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Subject: Re: [PATCH v2 1/2] gcov: fix clang-11+ support
To:     Nick Desaulniers <ndesaulniers@google.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Nathan Chancellor <nathan@kernel.org>,
        linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com,
        Fangrui Song <maskray@google.com>,
        Prasad Sodagudi <psodagud@quicinc.com>, stable@vger.kernel.org
References: <20210312220518.rz6cjh33bkwaumzz@archlinux-ax161>
 <20210312224132.3413602-1-ndesaulniers@google.com>
 <20210312224132.3413602-2-ndesaulniers@google.com>
From:   Peter Oberparleiter <oberpar@linux.ibm.com>
Message-ID: <67b313b0-3254-b394-7aa3-69113fe32838@linux.ibm.com>
Date:   Mon, 15 Mar 2021 14:51:36 +0100
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On 12.03.2021 23:41, Nick Desaulniers wrote:
> LLVM changed the expected function signatures for llvm_gcda_start_file()
> and llvm_gcda_emit_function() in the clang-11 release. Users of clang-11
> or newer may have noticed their kernels failing to boot due to a panic
> when enabling CONFIG_GCOV_KERNEL=y +CONFIG_GCOV_PROFILE_ALL=y.  Fix up
> the function signatures so calling these functions doesn't panic the
> kernel.
> 
> Link: https://reviews.llvm.org/rGcdd683b516d147925212724b09ec6fb792a40041
> Link: https://reviews.llvm.org/rG13a633b438b6500ecad9e4f936ebadf3411d0f44
> Cc: stable@vger.kernel.org # 5.4
> Reported-by: Prasad Sodagudi <psodagud@quicinc.com>
> Suggested-by: Nathan Chancellor <nathan@kernel.org>
> Reviewed-by: Fangrui Song <maskray@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> Tested-by: Nathan Chancellor <nathan@kernel.org>

Looks good to me (minus the code duplication - but that's IMO acceptable
since it's cleaned up again with patch 2).

Acked-by: Peter Oberparleiter <oberpar@linux.ibm.com>

That said, I'm currently thinking of adding a compile time check that
performs a dry-run gcov_info => gcda conversion in user space to detect
these kind of issues before kernels fail unpredictably [1]. I'm
confident that this could work for the GCC gcov kernel code, not sure
about the Clang version though. But if it's possible I guess it would
make sense to extend this to include the Clang code as well.

Note that this check wouldn't work for cross-compiles since the build
machine must be able to run code for the target machine.

[1]
https://lore.kernel.org/lkml/1c7a49e7-0e27-561b-a2f9-d42a83dc4c29@linux.ibm.com/


Regards,
  Peter

-- 
Peter Oberparleiter
Linux on Z Development - IBM Germany