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Subject[PATCH] scripts/decodecode: Add the capability to supply the program counter
From: Borislav Petkov <bp@suse.de>
Date: Tue, 29 Sep 2020 18:45:56 +0200

... so that comparing with objdump output from vmlinux can ease
pinpointing where the trapping instruction actually is. An example is
better than a thousand words:

$ PC=0xffffffff8329a927 ./scripts/decodecode < ~/tmp/syz/gfs2.splat
[ 477.379104][T23917] Code: 48 83 ec 28 48 89 3c 24 48 89 54 24 08 e8 c1 b4 4a fe 48 8d bb 00 01 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 97 05 00 00 48 8b 9b 00 01 00 00 48 85 db 0f 84
All code
========
ffffffff8329a8fd: 48 83 ec 28 sub $0x28,%rsp
ffffffff8329a901: 48 89 3c 24 mov %rdi,(%rsp)
ffffffff8329a905: 48 89 54 24 08 mov %rdx,0x8(%rsp)
ffffffff8329a90a: e8 c1 b4 4a fe callq 0xffffffff81745dd0
ffffffff8329a90f: 48 8d bb 00 01 00 00 lea 0x100(%rbx),%rdi
ffffffff8329a916: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax
ffffffff8329a91d: fc ff df
ffffffff8329a920: 48 89 fa mov %rdi,%rdx
ffffffff8329a923: 48 c1 ea 03 shr $0x3,%rdx
ffffffff8329a927:* 80 3c 02 00 cmpb $0x0,(%rdx,%rax,1) <-- trapping instruction
ffffffff8329a92b: 0f 85 97 05 00 00 jne 0xffffffff8329aec8
ffffffff8329a931: 48 8b 9b 00 01 00 00 mov 0x100(%rbx),%rbx
ffffffff8329a938: 48 85 db test %rbx,%rbx
ffffffff8329a93b: 0f .byte 0xf
ffffffff8329a93c: 84 .byte 0x84

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200929113238.GC21110@zn.tnic
---
scripts/decodecode | 29 ++++++++++++++++++++++-------
1 file changed, 22 insertions(+), 7 deletions(-)

diff --git a/scripts/decodecode b/scripts/decodecode
index fbdb325cdf4f..31d884e35f2f 100755
--- a/scripts/decodecode
+++ b/scripts/decodecode
@@ -6,6 +6,7 @@
# options: set env. variable AFLAGS=options to pass options to "as";
# e.g., to decode an i386 oops on an x86_64 system, use:
# AFLAGS=--32 decodecode < 386.oops
+# PC=hex - the PC (program counter) the oops points to

cleanup() {
rm -f $T $T.s $T.o $T.oo $T.aa $T.dis
@@ -67,15 +68,19 @@ if [ -z "$ARCH" ]; then
esac
fi

+# Params: (tmp_file, pc_sub)
disas() {
- ${CROSS_COMPILE}as $AFLAGS -o $1.o $1.s > /dev/null 2>&1
+ t=$1
+ pc_sub=$2
+
+ ${CROSS_COMPILE}as $AFLAGS -o $t.o $t.s > /dev/null 2>&1

if [ "$ARCH" = "arm" ]; then
if [ $width -eq 2 ]; then
OBJDUMPFLAGS="-M force-thumb"
fi

- ${CROSS_COMPILE}strip $1.o
+ ${CROSS_COMPILE}strip $t.o
fi

if [ "$ARCH" = "arm64" ]; then
@@ -83,11 +88,18 @@ disas() {
type=inst
fi

- ${CROSS_COMPILE}strip $1.o
+ ${CROSS_COMPILE}strip $t.o
fi

- ${CROSS_COMPILE}objdump $OBJDUMPFLAGS -S $1.o | \
- grep -v "/tmp\|Disassembly\|\.text\|^$" > $1.dis 2>&1
+ if [ $pc_sub -ne 0 ]; then
+ if [ $PC ]; then
+ adj_vma=$(( $PC - $pc_sub ))
+ OBJDUMPFLAGS="$OBJDUMPFLAGS --adjust-vma=$adj_vma"
+ fi
+ fi
+
+ ${CROSS_COMPILE}objdump $OBJDUMPFLAGS -S $t.o | \
+ grep -v "/tmp\|Disassembly\|\.text\|^$" > $t.dis 2>&1
}

marker=`expr index "$code" "\<"`
@@ -95,14 +107,17 @@ if [ $marker -eq 0 ]; then
marker=`expr index "$code" "\("`
fi

+
touch $T.oo
if [ $marker -ne 0 ]; then
+ # 2 opcode bytes and a single space
+ pc_sub=$(( $marker / 3 ))
echo All code >> $T.oo
echo ======== >> $T.oo
beforemark=`echo "$code"`
echo -n " .$type 0x" > $T.s
echo $beforemark | sed -e 's/ /,0x/g; s/[<>()]//g' >> $T.s
- disas $T
+ disas $T $pc_sub
cat $T.dis >> $T.oo
rm -f $T.o $T.s $T.dis

@@ -114,7 +129,7 @@ echo =========================================== >> $T.aa
code=`echo $code | sed -e 's/ [<(]/ /;s/[>)] / /;s/ /,0x/g; s/[>)]$//'`
echo -n " .$type 0x" > $T.s
echo $code >> $T.s
-disas $T
+disas $T 0
cat $T.dis >> $T.aa

# (lines of whole $T.oo) - (lines of $T.aa, i.e. "Code starting") + 3,
--
2.21.0
--
Regards/Gruss,
Boris.

https://people.kernel.org/tglx/notes-about-netiquette

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