Messages in this thread | | | From | Nick Desaulniers <> | Date | Mon, 24 Feb 2020 13:17:10 -0800 | Subject | Re: [PATCH 1/2] arch/x86: Use -fno-asynchronous-unwind-tables to suppress .eh_frame sections |
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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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