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    SubjectRe: [PATCH 1/2] arch/x86: Use -fno-asynchronous-unwind-tables to suppress .eh_frame sections
    On Mon, Feb 24, 2020 at 1:12 PM Fangrui Song <maskray@google.com> 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@alum.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.

    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

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