linux-kernel - Re: [PATCH] x86: fix .bss with -fdata-sections

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Date:   Mon, 15 Apr 2019 10:33:33 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] x86: fix .bss with -fdata-sections

On Mon, Apr 15, 2019 at 9:50 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
>
> With LD_DEAD_CODE_DATA_ELIMINATION, we compile the kernel with
> -fdata-sections, which also splits the .bss section. Use the
> common BSS_MAIN macro in the linker script to properly merge all
> the generated sections.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  arch/x86/kernel/vmlinux.lds.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index bad8c51fee6e..a5127b2c195f 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -362,7 +362,7 @@ SECTIONS
>         .bss : AT(ADDR(.bss) - LOAD_OFFSET) {
>                 __bss_start = .;
>                 *(.bss..page_aligned)
> -               *(.bss)
> +               *(BSS_MAIN)
>                 BSS_DECRYPTED
>                 . = ALIGN(PAGE_SIZE);
>                 __bss_stop = .;
> --
> 2.21.0.392.gf8f6787159e-goog
>

Thanks for the patch!  I checked as well that this issue doesn't exist
for .text, .data, .sdata, .rodata, and .sbss; just .bss had this
issue.  Looks like this is "no functional change" when
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is not set; it simply fixes the
case where CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

Nicholas,
I don't understand the comment about RODATA_MAIN from commit
266ff2a8f51f ("kbuild: Fix asm-generic/vmlinux.lds.h for
LD_DEAD_CODE_DATA_ELIMINATION").  It seems that there are no uses of
RODATA_MAIN in the kernel.  Can we remove its definition and that
comment? (That's neither here nor there for this patch).
-- 
Thanks,
~Nick Desaulniers