linux-kernel - Re: [PATCH v2] vmlinux.lds.h: Handle clang's module.{c,d}tor sections

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Message-ID: <CANpmjNMJR7A5FyPLuK+mWLKjZ7z4qJfygXWFpsADxicYE=Kx=g@mail.gmail.com>
Date:   Sat, 31 Jul 2021 11:08:54 +0200
From:   Marco Elver <elver@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>, Arnd Bergmann <arnd@...db.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Fangrui Song <maskray@...gle.com>, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        clang-built-linux@...glegroups.com, stable@...r.kernel.org
Subject: Re: [PATCH v2] vmlinux.lds.h: Handle clang's module.{c,d}tor sections

On Sat, 31 Jul 2021 at 04:33, Nathan Chancellor <nathan@...nel.org> wrote:
> A recent change in LLVM causes module_{c,d}tor sections to appear when
> CONFIG_K{A,C}SAN are enabled, which results in orphan section warnings
> because these are not handled anywhere:
>
> ld.lld: warning: arch/x86/pci/built-in.a(legacy.o):(.text.asan.module_ctor) is being placed in '.text.asan.module_ctor'
> ld.lld: warning: arch/x86/pci/built-in.a(legacy.o):(.text.asan.module_dtor) is being placed in '.text.asan.module_dtor'
> ld.lld: warning: arch/x86/pci/built-in.a(legacy.o):(.text.tsan.module_ctor) is being placed in '.text.tsan.module_ctor'
>
> Fangrui explains: "the function asan.module_ctor has the SHF_GNU_RETAIN
> flag, so it is in a separate section even with -fno-function-sections
> (default)".
>
> Place them in the TEXT_TEXT section so that these technologies continue
> to work with the newer compiler versions. All of the KASAN and KCSAN
> KUnit tests continue to pass after this change.
>
> Cc: stable@...r.kernel.org
> Link: https://github.com/ClangBuiltLinux/linux/issues/1432
> Link: https://github.com/llvm/llvm-project/commit/7b789562244ee941b7bf2cefeb3fc08a59a01865
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>

Acked-by: Marco Elver <elver@...gle.com>

For KASAN module_ctors are very much required to support detecting
globals out-of-bounds: https://reviews.llvm.org/D81390
For KASAN the test would have revealed that at the latest.

KCSAN does not yet have much use for the module_ctors, but it may
change in future, so keeping them all was the right call.

Thanks,
-- Marco

> ---
>
> v1 -> v2:
>
> * Fix inclusion of .text.tsan.* (Nick)
>
> * Drop .text.asan as it does not exist plus it would be handled by a
>   different line (Fangrui)
>
> * Add Fangrui's explanation about why the LLVM commit caused these
>   sections to appear.
>
>  include/asm-generic/vmlinux.lds.h | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 17325416e2de..62669b36a772 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -586,6 +586,7 @@
>                 NOINSTR_TEXT                                            \
>                 *(.text..refcount)                                      \
>                 *(.ref.text)                                            \
> +               *(.text.asan.* .text.tsan.*)                            \
>                 TEXT_CFI_JT                                             \
>         MEM_KEEP(init.text*)                                            \
>         MEM_KEEP(exit.text*)                                            \
>
> base-commit: 4669e13cd67f8532be12815ed3d37e775a9bdc16
> --
> 2.32.0.264.g75ae10bc75
>