linux-kernel - Re: [PATCH v2 13/28] kbuild: lto: merge module sections

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Message-ID: <20200908210748.GB1060586@google.com>
Date:   Tue, 8 Sep 2020 14:07:48 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pci@...r.kernel.org, X86 ML <x86@...nel.org>
Subject: Re: [PATCH v2 13/28] kbuild: lto: merge module sections

On Tue, Sep 08, 2020 at 12:25:54AM +0900, Masahiro Yamada wrote:
> On Fri, Sep 4, 2020 at 5:31 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
> >
> > LLD always splits sections with LTO, which increases module sizes. This
> > change adds a linker script that merges the split sections in the final
> > module.
> >
> > Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> > ---
> >  Makefile               |  2 ++
> >  scripts/module-lto.lds | 26 ++++++++++++++++++++++++++
> >  2 files changed, 28 insertions(+)
> >  create mode 100644 scripts/module-lto.lds
> >
> > diff --git a/Makefile b/Makefile
> > index c69e07bd506a..bb82a4323f1d 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -921,6 +921,8 @@ CC_FLAGS_LTO_CLANG += -fvisibility=default
> >  # Limit inlining across translation units to reduce binary size
> >  LD_FLAGS_LTO_CLANG := -mllvm -import-instr-limit=5
> >  KBUILD_LDFLAGS += $(LD_FLAGS_LTO_CLANG)
> > +
> > +KBUILD_LDS_MODULE += $(srctree)/scripts/module-lto.lds
> >  endif
> >
> >  ifdef CONFIG_LTO
> > diff --git a/scripts/module-lto.lds b/scripts/module-lto.lds
> > new file mode 100644
> > index 000000000000..cbb11dc3639a
> > --- /dev/null
> > +++ b/scripts/module-lto.lds
> > @@ -0,0 +1,26 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * With CONFIG_LTO_CLANG, LLD always enables -fdata-sections and
> > + * -ffunction-sections, which increases the size of the final module.
> > + * Merge the split sections in the final binary.
> > + */
> > +SECTIONS {
> > +       __patchable_function_entries : { *(__patchable_function_entries) }
> > +
> > +       .bss : {
> > +               *(.bss .bss.[0-9a-zA-Z_]*)
> > +               *(.bss..L*)
> > +       }
> > +
> > +       .data : {
> > +               *(.data .data.[0-9a-zA-Z_]*)
> > +               *(.data..L*)
> > +       }
> > +
> > +       .rodata : {
> > +               *(.rodata .rodata.[0-9a-zA-Z_]*)
> > +               *(.rodata..L*)
> > +       }
> > +
> > +       .text : { *(.text .text.[0-9a-zA-Z_]*) }
> > +}
> > --
> > 2.28.0.402.g5ffc5be6b7-goog
> >
> 
> 
> After I apply https://patchwork.kernel.org/patch/11757323/,
> is it possible to do like this ?
> 
> 
> #ifdef CONFIG_LTO
> SECTIONS {
>      ...
> };
> #endif
> 
> in scripts/module.lds.S

Yes, that should work. I'll change this in v3 after your change is
applied.

Sami