Re: [PATCH v6 14/25] kbuild: lto: remove duplicate dependencies from .mod files

From: Masahiro Yamada <masahiroy@kernel.org>
To: Kees Cook <keescook@chromium.org>
Cc: Sami Tolvanen <samitolvanen@google.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Will Deacon <will@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	linux-arch <linux-arch@vger.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-pci@vger.kernel.org, X86 ML <x86@kernel.org>
Subject: Re: [PATCH v6 14/25] kbuild: lto: remove duplicate dependencies from .mod files
Date: Fri, 4 Dec 2020 02:59:31 +0900	[thread overview]
Message-ID: <CAK7LNAT350QjusoYCQEHDdoxAfTZjj82xp86O1qoNF=0u0PN-g@mail.gmail.com> (raw)
In-Reply-To: <202010141549.412F2BF0@keescook>

On Thu, Oct 15, 2020 at 7:50 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Mon, Oct 12, 2020 at 05:31:52PM -0700, Sami Tolvanen wrote:
> > With LTO, llvm-nm prints out symbols for each archive member
> > separately, which results in a lot of duplicate dependencies in the
> > .mod file when CONFIG_TRIM_UNUSED_SYMS is enabled. When a module
> > consists of several compilation units, the output can exceed the
> > default xargs command size limit and split the dependency list to
> > multiple lines, which results in used symbols getting trimmed.
> >
> > This change removes duplicate dependencies, which will reduce the
> > probability of this happening and makes .mod files smaller and
> > easier to read.
> >
> > Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> > Reviewed-by: Kees Cook <keescook@chromium.org>
>
> Hi Masahiro,
>
> This appears to be a general improvement as well. This looks like it can
> land without depending on the rest of the series.

It cannot.
Adding "sort -u" is pointless without the rest of the series
since the symbol duplication happens only with Clang LTO.

This is not a solution.
"reduce the probability of this happening" well describes it.

I wrote a different patch.

> -Kees
>
> > ---
> >  scripts/Makefile.build | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> > index ab0ddf4884fd..96d6c9e18901 100644
> > --- a/scripts/Makefile.build
> > +++ b/scripts/Makefile.build
> > @@ -266,7 +266,7 @@ 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
> > +cmd_undef_syms = $(NM) $< | sed -n 's/^  *U //p' | sort -u | xargs echo
> >  else
> >  cmd_undef_syms = echo
> >  endif
> > --
> > 2.28.0.1011.ga647a8990f-goog
> >
>
> --
> Kees Cook
>
> --
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-- 
Best Regards
Masahiro Yamada