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

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Message-ID: <20170928143704.GC3611@e103592.cambridge.arm.com>
Date:   Thu, 28 Sep 2017 15:37:04 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Will Deacon <will.deacon@....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 03:14:47PM +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).

> > > This patch resolves both issues by using the .inst directive for 4-byte
> > > quantities on arm64 and stripping the resulting binaries (as is done on
> > > arm already) to remove the mapping symbols.
> > > 
> > > Signed-off-by: Will Deacon <will.deacon@....com>
> > > 
> > > ---
> > >  scripts/decodecode | 8 ++++++++
> > >  1 file changed, 8 insertions(+)
> > > 
> > > diff --git a/scripts/decodecode b/scripts/decodecode
> > > index d8824f37acce..67214ec5b2cb 100755
> > > --- a/scripts/decodecode
> > > +++ b/scripts/decodecode
> > > @@ -58,6 +58,14 @@ disas() {
> > >  		${CROSS_COMPILE}strip $1.o
> > >  	fi
> > >  
> > > +	if [ "$ARCH" = "arm64" ]; then
> > > +		if [ $width -eq 4 ]; then
> > > +			type=inst
> > 
> > Can we merge with arm here, or does arm still support toolchains that
> > don't have .inst?  Anyway, no big deal.
> 
> I thought we still supported those, so I'd be reluctant to merge the
> clauses unless it's broken (the script works as-is for me with arm). I'm
> also not sure what we should do for 16-bit Thumb-2 encodings, where we
> have inst.n and inst.w to contend with.

Fair enough.  I suspected as much, too.

Cheers
---Dave

> > > +		fi
> > > +
> > > +		${CROSS_COMPILE}strip $1.o
> > > +	fi
> > > +
> > >  	${CROSS_COMPILE}objdump $OBJDUMPFLAGS -S $1.o | \
> > >  		grep -v "/tmp\|Disassembly\|\.text\|^$" > $1.dis 2>&1
> > 
> > FWIW,
> > 
> > Reviewed-by: Dave Martin <Dave.Martin@....com>
> 
> Thanks!
> 
> Will
> 
> _______________________________________________
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> linux-arm-kernel@...ts.infradead.org
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