linux-kernel - Re: [PATCH 1/2] arch/x86: Use -fno-asynchronous-unwind-tables to suppress .eh

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Date:   Mon, 24 Feb 2020 16:05:22 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Borislav Petkov <bp@...en8.de>,
        Nathan Chancellor <natechancellor@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Michael Matz <matz@...e.de>, Fangrui Song <maskray@...gle.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 1/2] arch/x86: Use -fno-asynchronous-unwind-tables to
 suppress .eh_frame sections

On Mon, Feb 24, 2020 at 12:33:49PM -0800, Nick Desaulniers wrote:
> On Sun, Feb 23, 2020 at 11:37 AM Arvind Sankar <nivedita@...m.mit.edu> wrote:
> >
> > While discussing a patch to discard .eh_frame from the compressed
> > vmlinux using the linker script, Fangrui Song pointed out [1] that these
> > sections shouldn't exist in the first place because arch/x86/Makefile
> > uses -fno-asynchronous-unwind-tables.
> 
> Another benefit is that -fno-asynchronous-unwind-tables may help
> reduce the size of .text!
> https://stackoverflow.com/a/26302715/1027966

Hm I don't see any change in .text size.
> > diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> > index 98a81576213d..a1140c4ee478 100644
> > --- a/drivers/firmware/efi/libstub/Makefile
> > +++ b/drivers/firmware/efi/libstub/Makefile
> > @@ -12,7 +12,8 @@ cflags-$(CONFIG_X86)          += -m$(BITS) -D__KERNEL__ -O2 \
> >                                    -mno-mmx -mno-sse -fshort-wchar \
> >                                    -Wno-pointer-sign \
> >                                    $(call cc-disable-warning, address-of-packed-member) \
> > -                                  $(call cc-disable-warning, gnu)
> > +                                  $(call cc-disable-warning, gnu) \
> > +                                  -fno-asynchronous-unwind-tables
> 
> I think we want to add this flag a little lower, line 27 has:
> 
> KBUILD_CFLAGS     := $(cflags-y) -DDISABLE_BRANCH_PROFILING \
> 
> so the `cflags-y` variable you modify in this hunk will only set
> -fno-asynchronous-unwind-tables for CONFIG_X86, which I don't think is
> intentional.  Though when I run

It is intentional -- the other case is that we're building for ARM,
which only filters out the regular KBUILD_CFLAGS, so adding the flag for
it should not be necessary. The cflags for ARM are constructed by
manipulating KBUILD_CFLAGS. Besides it may or may not want unwind
tables. 32-bit ARM appears to have an option to enable -funwind-tables.

> 
> $ llvm-readelf -S drivers/firmware/efi/libstub/lib.a | grep eh_frame
> 
> after doing an x86_64 defconfig, I don't get any hits. Do you observe
> .eh_frame sections on any of these objects in this dir? (I'm fine
> adding it to be safe, but I'm curious why I'm not seeing any
> .eh_frame)
> 

You mean before this patch, right? I see hits on every .o file in there
(compiling with gcc 9.2.0).

> >
> >  # arm64 uses the full KBUILD_CFLAGS so it's necessary to explicitly
> >  # disable the stackleak plugin
> > --
> > 2.24.1
> >
> 
> 
> -- 
> Thanks,
> ~Nick Desaulniers