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