significantly increases compile time!)</flag> <flag name="go">Build the GCC Go language frontend.</flag> <flag name="graphite">Add support for the framework for loop optimizations based on a polyhedral intermediate representation</flag> <flag name="ieee-long-double">Use accelerated 128-bit IEEE long double ABI (ppc64le only)</flag> <flag name="jit">Enable libgccjit so other applications can embed gcc for Just-In-Time compilation.</flag> <flag name="libssp">Build SSP support into a dedicated library rather than use the code in the C library (DO NOT ENABLE THIS IF YOU DON'T KNOW WHAT IT DOES)</flag> - <flag name="lto">Build using Link Time Optimizations (LTO)</flag> + <flag name="lto"> + Build using Link Time Optimizations (LTO). + + Note that GCC is always built with support for building + other programs with LTO. This USE flag is for whether + GCC itself is built and optimized with LTO. + </flag> <flag name="mpx">Enable support for Intel Memory Protection Extensions (MPX)</flag> <flag name="nptl">Enable support for Native POSIX Threads Library, the new threading module (requires linux-2.6 or better usually)</flag> <flag name="objc">Build support for the Objective C code language</flag> <flag name="objc++">Build support for the Objective C++ language</flag> <flag name="objc-gc">Build support for the Objective C code language Garbage Collector</flag> - <flag name="pgo">Build GCC using Profile Guided Optimization (PGO)</flag> + <flag name="pgo"> + Build GCC using Profile Guided Optimization (PGO). + + GCC will build itself and then analyze the just-built + binary and then rebuild itself using the data obtained + from analysis of codepaths taken. + + It does not affect whether GCC itself supports PGO + when building other software. + + This substantially increases the build time needed for + building GCC itself. + </flag> <flag name="rust">Build support for the Rust language, installs gccrs.</flag> - <flag name="sanitize">Build support for various sanitizer functions (ASAN/TSAN/etc...)</flag> - <flag name="ssp">Build packages with stack smashing protector on by default</flag> + <flag name="sanitize"> + Build support for various sanitizer functions (ASAN/TSAN/etc...) + to find runtime problems in applications. + </flag> + <flag name="ssp">Build packages with stack smashing protection on by default</flag> <flag name="systemtap">enable systemtap static probe points</flag> <flag name="valgrind">Enable valgrind annotations for gcc internals (useful for gcc debugging).</flag> - <flag name="vtv">Build support for virtual table verification (a C++ hardening feature)</flag> + <flag name="vtv"> + Build support for virtual table verification (a C++ hardening feature). + + This does not control whether GCC defaults to using VTV> + + Note that actually using VTV breaks ABI and hence the whole + system must be built with -fvtable-verify. + </flag> </use> <upstream> <remote-id type="cpe">cpe:/a:gnu:gcc</remote-id>
