| Message ID | 20210709000627.3183718-1-nathan@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | arm64: Restrict ARM64_BTI_KERNEL to clang 12.0.0 and newer | expand |
Hi Nathan, On Thu, Jul 08, 2021 at 05:06:27PM -0700, Nathan Chancellor wrote: > Commit 97fed779f2a6 ("arm64: bti: Provide Kconfig for kernel mode BTI") > disabled CONFIG_ARM64_BTI_KERNEL when CONFIG_GCOV_KERNEL was enabled and > compilign with clang because of warnings that occur because LLVM was not Typo: "compilign" > emitting PAC/BTI instructions for compiler generated functions. > > This was an adequate fix to avoid the warnings with allmodconfig until > commit 51c2ee6d121c ("Kconfig: Introduce ARCH_WANTS_NO_INSTR and > CC_HAS_NO_PROFILE_FN_ATTR"), which prevented CONFIG_GCOV_KERNEL from > being selected with clang 12.0.0 and older because it does not support > the no_profile_instrument_function attribute. As a result, > CONFIG_ARM64_BTI_KERNEL gets enabled and there are more warnings of this > nature. I'm half asleep today, but I'm struggling to follow what GCOV_KERNEL has to do with the warnings here. Prior to 51c2ee6d121c, you could still go and turn that option off and so wouldn't the same warnings be triggered in that configuration? In other words, I think there are two aspects to this patch: 1. Removing the (now useless) !GCOV_KERNEL dependency 2. Requiring Clang >= version 12 but the commit message doesn't really say where these warnings are coming from or why Clang 12 gets rid of them. Thanks, Will
On Mon, Jul 12, 2021 at 5:24 AM Will Deacon <will@kernel.org> wrote: > > Hi Nathan, > > On Thu, Jul 08, 2021 at 05:06:27PM -0700, Nathan Chancellor wrote: > > Commit 97fed779f2a6 ("arm64: bti: Provide Kconfig for kernel mode BTI") > > disabled CONFIG_ARM64_BTI_KERNEL when CONFIG_GCOV_KERNEL was enabled and > > compilign with clang because of warnings that occur because LLVM was not > > Typo: "compilign" > > > emitting PAC/BTI instructions for compiler generated functions. > > > > This was an adequate fix to avoid the warnings with allmodconfig until > > commit 51c2ee6d121c ("Kconfig: Introduce ARCH_WANTS_NO_INSTR and > > CC_HAS_NO_PROFILE_FN_ATTR"), which prevented CONFIG_GCOV_KERNEL from > > being selected with clang 12.0.0 and older because it does not support > > the no_profile_instrument_function attribute. As a result, > > CONFIG_ARM64_BTI_KERNEL gets enabled and there are more warnings of this > > nature. > > I'm half asleep today, but I'm struggling to follow what GCOV_KERNEL has > to do with the warnings here. Prior to 51c2ee6d121c, you could still go > and turn that option off and so wouldn't the same warnings be triggered > in that configuration? Nathan, consider if any of the below can be reused to reword the commit message? ``` Prior to 51c2ee6d121c, allmodconfig with clang-11 would not select ARM64_BTI_KERNEL because GCOV_KERNEL already had been. After 51c2ee6d121c, allmodconfig with clang-11 does select ARM64_BTI_KERNEL because GCOV_KERNEL cannot be selected. clang-11 had basic support for BTI, but failed to instrument compiler generated functions properly. This produces the following stream of warnings for clang-11 allmodconfig builds of ARCH=arm64: warning: some functions compiled with BTI and some compiled without BTI warning: not setting BTI in feature flags This was fixed in clang-12 via https://reviews.llvm.org/D85649. ``` Perhaps its of interest to find which config in (allmodconfig minus GCOV_KERNEL) but not in (defconfig) produces these, since I don't think we see them in defconfig builds of clang-11? https://github.com/ClangBuiltLinux/continuous-integration2/runs/3009146591?check_suite_focus=true > > In other words, I think there are two aspects to this patch: > > 1. Removing the (now useless) !GCOV_KERNEL dependency > 2. Requiring Clang >= version 12 > > but the commit message doesn't really say where these warnings are coming > from or why Clang 12 gets rid of them. > > Thanks, > > Will
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index e07e7de9ac49..c2127595980f 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -1605,7 +1605,8 @@ config ARM64_BTI_KERNEL depends on CC_HAS_BRANCH_PROT_PAC_RET_BTI # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94697 depends on !CC_IS_GCC || GCC_VERSION >= 100100 - depends on !(CC_IS_CLANG && GCOV_KERNEL) + # https://reviews.llvm.org/D85649 + depends on !CC_IS_CLANG || CLANG_VERSION >= 120000 depends on (!FUNCTION_GRAPH_TRACER || DYNAMIC_FTRACE_WITH_REGS) help Build the kernel with Branch Target Identification annotations
Commit 97fed779f2a6 ("arm64: bti: Provide Kconfig for kernel mode BTI") disabled CONFIG_ARM64_BTI_KERNEL when CONFIG_GCOV_KERNEL was enabled and compilign with clang because of warnings that occur because LLVM was not emitting PAC/BTI instructions for compiler generated functions. This was an adequate fix to avoid the warnings with allmodconfig until commit 51c2ee6d121c ("Kconfig: Introduce ARCH_WANTS_NO_INSTR and CC_HAS_NO_PROFILE_FN_ATTR"), which prevented CONFIG_GCOV_KERNEL from being selected with clang 12.0.0 and older because it does not support the no_profile_instrument_function attribute. As a result, CONFIG_ARM64_BTI_KERNEL gets enabled and there are more warnings of this nature. Rather than play whack-a-mole with the config options, just update this condition to require clang 12.0.0, which should have all of the issues ironed out. Link: https://github.com/ClangBuiltLinux/linux/issues/1428 Link: https://github.com/ClangBuiltLinux/continuous-integration2/runs/3010034706?check_suite_focus=true Link: https://github.com/ClangBuiltLinux/continuous-integration2/runs/3010035725?check_suite_focus=true Signed-off-by: Nathan Chancellor <nathan@kernel.org> --- arch/arm64/Kconfig | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) base-commit: f55966571d5eb2876a11e48e798b4592fa1ffbb7