linux-kernel - Re: [PATCH 1/2] x86/boot/compressed/64: Remove .bss/.pgtable from bzImage

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Message-ID: <20200205162921.GA318609@rani.riverdale.lan>
Date:   Wed, 5 Feb 2020 11:29:22 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Arvind Sankar <nivedita@...m.mit.edu>
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,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/boot/compressed/64: Remove .bss/.pgtable from
 bzImage

On Thu, Jan 09, 2020 at 10:02:17AM -0500, Arvind Sankar wrote:
> Commit 5b11f1cee579 ("x86, boot: straighten out ranges to copy/zero in
> compressed/head*.S") introduced a separate .pgtable section, splitting
> it out from the rest of .bss. This section was added without the
> writeable flag, marking it as read-only. This results in the linker
> putting the .rela.dyn section (containing bogus dynamic relocations from
> head_64.o) after the .bss and .pgtable sections.
> 
> When we use objcopy to convert compressed/vmlinux into a binary for the
> bzImage, the .bss and .pgtable sections get materialized as ~176KiB of
> zero bytes in the binary in order to place .rela.dyn at the correct
> location.
> 
> Fix this by marking .pgtable as writeable. This moves the .rela.dyn
> section earlier so that .bss and .pgtable are the last allocated
> sections and so don't appear in bzImage.
> 
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> ---
>  arch/x86/boot/compressed/head_64.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index 58a512e33d8d..6eb30f8a3ce7 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -709,7 +709,7 @@ SYM_DATA_END_LABEL(boot_stack, SYM_L_LOCAL, boot_stack_end)
>  /*
>   * Space for page tables (not in .bss so not zeroed)
>   */
> -	.section ".pgtable","a",@nobits
> +	.section ".pgtable","aw",@nobits
>  	.balign 4096
>  SYM_DATA_LOCAL(pgtable,		.fill BOOT_PGT_SIZE, 1, 0)
>  
> -- 
> 2.24.1
> 

Gentle reminder.

https://lore.kernel.org/lkml/20200109150218.16544-1-nivedita@alum.mit.edu