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

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Message-ID: <20200219120938.GB30966@zn.tnic>
Date:   Wed, 19 Feb 2020 13:09:38 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "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 Tue, Feb 18, 2020 at 01:03:54PM -0500, Arvind Sankar wrote:
> On Wed, Feb 05, 2020 at 11:29:21AM -0500, Arvind Sankar wrote:
> > 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
> 
> Ping.

You keep pinging. Why? Is it a showstopper or what is the urgency here?

This is shaving off some 100 - 200 KiB from the final bzImage, AFAICT. Or
is there something more broken this is fixing?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette