On Tue, Nov 21, 2017 at 06:27:51PM +0100, Jason A. Donenfeld wrote:
> On older versions of binutils, \sym points to an aligned address. On
> newer versions of binutils, \sym sometimes points to the unaligned thumb
> address in mysterious and buggy circumstances. In order to homogenize
> this behavior, rather than adding 1, we simply OR in 1, so that already
> unaligned instructions don't change. This fix is required for a
> pedestrian THUMB2_KERNEL to boot without crashing when built with
> non-old binutils.
>
> While it works, the downside is that we have to add an `orr` instruction
> to a fast path. The assembler can't do this at assemble time via "|1"
> because "invalid operands (.text and *ABS* sections) for `|'", so we're
> forced to do this. A better solution would be to have consistent
> binutils behavior, or to have some kind of \sym feature detection that
> won't turn into a maze of version comparisons. However, it's at the
> moment unclear how to achieve this.
>
> The rest of this commit message contains all of the relevant
> information.
>
> My tests concerned these versions:
> broken: GNU ld (Gentoo 2.29.1 p3) 2.29.1
> working: GNU ld (GNU Binutils for Ubuntu) 2.26.1
>
> These produced the following code:
> --- broken 2017-11-21 17:44:14.523416082 +0100
> +++ working 2017-11-21 17:44:44.548461234 +0100
> @@ -133,7 +133,7 @@
> 160: f01a 0ff0 tst.w sl, #240 ; 0xf0
> 164: d111 bne.n 18a <__sys_trace>
> 166: f5b7 7fc8 cmp.w r7, #400 ; 0x190
> - 16a: f2af 1e6a subw lr, pc, #362 ; 0x16a
> + 16a: f2af 1e6b subw lr, pc, #363 ; 0x16b
> 16e: bf38 it cc
> 170: f858 f027 ldrcc.w pc, [r8, r7, lsl #2]
> 174: a902 add r1, sp, #8
>
> The differing instruction corresponds with this actual line in
> arch/arm/kernel/entry-common.S:
> badr lr, ret_fast_syscall @ return address
>
> Running the broken kernel results in a runtime OOPS with:
> PC is at ret_fast_syscall+0x4/0x52
> LR is at ret_fast_syscall+0x2/0x52
>
> The disassembly of that function for the crashing kernel is:
> .text:00000000 ret_fast_syscall ; CODE XREF: sys_syscall+1C↓j
> .text:00000000 CPSID I ; jumptable 00000840 cases 15,18-376
> .text:00000002
> .text:00000002 loc_2 ; DATA XREF: sys_syscall-6BA↓o
> .text:00000002 LDR.W R2, [R9,#8]
> .text:00000006 CMP.W R2, #0xBF000000
>
> Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
As it just seems to be a limited range of binutils versions that are
affected, I'd rather not impact the kernel fast-paths with extra
cycles just because binutils decided to change behaviour. I'd prefer
to inform people about the problem and get them to change to a non-
buggy binutils.
This seems to be the second binutils bug that's biting us within the
last month... what's going on with binutils QA?
arch/arm/Makefile | 7 +++++--
arch/arm/tools/Makefile | 5 ++++-
arch/arm/tools/toolcheck | 24 ++++++++++++++++++++++++
3 files changed, 33 insertions(+), 3 deletions(-)
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 1cfac5119545..9e70d0435121 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -319,16 +319,19 @@ all: $(notdir $(KBUILD_IMAGE)) $(KBUILD_DTBS)
archheaders:
$(Q)$(MAKE) $(build)=arch/arm/tools uapi
-archprepare:
+archprepare: toolcheck
$(Q)$(MAKE) $(build)=arch/arm/tools kapi
+toolcheck:
+ $(Q)$(MAKE) $(build)=arch/arm/tools $@
+
# Convert bzImage to zImage
bzImage: zImage
BOOT_TARGETS = zImage Image xipImage bootpImage uImage
INSTALL_TARGETS = zinstall uinstall install
-PHONY += bzImage $(BOOT_TARGETS) $(INSTALL_TARGETS)
+PHONY += bzImage $(BOOT_TARGETS) $(INSTALL_TARGETS) toolcheck
bootpImage uImage: zImage
zImage: Image
diff --git a/arch/arm/tools/Makefile b/arch/arm/tools/Makefile
index ddb89a7db36f..fa77351ccefd 100644
--- a/arch/arm/tools/Makefile
+++ b/arch/arm/tools/Makefile
@@ -23,12 +23,15 @@ uapi-hdrs-y += $(uapi)/unistd-eabi.h
targets += $(addprefix ../../../,$(gen-y) $(kapi-hdrs-y) $(uapi-hdrs-y))
-PHONY += kapi uapi
+PHONY += kapi uapi toolcheck
kapi: $(kapi-hdrs-y) $(gen-y)
uapi: $(uapi-hdrs-y)
+toolcheck:
+ @$(CONFIG_SHELL) '$(srctree)/$(src)/toolcheck'
+
# Create output directory if not already present
_dummy := $(shell [ -d '$(kapi)' ] || mkdir -p '$(kapi)') \
$(shell [ -d '$(uapi)' ] || mkdir -p '$(uapi)')
diff --git a/arch/arm/tools/toolcheck b/arch/arm/tools/toolcheck
index e69de29bb2d1..97bbeeb691da 100644
--- a/arch/arm/tools/toolcheck
+++ b/arch/arm/tools/toolcheck
@@ -0,0 +1,24 @@
+#!/bin/sh -ex
+if grep -q 'CONFIG_THUMB2_KERNEL=y' .config; then
+ tmp=$(mktemp -d /tmp/binutils-test.XXXXXXXXXX)
+ cat <<EOF | $AS $ASFLAGS -o $tmp/test.o
+ .syntax unified
+ .thumb
+ .macro badr, reg, sym
+ adr \reg, \sym + 1
+ .endm
+
+test:
+ mov r0, #0
+ badr lr, test
+EOF
+ if ! $OBJDUMP -d $tmp/test.o | grep -q '4:\s*f2af 0e07'; then
+ echo "Error: your assembler version produces buggy kernels" >&2
+ $AS --version | head -n1 >&2
+ rm $tmp/*.o
+ rmdir $tmp
+ exit 1
+ fi
+ rm $tmp/*.o
+ rmdir $tmp
+fi
--
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up
According to speedtest.net: 8.21Mbps down 510kbps up