Fwd: [PATCH] arm: ensure symbol is a thumb symbol in new binutils

Fwd: [PATCH] arm: ensure symbol is a thumb symbol in new binutils

[Jason A. Donenfeld]

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.

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, 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

----------
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.html

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.

[PATCH] arm: detect buggy binutils when in thumb2 mode

[Jason A. Donenfeld]

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 certain circumstances. In order to homogenize this behavior,
rather than adding 1, we could simply OR in 1, so that already unaligned
instructions don't change. While that 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 `|'". A better solution would be to have consistent binutils behavior,
but that ship has sailed.

So, this commit adds a detection mechanism, which began as a small thing
from Russell King that I then rewrote to use pure bash instead of
shelling out, so that it doesn't slow down the build process. The detection
mechanism _could_ be used to modify the assembly we generate, but for now
it's just being used to catch buggy binutils and abort the build process in
that case.

The rest of this commit message contains all of the relevant information
about the boot bug when compiled in thumb2 mode.

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: Russell King <linux@armlinux.org.uk>
Cc: nickc@redhat.com
Cc: stable@vger.kernel.org
---
 arch/arm/Makefile        |  7 +++++--
 arch/arm/tools/Makefile  |  5 ++++-
 arch/arm/tools/toolcheck | 44 ++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 53 insertions(+), 3 deletions(-)
 create mode 100644 arch/arm/tools/toolcheck

diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 80351e505fd5..bd4e248a7f8f 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..0a283756f1c5 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:
+	@'$(srctree)/$(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
new file mode 100644
index 000000000000..04fc44b750d2
--- /dev/null
+++ b/arch/arm/tools/toolcheck
@@ -0,0 +1,44 @@
+#!/bin/bash
+#
+# Copyright 2017 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+#
+
+set -e
+
+cleanup() {
+	[[ ! -d $temp ]] || rm -rf "$temp"
+	exit
+}
+trap cleanup INT TERM EXIT
+temp="$(mktemp -d)"
+
+check_thumb2_address() {
+	local disassembly
+
+	$CC $KBUILD_AFLAGS -o "$temp/a.out" -c -xassembler - <<-_EOF
+		.syntax unified
+		.thumb
+		.macro	badr, reg, sym
+		adr	\reg, \sym + 1
+		.endm
+
+		.type test, %function
+		.thumb_func
+		test:
+		mov	r0, #0
+		badr	lr, test
+	_EOF
+	disassembly="$($OBJDUMP -d "$temp/a.out")"
+
+	[[ $disassembly =~ 4:[[:space:]]*f2af\ 0e07 ]] && return 0
+
+	echo "Error: your assembler version produces buggy kernels:" >&2
+	read < <($AS --version) && echo "$REPLY" >&2
+	[[ $disassembly =~ 4:[[:space:]].*$ ]] && echo "${BASH_REMATCH[0]}" >&2 || echo "$disassembly" >&2
+	return 1
+}
+
+config="$(< .config)"
+[[ $config == *CONFIG_THUMB2_KERNEL=y* ]] && check_thumb2_address
+
+exit 0
-- 
2.15.0

[PATCH v2] arm: detect buggy binutils when in thumb2 mode

[Jason A. Donenfeld]

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 certain circumstances. In order to homogenize this behavior,
rather than adding 1, we could simply OR in 1, so that already unaligned
instructions don't change. While that 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 `|'". A better solution would be to have consistent binutils behavior,
but that ship has sailed.

So, this commit adds a detection mechanism, which began as a small thing
from Russell King that I then rewrote to use pure bash instead of
shelling out, so that it doesn't slow down the build process. The detection
mechanism _could_ be used to modify the assembly we generate, but for now
it's just being used to catch buggy binutils and abort the build process in
that case.

The rest of this commit message contains all of the relevant information
about the boot bug when compiled in thumb2 mode.

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: Russell King <linux@armlinux.org.uk>
Cc: nickc@redhat.com
Cc: stable@vger.kernel.org
---
Had the file mode wrong on the submission from a second ago, sorry about
that.

 arch/arm/Makefile        |  7 +++++--
 arch/arm/tools/Makefile  |  5 ++++-
 arch/arm/tools/toolcheck | 44 ++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 53 insertions(+), 3 deletions(-)
 create mode 100755 arch/arm/tools/toolcheck

diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 80351e505fd5..bd4e248a7f8f 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..0a283756f1c5 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:
+	@'$(srctree)/$(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
new file mode 100755
index 000000000000..04fc44b750d2
--- /dev/null
+++ b/arch/arm/tools/toolcheck
@@ -0,0 +1,44 @@
+#!/bin/bash
+#
+# Copyright 2017 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+#
+
+set -e
+
+cleanup() {
+	[[ ! -d $temp ]] || rm -rf "$temp"
+	exit
+}
+trap cleanup INT TERM EXIT
+temp="$(mktemp -d)"
+
+check_thumb2_address() {
+	local disassembly
+
+	$CC $KBUILD_AFLAGS -o "$temp/a.out" -c -xassembler - <<-_EOF
+		.syntax unified
+		.thumb
+		.macro	badr, reg, sym
+		adr	\reg, \sym + 1
+		.endm
+
+		.type test, %function
+		.thumb_func
+		test:
+		mov	r0, #0
+		badr	lr, test
+	_EOF
+	disassembly="$($OBJDUMP -d "$temp/a.out")"
+
+	[[ $disassembly =~ 4:[[:space:]]*f2af\ 0e07 ]] && return 0
+
+	echo "Error: your assembler version produces buggy kernels:" >&2
+	read < <($AS --version) && echo "$REPLY" >&2
+	[[ $disassembly =~ 4:[[:space:]].*$ ]] && echo "${BASH_REMATCH[0]}" >&2 || echo "$disassembly" >&2
+	return 1
+}
+
+config="$(< .config)"
+[[ $config == *CONFIG_THUMB2_KERNEL=y* ]] && check_thumb2_address
+
+exit 0
-- 
2.15.0

Re: [PATCH v2] arm: detect buggy binutils when in thumb2 mode

[Martin Storsjö]

On Thu, 23 Nov 2017, 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 certain circumstances. In order to homogenize this behavior,
> rather than adding 1, we could simply OR in 1, so that already unaligned
> instructions don't change. While that 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 `|'". A better solution would be to have consistent binutils behavior,
> but that ship has sailed.
>
> So, this commit adds a detection mechanism, which began as a small thing
> from Russell King that I then rewrote to use pure bash instead of
> shelling out, so that it doesn't slow down the build process. The detection
> mechanism _could_ be used to modify the assembly we generate, but for now
> it's just being used to catch buggy binutils and abort the build process in
> that case.
>
> The rest of this commit message contains all of the relevant information
> about the boot bug when compiled in thumb2 mode.
>
> 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

FWIW, this issue stems from this change: 
https://sourceware.org/bugzilla/show_bug.cgi?id=21458

The same issue caused problems in libavcodec as well, where we chose to 
work around the issue in this fashion:
https://git.libav.org/?p=libav.git;a=commitdiff;h=9dde6ab06c48f9447cd16f39bee33569cddb7be4;hp=547db1eaecd597031165a2bf637acaaacde52788

Related debian bug report, with a different workaround: 
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=870622

(In libav, we chose the workaround since the .eqv one suggested in the 
debian bug report didn't really work well with assemblers for other 
platforms.)

// Martin

Re: Fwd: [PATCH] arm: ensure symbol is a thumb symbol in new binutils

[Russell King - ARM Linux]

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, 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
> 
> ----------
> 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.html
> 
> 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.

-- 
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
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Re: Fwd: [PATCH] arm: ensure symbol is a thumb symbol in new binutils

[Ard Biesheuvel]

On 23 November 2017 at 14:02, Russell King - ARM Linux
<linux@armlinux.org.uk> wrote:
> 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 think part of the confusion comes from the fact that ADR is no
longer a pseudo-instruction on Thumb2, and can be used to create
cross-section symbol references that are fixed up by the linker (using
a dedicated relocation type)

So before the change, we were in the situation where ADR behaved
differently when used on symbols, depending on whether they were
defined in the same section or not:
- cross section symbol references are fixed up by the linker, and do
take the Thumb bit into account for function symbols,
- local symbol references and label references are not fixed up at
all, regardless of their type annotation.

Note that external symbol references are hardly ever used, given that
they don't work in ARM mode (since it is a pseudo op there), so in the
kernel, we only care about local symbol references and label
references.

After the change, internal symbol references behave differently, but
label references behave the same. So the uniform behavior we care
about and rely on is no longer there, and even worse, we cannot deal
with this in the code because we cannot detect which binutils version
we are using without intrusive workarounds)

-- 
Ard.