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Message-ID: <CAMj1kXGnm=uujNfNGJzQxd7BSF0qT2n6jPX1OqwnGov1nKC_cg@mail.gmail.com>
Date: Tue, 23 Jun 2020 23:21:16 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Kees Cook <keescook@...omium.org>
Cc: Will Deacon <will@...nel.org>,
Catalin Marinas <catalin.marinas@....com>,
Mark Rutland <mark.rutland@....com>,
Arnd Bergmann <arnd@...db.de>,
Peter Collingbourne <pcc@...gle.com>,
James Morse <james.morse@....com>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Nathan Chancellor <natechancellor@...il.com>,
clang-built-linux <clang-built-linux@...glegroups.com>,
Linux ARM <linux-arm-kernel@...ts.infradead.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] arm64/build: Warn on orphan section placement
On Tue, 23 Jun 2020 at 23:06, Kees Cook <keescook@...omium.org> wrote:
>
> On Tue, Jun 23, 2020 at 04:59:39PM +0200, Ard Biesheuvel wrote:
> > On Tue, 23 Jun 2020 at 16:52, Will Deacon <will@...nel.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.
>
> Is there a way to suppress the generation? I haven't found a way, so I'd
> left it as-is.
>
Not really. What we could do is assert that it is empty, and emit it
as info, so the 24 bytes are not emitted into the image.
This change
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 6827da7f3aa5..9e13b371559f 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -244,6 +244,9 @@ SECTIONS
__pecoff_data_size = ABSOLUTE(. - __initdata_begin);
_end = .;
+ .got.plt (INFO) : { *(.got.plt) }
+ ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18,
".got.plt not empty")
+
STABS_DEBUG
HEAD_SYMBOLS
results in
[28] .bss NOBITS ffff800010d71000 00d70200
0000000000084120 0000000000000000 WA 0 0 4096
[29] .got.plt PROGBITS ffff800010e00000 00d70200
0000000000000018 0000000000000008 W 0 0 8
[30] .comment PROGBITS 0000000000000000 00d70218
000000000000001c 0000000000000001 MS 0 0 1
in the ELF output, so it will be emitted from the image, unless it
actually have any entries, in which case we fail the build.
> > 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.
>
That would prevent us from doing the assert on its size, so i think
the above is more suitable in this case
> > > > 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
>
That is because that file has CFI annotations which it doesn't need
(since we don't unwind data).
The below should fix that - I guess this may have been inherited from
32-bit ARM, where we do use unwind data in the kernel?
diff --git a/arch/arm64/kernel/smccc-call.S b/arch/arm64/kernel/smccc-call.S
index 1f93809528a4..d62447964ed9 100644
--- a/arch/arm64/kernel/smccc-call.S
+++ b/arch/arm64/kernel/smccc-call.S
@@ -9,7 +9,6 @@
#include <asm/assembler.h>
.macro SMCCC instr
- .cfi_startproc
\instr #0
ldr x4, [sp]
stp x0, x1, [x4, #ARM_SMCCC_RES_X0_OFFS]
@@ -21,7 +20,6 @@
b.ne 1f
str x6, [x4, ARM_SMCCC_QUIRK_STATE_OFFS]
1: ret
- .cfi_endproc
.endm
/*
> > > > }
> > > >
> > > > . = 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
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