Messages in this thread | | | From | Arvind Sankar <> | Date | Mon, 24 Feb 2020 16:05:22 -0500 | 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 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.
> > $ 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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