Messages in this thread | | | Date | Mon, 24 Feb 2020 13:12:09 -0800 | From | Fangrui Song <> | Subject | Re: [PATCH 1/2] arch/x86: Use -fno-asynchronous-unwind-tables to suppress .eh_frame sections |
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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.
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 >> >
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