linux-kernel - Re: [PATCH] scripts/decodecode: Fix decoding for AArch64 (arm64) instructions

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Message-ID: <20170928180135.GH9892@arm.com>
Date:   Thu, 28 Sep 2017 19:01:35 +0100
From:   Will Deacon <will.deacon@....com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     mmarek@...e.cz, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] scripts/decodecode: Fix decoding for AArch64 (arm64)
 instructions

On Thu, Sep 28, 2017 at 03:37:04PM +0100, Dave Martin wrote:
> On Thu, Sep 28, 2017 at 03:14:47PM +0100, Will Deacon wrote:
> > On Thu, Sep 28, 2017 at 01:42:31PM +0100, Dave Martin wrote:
> > > On Thu, Sep 28, 2017 at 11:55:47AM +0100, Will Deacon wrote:
> > > > There are a couple of problems with the decodecode script and arm64:
> > > > 
> > > > 1. AArch64 objdump refuses to disassemble .4byte directives as instructions,
> > > >    insisting that they are data values and displaying them as:
> > > > 
> > > > 	a94153f3	.word	0xa94153f3		<-- trapping instruction
> > > > 
> > > >    This is resolved by using the .inst directive instead.
> > > > 
> > > > 2. Disassembly of branch instructions attempts to provide the target as
> > > >    an offset from a symbol, e.g.:
> > > > 
> > > >    0:	34000082	cbz	w2, 10 <.text+0x10>
> > > > 
> > > >   however this falls foul of the grep -v, which matches lines containing
> > > >   ".text" and ends up removing all branch instructions from the dump.
> > > 
> > > Any idea why this doesn't affect other arches too ... or does it?
> > 
> > I'm not sure, although I don't know how .inst works for architectures
> > with variable-length instructions and I *guess* the disassembly is less
> > fussy about data vs text for those targets.
> 
> I rather meant the target disassembly for relative branches in the
> absence of labels.
> 
> Anyway, I think this is at least harmless to other arches, and possibly
> helpful to them (if they disassemble those branch targets in the same
> sort of way).

Ah, I see what you mean. Something like the fixup below on top.

Will

--->8

diff --git a/scripts/decodecode b/scripts/decodecode
index 67214ec5b2cb..f1ec57c3cbf7 100755
--- a/scripts/decodecode
+++ b/scripts/decodecode
@@ -49,21 +49,14 @@ esac
 
 disas() {
 	${CROSS_COMPILE}as $AFLAGS -o $1.o $1.s > /dev/null 2>&1
+	${CROSS_COMPILE}strip $1.o
 
-	if [ "$ARCH" = "arm" ]; then
-		if [ $width -eq 2 ]; then
-			OBJDUMPFLAGS="-M force-thumb"
-		fi
-
-		${CROSS_COMPILE}strip $1.o
+	if [ "$ARCH" = "arm" -a $width -eq 2 ]; then
+		OBJDUMPFLAGS="-M force-thumb"
 	fi
 
-	if [ "$ARCH" = "arm64" ]; then
-		if [ $width -eq 4 ]; then
-			type=inst
-		fi
-
-		${CROSS_COMPILE}strip $1.o
+	if [ "$ARCH" = "arm64" -a $width -eq 4 ]; then
+		type=inst
 	fi
 
 	${CROSS_COMPILE}objdump $OBJDUMPFLAGS -S $1.o | \