linux-kernel - Re: [PATCH] x86/purgatory: strip debug info

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Date:   Fri, 31 Jul 2020 11:18:18 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Pingfan Liu <kernelfans@...il.com>, Steve Wahl <steve.wahl@....com>
Cc:     "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/purgatory: strip debug info

On Fri, Jul 31, 2020 at 2:36 AM Pingfan Liu <kernelfans@...il.com> wrote:
>
> On Fri, Jul 31, 2020 at 7:11 AM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> >
> > On Thu, Jul 30, 2020 at 1:27 AM Pingfan Liu <kernelfans@...il.com> wrote:
> > >
> > > It is useless to keep debug info in purgatory. And discarding them saves
> > > about 200K space.
> > >
> > > Original:
> > >   259080  kexec-purgatory.o
> > > Stripped:
> > >    29152  kexec-purgatory.o
> > >
> > > Signed-off-by: Pingfan Liu <kernelfans@...il.com>
> > > Cc: Thomas Gleixner <tglx@...utronix.de>
> > > Cc: Ingo Molnar <mingo@...hat.com>
> > > Cc: Borislav Petkov <bp@...en8.de>
> > > Cc: "H. Peter Anvin" <hpa@...or.com>
> > > Cc: Hans de Goede <hdegoede@...hat.com>
> > > Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> > > Cc: Arvind Sankar <nivedita@...m.mit.edu>
> > > Cc: Steve Wahl <steve.wahl@....com>
> > > Cc: linux-kernel@...r.kernel.org
> > > To: x86@...nel.org
> >
> > I don't see any code in
> > arch/x86/purgatory/
> > arch/x86/include/asm/purgatory.h
> > include/linux/purgatory.h
> > include/uapi/linux/kexec.h
> > kernel/kexec*
> > include/linux/kexec.h
> > include/linux/crash_dump.h
> > kernel/crash_dump.c
> > arch/x86/kernel/crash*
> > https://github.com/horms/kexec-tools/tree/master/kexec/arch/x86_64
> > that mentions any kind of debug info section.  I'm not sure what you'd
> > do with the debug info anyway for this binary.  So I suspect this
> > information should ok to discard.
> >
> > This works, but it might be faster to build to not generate the
> > compile info in the first place via compile flag `-g0`, which could be
> > added `ifdef CONFIG_DEBUG_INFO` or even just unconditionally.  That
> > way we're not doing additional work to generate debug info, then
> > additional work to throw it away.
> What about:
> diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
> index 088bd76..7e1ad9e 100644
> --- a/arch/x86/purgatory/Makefile
> +++ b/arch/x86/purgatory/Makefile
> @@ -32,7 +32,7 @@ KCOV_INSTRUMENT := n
>  # make up the standalone purgatory.ro
>
>  PURGATORY_CFLAGS_REMOVE := -mcmodel=kernel
> -PURGATORY_CFLAGS := -mcmodel=large -ffreestanding -fno-zero-initialized-in-bss
> +PURGATORY_CFLAGS := -mcmodel=large -ffreestanding
> -fno-zero-initialized-in-bss -g0
>  PURGATORY_CFLAGS += $(DISABLE_STACKLEAK_PLUGIN) -DDISABLE_BRANCH_PROFILING
>  PURGATORY_CFLAGS += $(call cc-option,-fno-stack-protector)

I tested your patch but still see .debug_* sections in the .ro from a few .o.

At least on
* setup-x86_64.o
* entry64.o

If you add the following hunk to your diff:
```
@@ -64,6 +64,9 @@ CFLAGS_sha256.o                       += $(PURGATORY_CFLAGS)
 CFLAGS_REMOVE_string.o         += $(PURGATORY_CFLAGS_REMOVE)
 CFLAGS_string.o                        += $(PURGATORY_CFLAGS)

+AFLAGS_REMOVE_setup-x86_$(BITS).o      += -Wa,-gdwarf-2
+AFLAGS_REMOVE_entry64.o                += -Wa,-gdwarf-2
+
 $(obj)/purgatory.ro: $(PURGATORY_OBJS) FORCE
                $(call if_changed,ld)
```
then that should do it.  Then you can verify the .ro file via:
$ llvm-readelf -S arch/x86/purgatory/purgatory.ro | not grep debug_
(no output, should return zero)
-- 
Thanks,
~Nick Desaulniers