Commit 262448f3 authored Mar 18, 2022 by Nathan Chancellor Committed by Peter Zijlstra Mar 22, 2022

x86/Kconfig: Only enable CONFIG_CC_HAS_IBT for clang >= 14.0.0

Commit 156ff4a5 ("x86/ibt: Base IBT bits") added a check for a crash
with 'clang -fcf-protection=branch -mfentry -pg', which intended to
exclude Clang versions older than 14.0.0 from selecting
CONFIG_X86_KERNEL_IBT.

clang-11 does not have the issue that the check is testing for, so
CONFIG_X86_KERNEL_IBT is selectable. Unfortunately, there is a different
crash in clang-11 that was fixed in clang-12. To make matters worse,
that crash does not appear to be entirely deterministic, as the same
input to the compiler will sometimes crash and other times not, which
makes dynamically checking for the crash like the '-pg' one unreliable.

To make everything work properly for all common versions of clang, use a
hard version check of 14.0.0, as that will be the first release upstream
that has both bugs properly fixed.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220318230747.3900772-2-nathan@kernel.org

parent d31ed5d7

arch/x86/Kconfig

+3 −2

Original line number	Diff line number	Diff line
		@@ -1865,9 +1865,10 @@ config CC_HAS_IBT
		# GCC >= 9 and binutils >= 2.29
		# Retpoline check to work around https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93654
		# Clang/LLVM >= 14
		# fentry check to work around https://reviews.llvm.org/D111108
		# https://github.com/llvm/llvm-project/commit/e0b89df2e0f0130881bf6c39bf31d7f6aac00e0f
		# https://github.com/llvm/llvm-project/commit/dfcf69770bc522b9e411c66454934a37c1f35332
		def_bool ((CC_IS_GCC && $(cc-option, -fcf-protection=branch -mindirect-branch-register)) \|\| \
		(CC_IS_CLANG && $(success,echo "void a(void) {}" \| $(CC) -Werror $(CLANG_FLAGS) -fcf-protection=branch -mfentry -pg -x c - -c -o /dev/null))) && \
		(CC_IS_CLANG && CLANG_VERSION >= 140000)) && \
		$(as-instr,endbr64)

		config X86_KERNEL_IBT