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Date:   Wed, 16 Dec 2020 15:19:14 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     linux-kbuild <linux-kbuild@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kbuild: avoid split lines in .mod files

On Fri, Dec 4, 2020 at 3:46 AM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@...glegroups.com> wrote:
>
> Hi Masahiro,
>
> On Thu, Dec 3, 2020 at 9:56 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
> >
> > "xargs echo" is not a safe way to remove line breaks because the input
> > may exceed the command line limit and xargs may break it up into
> > multiple invocations of echo. This should never happen because
> > scripts/gen_autoksyms.sh expects all undefined symbols are placed in
> > the second line of .mod files.
> >
> > One possible way is to replace "xargs echo" with
> > "sed ':x;N;$!bx;s/\n/ /g'" or something, but I rewrote the code by
> > using awk because it is more readable.
> >
> > This issue was reported by Sami Tolvanen; in his Clang LTO patch set,
> > $(multi-used-m) is no longer an ELF object, but a thin archive that
> > contains LLVM bitcode files. llvm-nm prints out symbols for each
> > archive member separately, which results a lot of dupications, in some
> > places, beyond the system-defined limit.
> >
> > This problem must be fixed irrespective of LTO, and we must ensure
> > zero possibility of having this issue.
> >
> > Link: https://lkml.org/lkml/2020/12/1/1658
> > Reported-by: Sami Tolvanen <samitolvanen@...gle.com>
> > Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> > ---
> >
> >  scripts/Makefile.build | 12 ++++--------
> >  1 file changed, 4 insertions(+), 8 deletions(-)
> >
> > diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> > index ae647379b579..4c058f12dd73 100644
> > --- a/scripts/Makefile.build
> > +++ b/scripts/Makefile.build
> > @@ -252,6 +252,9 @@ objtool_dep = $(objtool_obj)                                        \
> >  ifdef CONFIG_TRIM_UNUSED_KSYMS
> >  cmd_gen_ksymdeps = \
> >         $(CONFIG_SHELL) $(srctree)/scripts/gen_ksymdeps.sh $@ >> $(dot-target).cmd
> > +
> > +# List module undefined symbols
> > +undefined_syms = $(NM) $< | $(AWK) '$$1 == "U" { printf("%s%s", x++ ? " " : "", $$2) }';
> >  endif
> >
> >  define rule_cc_o_c
> > @@ -271,13 +274,6 @@ define rule_as_o_S
> >         $(call cmd,modversions_S)
> >  endef
> >
> > -# List module undefined symbols (or empty line if not enabled)
> > -ifdef CONFIG_TRIM_UNUSED_KSYMS
> > -cmd_undef_syms = $(NM) $< | sed -n 's/^  *U //p' | xargs echo
> > -else
> > -cmd_undef_syms = echo
> > -endif
> > -
> >  # Built-in and composite module parts
> >  $(obj)/%.o: $(src)/%.c $(recordmcount_source) $(objtool_dep) FORCE
> >         $(call if_changed_rule,cc_o_c)
> > @@ -285,7 +281,7 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) $(objtool_dep) FORCE
> >
> >  cmd_mod = { \
> >         echo $(if $($*-objs)$($*-y)$($*-m), $(addprefix $(obj)/, $($*-objs) $($*-y) $($*-m)), $(@:.mod=.o)); \
> > -       $(cmd_undef_syms); \
> > +       $(undefined_syms) echo; \
> >         } > $@
> >
> >  $(obj)/%.mod: $(obj)/%.o FORCE
>
> Thanks for the patch! I confirmed that this works with llvm-nm and
> bitcode files, but it does still produce plenty of duplicates,

Actually, the duplication does not matter
because scripts/gen_autoksyms.sh line 46
calls 'sort -u' anyway.
Only the problem is we have bigger .mod files, though.

We do not have a good reason
to move 'sort -u' for now.




> even
> though they now stay on one line. I'm not sure if the readability of
> the .mod file matters though. Please feel free to add:
>
> Reviewed-by: Sami Tolvanen <samitolvanen@...gle.com>
>
> Sami
>
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-- 
Best Regards
Masahiro Yamada

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