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

[Nick Desaulniers]

On Mon, Feb 24, 2020 at 1:12 PM Fangrui Song <maskray@xxxxxxxxxx> wrote:
>
> 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.

Ah, right the `subst` from `KBUILD_CFLAGS`.
Are there other architectures that care about EFI beyond x86 and ARM? IA64?

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

Ah, thanks for the clarification.

>
> >>
> >> $ 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