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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 12 Jul 2021 16:15:47 -0700
Message-ID: <CAKwvOdmDOSY4du_6V0Mh5WdqWWU8nC_b-2Eii7Pa5fVhZXbG9A@mail.gmail.com>
Subject: Re: [PATCH v2] arm64: Restrict ARM64_BTI_KERNEL to clang 12.0.0 and newer
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com
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On Mon, Jul 12, 2021 at 2:51 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> Commit 97fed779f2a6 ("arm64: bti: Provide Kconfig for kernel mode BTI")
> disabled CONFIG_ARM64_BTI_KERNEL when CONFIG_GCOV_KERNEL was enabled and
> compiling with clang because of warnings that were seen with
> allmodconfig because LLVM was not emitting PAC/BTI instructions for
> compiler generated functions:
>
> warning: some functions compiled with BTI and some compiled without BTI
> warning: not setting BTI in feature flags
>
> This depedency was fine for avoiding the warnings with allmodconfig
> until commit 51c2ee6d121c ("Kconfig: Introduce ARCH_WANTS_NO_INSTR and
> CC_HAS_NO_PROFILE_FN_ATTR"), which prevents CONFIG_GCOV_KERNEL from
> being enabled with clang 12.0.0 or older because those versions do not
> support the no_profile_instrument_function attribute.
>
> As a result, CONFIG_ARM64_BTI_KERNEL gets enabled with allmodconfig and
> there are more warnings like the ones above due to CONFIG_KASAN, which
> suffers from the same problem as CONFIG_GCOV_KERNEL. This was most
> likely not noticed at the time because allmodconfig +
> CONFIG_GCOV_KERNEL=n was not tested. defconfig + CONFIG_KASAN=y is
> enough to reproduce the same warnings as above.
>
> The root cause of the warnings was resolved in LLVM during the 12.0.0
> release so rather than play whack-a-mole with the dependencies, just
> update CONFIG_ARM64_BTI_KERNEL to require clang 12.0.0, which will 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
> Link: https://github.com/llvm/llvm-project/commit/a88c722e687e6780dcd6a58718350dc76fcc4cc9
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>
> v1 -> v2:
>
> * Fix typo ("compilign" to "compiling") per Will.
>
> * Clarify that these warnings are reproducible back at the time of
>   97fed779f2a6 with allmodconfig + CONFIG_GCOV_KERNEL=n but that
>   configuration was most likely not tested.
>
> * Clarify that the warnings come from CONFIG_KASAN, which suffers from
>   the same issue at CONFIG_GCOV_KERNEL because there are compiler
>   generated functions.
>
> * Clarify that the root cause of the warnings is resolved in 12.0.0 and
>   link to the GitHub commit rather than the Phabricator instance, in
>   case LLVM moves to GitHub reviews in the future.
>
> Hopefully this is adequate explanation for what is going on, thanks to
> Will and Nick for providing input on the commit message.

Perfect, sorry for being a pedant on the commit message, but this
provides much more context should we need to revisit this again in the
future.  Never say never, (again)!  Thanks very much for this patch.

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

>
>  arch/arm64/Kconfig | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index e07e7de9ac49..b5b13a932561 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://github.com/llvm/llvm-project/commit/a88c722e687e6780dcd6a58718350dc76fcc4cc9
> +       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
>
> base-commit: e73f0f0ee7541171d89f2e2491130c7771ba58d3
> --
> 2.32.0.93.g670b81a890
>


-- 
Thanks,
~Nick Desaulniers