linux-kernel - Re: [PATCH] gendwarfksyms: define __GENKSYMS_

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Message-ID: <CABCJKueFujwW_=JMZHTf+Y3qN9=uvQYSoDcg_6v=9svUYTsMGQ@mail.gmail.com>
Date: Thu, 20 Mar 2025 09:22:02 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kbuild@...r.kernel.org, brgerst@...il.com, 
	x86@...nel.org, masahiroy@...nel.org, nathan@...nel.org, 
	linux-kernel@...r.kernel.org, nogikh@...gle.com, 
	syzbot+06fd1a3613c50d36129e@...kaller.appspotmail.com
Subject: Re: [PATCH] gendwarfksyms: define __GENKSYMS__ when processing asm-protoypes.h

On Thu, Mar 20, 2025 at 9:09 AM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Thu, 20 Mar 2025 at 15:48, Sami Tolvanen <samitolvanen@...gle.com> wrote:
> >
> > Hi Ard,
> >
> > On Thu, Mar 20, 2025 at 12:07 AM Ard Biesheuvel <ardb+git@...gle.com> wrote:
> > >
> > > From: Ard Biesheuvel <ardb@...nel.org>
> > >
> > > Ensure that __GENKSYMS__ is #define'd when passing asm/asm-prototypes.h
> > > through the compiler to capture the exported symbols. This ensures that
> > > exported symbols such as __ref_stack_chk_guard on x86, which is declared
> > > conditionally, is visible to the tool.
> > >
> > > Otherwise, an error such as the below may be raised, breaking the build
> > > when CONFIG_GENDWARFKSYMS=y
> > >
> > >   <stdin>:4:15: error: use of undeclared identifier '__ref_stack_chk_guard'
> > >
> > > Cc: Sami Tolvanen <samitolvanen@...gle.com>
> > > Reported-by: syzbot+06fd1a3613c50d36129e@...kaller.appspotmail.com
> > > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > > ---
> > >  scripts/Makefile.build | 1 +
> > >  1 file changed, 1 insertion(+)
> > >
> > > diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> > > index 993708d11874..7855cdc4e763 100644
> > > --- a/scripts/Makefile.build
> > > +++ b/scripts/Makefile.build
> > > @@ -305,6 +305,7 @@ $(obj)/%.rs: $(obj)/%.rs.S FORCE
> > >  getasmexports =                                                                \
> > >     { echo "\#include <linux/kernel.h>" ;                               \
> > >       echo "\#include <linux/string.h>" ;                               \
> > > +     echo "\#define  __GENKSYMS__" ;                                   \
> > >       echo "\#include <asm/asm-prototypes.h>" ;                         \
> > >       $(call getexportsymbols,EXPORT_SYMBOL(\1);) ; }
> >
> > This works with gendwarfksyms since __GENKSYMS__ is defined after the
> > EXPORT_SYMBOL() definition, but I'm now getting warnings with
> > genksyms:
> >
> >   AS      arch/x86/lib/clear_page_64.o
> > <stdin>:3:10: warning: "__GENKSYMS__" redefined
> > <command-line>: note: this is the location of the previous definition
> >
>
> Oops.
>
> Do you think the fix below should be sufficient?
>
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -314,7 +314,7 @@
>  else
>  cmd_gensymtypes_S =                                                    \
>         $(getasmexports) |                                              \
> -       $(CPP) -D__GENKSYMS__ $(c_flags) -xc - | $(genksyms)
> +       $(CPP) $(c_flags) -xc - | $(genksyms)
>  endif # CONFIG_GENDWARFKSYMS
>
>  quiet_cmd_cpp_s_S = CPP $(quiet_modtag) $@

EXPORT_SYMBOL() has a different definition when __GENKSYMS__ is
defined, so I think with genksyms we actually do need this on the
command line. I suppose you could wrap the getasmexports definition in
#ifndef __GENKSYMS__ to avoid the warning, or just use
__GENDWARFKSYMS__ like you suggested earlier.

Sami