Messages in this thread | | | From | Nick Desaulniers <> | Date | Mon, 24 Feb 2020 13:58:35 -0800 | Subject | Re: [PATCH 2/2] arch/x86: Drop unneeded linker script discard of .eh_frame |
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On Mon, Feb 24, 2020 at 1:33 PM Arvind Sankar <nivedita@alum.mit.edu> wrote: > > On Mon, Feb 24, 2020 at 12:45:51PM -0800, Nick Desaulniers wrote: > > > > grepping for eh_frame in arch/x86/ there's a comment in > > arch/x86/include/asm/dwarf2.h: > > 40 #ifndef BUILD_VDSO > > 41 /* > > 42 * Emit CFI data in .debug_frame sections, not .eh_frame > > sections. > > 43 * The latter we currently just discard since we don't do DWARF > > 44 * unwinding at runtime. So only the offline DWARF information is > > 45 * useful to anyone. Note we should not use this directive if > > 46 * vmlinux.lds.S gets changed so it doesn't discard .eh_frame. > > 47 */ > > 48 .cfi_sections .debug_frame > > > > add via: > > commit 7b956f035a9ef ("x86/asm: Re-add parts of the manual CFI infrastructure") > > > > https://sourceware.org/binutils/docs/as/CFI-directives.html#g_t_002ecfi_005fsections-section_005flist > > is the manual's section on .cfi_sections directives, and states `The > > default if this directive is not used is .cfi_sections .eh_frame.`. > > So the comment is slightly stale since we're no longer explicitly > > discarding .eh_frame in arch/x86/kernel/vmlinux.lds.S, rather > > preventing the generation via -fno-asynchronous-unwind-tables in > > KBUILD_CFLAGS (across a few different Makefiles). Would you mind also > > updating the comment in arch/x86/include/asm/dwarf2.h in a V2? The > > rest of this patch LGTM. > > > > i.e. just replace that last sentence with "Note ... if we decide to use > runtime DWARF unwinding again"?
Yeah that should be good. Maybe these cleanups could be a separate patch, if you prefer?
> > The whole ifdef-ery machinery there is obsolete, all the directives its > checking support for have been there since binutils-2.18, so should > probably also clean it up to just unconditionally define them.
arch/x86/Makefile: 184 # do binutils support CFI? 185 cfi := $(call as-instr,.cfi_startproc\n.cfi_rel_offset $(sp-y)$(comma)0\n.cfi_endproc,-DCONFIG_AS_CFI=1) 186 # is .cfi_signal_frame supported too? 187 cfi-sigframe := $(call as-instr,.cfi_startproc\n.cfi_signal_frame\n.cfi_endproc,-DCONFIG_AS_CFI_SIGNAL_FRAME=1) 188 cfi-sections := $(call as-instr,.cfi_sections .debug_frame,-DCONFIG_AS_CFI_SECTIONS=1)
2.18? Oh, yeah, we can clean that up to. Documentation/process/changes.rst list binutils 2.21 as the minimum supported version. Then I assume that code that uses those -D flags can go, too. -- Thanks, ~Nick Desaulniers
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