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From:   Sedat Dilek <sedat.dilek@gmail.com>
Date:   Wed, 15 Jul 2020 10:52:20 +0200
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Subject: Re: [PATCH v5 1/7] x86/boot/compressed: Move .got.plt entries out of
 the .got section
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Nick Desaulniers <ndesaulniers@google.com>,
        Fangrui Song <maskray@google.com>,
        Dmitry Golovin <dima@golovin.in>,
        Clang-Built-Linux ML <clang-built-linux@googlegroups.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Daniel Kiper <daniel.kiper@oracle.com>,
        Kees Cook <keescook@chromium.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Arnd Bergmann <arnd@arndb.de>,
        "H . J . Lu" <hjl@sourceware.org>, linux-kernel@vger.kernel.org
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On Wed, Jul 15, 2020 at 2:41 AM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> From: Ard Biesheuvel <ardb@kernel.org>
>
> The .got.plt section contains the part of the GOT which is used by PLT
> entries, and which gets updated lazily by the dynamic loader when
> function calls are dispatched through those PLT entries.
>
> On fully linked binaries such as the kernel proper or the decompressor,
> this never happens, and so in practice, the .got.plt section consists
> only of the first 3 magic entries that are meant to point at the _DYNAMIC
> section and at the fixup routine in the loader. However, since we don't
> use a dynamic loader, those entries are never populated or used.
>
> This means that treating those entries like ordinary GOT entries, and
> updating their values based on the actual placement of the executable in
> memory is completely pointless, and we can just ignore the .got.plt
> section entirely, provided that it has no additional entries beyond
> the first 3 ones.
>
> So add an assertion in the linker script to ensure that this assumption
> holds, and move the contents out of the [_got, _egot) memory range that
> is modified by the GOT fixup routines.
>
> While at it, drop the KEEP(), since it has no effect on the contents
> of output sections that are created by the linker itself.
>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> Acked-by: Arvind Sankar <nivedita@alum.mit.edu>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> From: Ard Biesheuvel <ardb@kernel.org>
> Link: https://lore.kernel.org/r/20200523120021.34996-2-ardb@kernel.org

Tested-by: Sedat Dilek <sedat.dilek@gmail.com>

- Sedat -

> ---
>  arch/x86/boot/compressed/vmlinux.lds.S | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index 8f1025d1f681..b17d218ccdf9 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -44,10 +44,13 @@ SECTIONS
>         }
>         .got : {
>                 _got = .;
> -               KEEP(*(.got.plt))
>                 KEEP(*(.got))
>                 _egot = .;
>         }
> +       .got.plt : {
> +               *(.got.plt)
> +       }
> +
>         .data : {
>                 _data = . ;
>                 *(.data)
> @@ -77,3 +80,9 @@ SECTIONS
>
>         DISCARDS
>  }
> +
> +#ifdef CONFIG_X86_64
> +ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, "Unexpected GOT/PLT entries detected!")
> +#else
> +ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0xc, "Unexpected GOT/PLT entries detected!")
> +#endif
> --
> 2.26.2
>