Messages in this thread | | | From | Ard Biesheuvel <> | Date | Tue, 23 Jun 2020 16:59:39 +0200 | Subject | Re: [PATCH v2 2/2] arm64/build: Warn on orphan section placement |
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On Tue, 23 Jun 2020 at 16:52, Will Deacon <will@kernel.org> wrote: > > On Mon, Jun 22, 2020 at 01:58:15PM -0700, Kees Cook wrote: > > We don't want to depend on the linker's orphan section placement > > heuristics as these can vary between linkers, and may change between > > versions. All sections need to be explicitly named in the linker > > script. > > > > Explicitly include debug sections when they're present. Add .eh_frame* > > to discard as it seems that these are still generated even though > > -fno-asynchronous-unwind-tables is being specified. Add .plt and > > .data.rel.ro to discards as they are not actually used. Add .got.plt > > to the image as it does appear to be mapped near .data. Finally enable > > orphan section warnings. > > Can you elaborate a bit on what .got.plt is being used for, please? I > wonder if there's an interaction with an erratum workaround in the linker > or something. >
.got.plt is not used at all, but it has three magic entries at the start that the dynamic linker uses for lazy dispatch, so it turns up as a non-empty section of 0x18 bytes.
We should be able to discard it afaict, but given that it does not actually take up any space, it doesn't really matter either way.
> > diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile > > index a0d94d063fa8..3e628983445a 100644 > > --- a/arch/arm64/Makefile > > +++ b/arch/arm64/Makefile > > @@ -29,6 +29,10 @@ LDFLAGS_vmlinux += --fix-cortex-a53-843419 > > endif > > endif > > > > +# We never want expected sections to be placed heuristically by the > > +# linker. All sections should be explicitly named in the linker script. > > +LDFLAGS_vmlinux += --orphan-handling=warn > > + > > ifeq ($(CONFIG_ARM64_USE_LSE_ATOMICS), y) > > ifneq ($(CONFIG_ARM64_LSE_ATOMICS), y) > > $(warning LSE atomics not supported by binutils) > > diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S > > index 5427f502c3a6..c9ecb3b2007d 100644 > > --- a/arch/arm64/kernel/vmlinux.lds.S > > +++ b/arch/arm64/kernel/vmlinux.lds.S > > @@ -94,7 +94,8 @@ SECTIONS > > /DISCARD/ : { > > *(.interp .dynamic) > > *(.dynsym .dynstr .hash .gnu.hash) > > - *(.eh_frame) > > + *(.plt) *(.data.rel.ro) > > + *(.eh_frame) *(.init.eh_frame) > > Do we need to include .eh_frame_hdr here too? >
It would be better to build with -fno-unwind-tables, in which case these sections should not even exist.
> > } > > > > . = KIMAGE_VADDR + TEXT_OFFSET; > > @@ -209,6 +210,7 @@ SECTIONS > > _data = .; > > _sdata = .; > > RW_DATA(L1_CACHE_BYTES, PAGE_SIZE, THREAD_ALIGN) > > + .got.plt : ALIGN(8) { *(.got.plt) } > > > > /* > > * Data written with the MMU off but read with the MMU on requires > > @@ -244,6 +246,7 @@ SECTIONS > > _end = .; > > > > STABS_DEBUG > > + DWARF_DEBUG > > I know you didn't add it, but do we _really_ care about stabs debug? Who > generates that for arm64? > > Will
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