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

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Message-ID: <CAMj1kXFZFgYp40xD72s0oMThgxK7J-yyCYUQkkBQjh3xR+fNeQ@mail.gmail.com>
Date: Thu, 20 Mar 2025 17:08:54 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Sami Tolvanen <samitolvanen@...gle.com>
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, 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) $@