On Thu, Nov 23, 2017 at 11:47:56AM +0100, Jason A. Donenfeld wrote:
Hello Nick & Binutils developers,
It would appear that recent changes in the binutils assembler (perhaps 52a86f843b6dee1de9977293da9786649b146b05? perhaps not?) have broken the kernel when compiled in thumb2 mode. We currently do not have a way of working around your breaking changes without adding additional runtime instructions, which isn't acceptable for us. Details are included in the thread below.
Hi Nick,
I have to ask why it was thought a good idea to change the behaviour of the assembler's "adr" pseudo-instruction after it's had the existing behaviour for such a long time.
I see in the bug report https://sourceware.org/bugzilla/show_bug.cgi?id=21458 that guidance was sort from ARM Ltd, but none was forthcoming - which implies that there was little technical input to make the change. Given that the change has been known to break at least three programs (the Linux kernel, libavcodec and openssl), that there's no technical reason to make the change, and that it introduces inconsistency in the assembler, is there enough reason to get the change at least partially removed - restoring the long-standing "adr" behaviour?
The inconsistency is introduced in that "adr" now behaves differently depending on whether the symbol is marked as a thumb function (which requires an explicit pseudo-op) or not. This means that either we need additional labels that are not marked as a thumb function for use with "adr" or we need to mark _every_ symbol in thumb code that is used with "adr" with a .thumb_func annotation, including the numeric local labels.
This makes it much harder to review patches for projects and ensure that they are correct as it means that reviewers now need to do detailed in-depth analysis of the code after /any/ patch that uses "adr" has been applied. Given that the failure is will only be seen at runtime, this is particularly bad.
Thanks, Jason
Forwarded conversation Subject: [PATCH] arm: ensure symbol is a thumb symbol in new binutils
From: Jason A. Donenfeld Jason@zx2c4.com Date: Tue, Nov 21, 2017 at 6:27 PM To: linux@armlinux.org.uk, linux-arm-kernel@lists.infradead.org Cc: "Jason A. Donenfeld" Jason@zx2c4.com, stable@vger.kernel.org
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@zx2c4.com Cc: stable@vger.kernel.org
arch/arm/include/asm/assembler.h | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h index ad301f107dd2..c62a3b6b0a3e 100644 --- a/arch/arm/include/asm/assembler.h +++ b/arch/arm/include/asm/assembler.h @@ -194,10 +194,9 @@ */ .irp c,,eq,ne,cs,cc,mi,pl,vs,vc,hi,ls,ge,lt,gt,le,hs,lo .macro badr\c, rd, sym -#ifdef CONFIG_THUMB2_KERNEL
adr\c \rd, \sym + 1-#else adr\c \rd, \sym +#ifdef CONFIG_THUMB2_KERNEL
orr\c \rd, \rd, 1#endif .endm .endr -- 2.15.0
From: Russell King - ARM Linux linux@armlinux.org.uk Date: Tue, Nov 21, 2017 at 6:38 PM To: "Jason A. Donenfeld" Jason@zx2c4.com, Will Deacon will.deacon@arm.com Cc: linux-arm-kernel@lists.infradead.org, lkml@vger.kernel.org, stable@vger.kernel.org
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, symadr \reg, \sym + 1.endm+test:
mov r0, #0badr 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 >&2rm $tmp/*.ormdir $tmpexit 1- fi
- rm $tmp/*.o
- rmdir $tmp
+fi
From: Jason A. Donenfeld Jason@zx2c4.com Date: Tue, Nov 21, 2017 at 6:46 PM To: Russell King - ARM Linux linux@armlinux.org.uk Cc: Will Deacon will.deacon@arm.com, linux-arm-kernel@lists.infradead.org, lkml@vger.kernel.org, stable@vger.kernel.org
Hi Russell,
This doesn't actually catch the issues. In the buggy binutils, it appears that sometimes adr grabs the right symbol and sometimes it doesn't. I'm not yet able to figure out a minimal condition for it going one way or the other.
Jason
From: Russell King - ARM Linux linux@armlinux.org.uk Date: Thu, Nov 23, 2017 at 11:35 AM To: "Jason A. Donenfeld" Jason@zx2c4.com Cc: Will Deacon will.deacon@arm.com, linux-arm-kernel@lists.infradead.org, lkml@vger.kernel.org, stable@vger.kernel.org
On Thu, Nov 23, 2017 at 12:34:08AM +0100, Jason A. Donenfeld wrote:
On Tue, Nov 21, 2017 at 6:49 PM, Russell King - ARM Linux linux@armlinux.org.uk wrote:
What if we locate the "badr" instruction to the same offset - does that trigger the binutils bug? Note that the grep expression will need updating...
Nope, this too does not reproduce it. I'm having a bit of trouble making a minimal test case to reproduce it. But I can reproduce it everytime by simply assembling the file in question using that binutils.
If it's that hard to reproduce it, it makes me wonder if it's being caused by some memory allocation being re-used without full initialisation, and it's reading stale data.
We can't easily build entry-common.o and check it for the problem as the position of the "local_restart" code depends on various configuration options.
I found this URL:
https://fossies.org/diffs/binutils/2.28_vs_2.29/gas/config/tc-arm.c-diff.htm...
and if you search down to around "line 8358", which is in do_adr(), there is this new code added:
if (inst.reloc.exp.X_op == O_symbol && inst.reloc.exp.X_add_symbol != NULL && S_IS_DEFINED (inst.reloc.exp.X_add_symbol) && THUMB_IS_FUNC (inst.reloc.exp.X_add_symbol)) inst.reloc.exp.X_add_number += 1;which would account for the issue you're seeing.
Given that this change breaks openssl, and breaks the kernel, and this behaviour is something that we've come to rely upon in the kernel since T2 was introduced, and there's no way around it without adding additional instructions, I have to ask what the hell binutils people think they are doing changing the behaviour of the assembler in such a gratuitous way, and how they think that users of their crapware are going to be able to write code that assembles correctly on both pre-2.29 assemblers and post-2.29 assemblers.
Sorry, but gratuitous changes like this in the toolchain really annoy me, and just give me more reason to stick with my old known working versions (binutils 2.25, gcc 4.7.4!) rather than move forward and then not know whether bugs are due to crappy toolchains or really something wrong in the program.
binutils people need to realise that what they offer is an interface for converting assembly code into opcodes and if they change the translation of that in a visible way, people are going to get annoyed - just like if we in the kernel change the kernel's user visible API.
IMHO, binutils should have exactly the same rules - if a change causes a regression for a user, the change is wrong and should be reverted.