Re: [PATCH 1/2] arch/x86: Use -fno-asynchronous-unwind-tables to suppress .eh_frame sections

[Fangrui Song]

On 2020-02-24, Arvind Sankar wrote:
> 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@xxxxxxxxxxxx> 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.

clang (as of today) has not implemented the
-funwind-tables/-fasynchronous-unwind-tables distinction as GCC does..
(probably because not many people care..)

> > $ 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
> > >