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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id g17sm3167286pju.11.2020.06.23.14.06.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 23 Jun 2020 14:06:39 -0700 (PDT) Date: Tue, 23 Jun 2020 14:06:37 -0700 From: Kees Cook To: Ard Biesheuvel Cc: Will Deacon , Catalin Marinas , Mark Rutland , Arnd Bergmann , Peter Collingbourne , James Morse , Nick Desaulniers , Nathan Chancellor , clang-built-linux , Linux ARM , Linux Kernel Mailing List Subject: Re: [PATCH v2 2/2] arm64/build: Warn on orphan section placement Message-ID: <202006231208.F3DA600E18@keescook> References: <20200622205815.2988115-1-keescook@chromium.org> <20200622205815.2988115-3-keescook@chromium.org> <20200623145218.GC4336@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 23, 2020 at 04:59:39PM +0200, Ard Biesheuvel wrote: > On Tue, 23 Jun 2020 at 16:52, Will Deacon 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. Is there a way to suppress the generation? I haven't found a way, so I'd left it as-is. > 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. I will add it to the discards then. > > > 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. Nothing seems to help with the .eh_frame issue (even with -fno-asynchronous-unwind-tables and -fno-unwind-tables): $ aarch64-linux-gnu-gcc -Wp,-MMD,arch/arm64/kernel/.smccc-call.o.d \ -nostdinc -isystem /usr/lib/gcc-cross/aarch64-linux-gnu/9/include \ -I./arch/arm64/include -I./arch/arm64/include/generated -I./include \ -I./arch/arm64/include/uapi -I./arch/arm64/include/generated/uapi \ -I./include/uapi -I./include/generated/uapi -include \ ./include/linux/kconfig.h -D__KERNEL__ -mlittle-endian \ -DCC_USING_PATCHABLE_FUNCTION_ENTRY -DKASAN_SHADOW_SCALE_SHIFT=3 \ -D__ASSEMBLY__ -fno-PIE -mabi=lp64 -fno-asynchronous-unwind-tables \ -fno-unwind-tables -DKASAN_SHADOW_SCALE_SHIFT=3 -Wa,-gdwarf-2 -c -o \ arch/arm64/kernel/smccc-call.o arch/arm64/kernel/smccc-call.S $ readelf -S arch/arm64/kernel/smccc-call.o | grep eh_frame [17] .eh_frame PROGBITS 0000000000000000 000001f0 [18] .rela.eh_frame RELA 0000000000000000 00000618 > > > } > > > > > > . = 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? It's also where .comment and the .symtab ends up living. As a result, I think it's correct to keep it. (If we wanted to split .stabs from .comment/.symtab, we could do that, but I'd kind of like to avoid it for this series, as it feels kind of unrelated.) -- Kees Cook