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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 29 Jan 2021 12:18:28 -0800
Message-ID: <CAKwvOdkoivL4+HjXi4aKQbQ7R2J_oKBcMKrSsdxuOGLpseQZcw@mail.gmail.com>
Subject: Re: [PATCH] vmlinux.lds.h: Define SANTIZER_DISCARDS with CONFIG_GCOV_KERNEL=y
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Arnd Bergmann <arnd@arndb.de>, Kees Cook <keescook@chromium.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        kernel test robot <lkp@intel.com>,
        Fangrui Song <maskray@google.com>
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On Fri, Jan 29, 2021 at 12:11 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> clang produces .eh_frame sections when CONFIG_GCOV_KERNEL is enabled,
> even when -fno-asynchronous-unwind-tables is in KBUILD_CFLAGS:
>
> $ make CC=clang vmlinux
> ...
> ld: warning: orphan section `.eh_frame' from `init/main.o' being placed in section `.eh_frame'
> ld: warning: orphan section `.eh_frame' from `init/version.o' being placed in section `.eh_frame'
> ld: warning: orphan section `.eh_frame' from `init/do_mounts.o' being placed in section `.eh_frame'
> ld: warning: orphan section `.eh_frame' from `init/do_mounts_initrd.o' being placed in section `.eh_frame'
> ld: warning: orphan section `.eh_frame' from `init/initramfs.o' being placed in section `.eh_frame'
> ld: warning: orphan section `.eh_frame' from `init/calibrate.o' being placed in section `.eh_frame'
> ld: warning: orphan section `.eh_frame' from `init/init_task.o' being placed in section `.eh_frame'
> ...
>
> $ rg "GCOV_KERNEL|GCOV_PROFILE_ALL" .config
> CONFIG_GCOV_KERNEL=y
> CONFIG_ARCH_HAS_GCOV_PROFILE_ALL=y
> CONFIG_GCOV_PROFILE_ALL=y
>
> This was already handled for a couple of other options in
> commit d812db78288d ("vmlinux.lds.h: Avoid KASAN and KCSAN's unwanted
> sections") and there is an open LLVM bug for this issue. Take advantage
> of that section for this config as well so that there are no more orphan
> warnings.
>
> Link: https://bugs.llvm.org/show_bug.cgi?id=46478
> Link: https://github.com/ClangBuiltLinux/linux/issues/1069
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

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

I suspect we're going to need to add module level attributes in LLVM
IR for these options, then check those when synthesizing new function
definitions within LLVM.  At least we'll be able to point to this file
and say "hey, this is a general problem in LLVM, and here are 3
specific cases now where it's a problem."  Not a large problem, but
would help us save some bytes in the final object.  LLVM is not
producing data in this section for all code, just these synthesized
routines.

> ---
>  include/asm-generic/vmlinux.lds.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index b2b3d81b1535..f753fd449436 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -988,12 +988,13 @@
>  #endif
>
>  /*
> - * Clang's -fsanitize=kernel-address and -fsanitize=thread produce
> + * Clang's -fsanitize=kernel-address, -fsanitize=thread,
> + * and -fprofile-arcs -ftest-coverage produce unwanted
>   * unwanted sections (.eh_frame and .init_array.*), but
>   * CONFIG_CONSTRUCTORS wants to keep any .init_array.* sections.
>   * https://bugs.llvm.org/show_bug.cgi?id=46478
>   */
> -#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KCSAN)
> +#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KCSAN) || defined(CONFIG_GCOV_KERNEL)
>  # ifdef CONFIG_CONSTRUCTORS
>  #  define SANITIZER_DISCARDS                                           \
>         *(.eh_frame)
>
> base-commit: bec4c2968fce2f44ce62d05288a633cd99a722eb
> --
> 2.30.0
>


-- 
Thanks,
~Nick Desaulniers