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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 12 Jul 2021 10:04:54 -0700
Message-ID: <CAKwvOdmGhMbvtko2EVARg2E8zSX-LQKSPfdj6fbbAsCK4nw_Qg@mail.gmail.com>
Subject: Re: [PATCH] arm64: Restrict ARM64_BTI_KERNEL to clang 12.0.0 and newer
To:     Will Deacon <will@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        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 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


-- 
Thanks,
~Nick Desaulniers