linux-kernel - Re: [PATCH v2] arm64: bpf: Fix branch offset in JIT

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Message-ID: <20200916160404.GA153139@apalos.home>
Date:   Wed, 16 Sep 2020 19:04:04 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Will Deacon <will@...nel.org>
Cc:     bpf@...r.kernel.org, ardb@...nel.org, naresh.kamboju@...aro.org,
        Jiri Olsa <jolsa@...nel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Yauheni Kaliuta <yauheni.kaliuta@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Zi Shen Lim <zlim.lnx@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64: bpf: Fix branch offset in JIT

Hi Will, 

On Tue, Sep 15, 2020 at 02:11:03PM +0100, Will Deacon wrote:
[...]
> >  			continue;
> >  		}
> > -		if (ctx->image == NULL)
> > -			ctx->offset[i] = ctx->idx;
> >  		if (ret)
> >  			return ret;
> >  	}
> > +	if (ctx->image == NULL)
> > +		ctx->offset[i] = ctx->idx;
> 
> I think it would be cleared to set ctx->offset[0] before the for loop (with
> a comment about what it is) and then change the for loop to iterate from 1
> all the way to prog->len.

On a second thought while trying to code this, I'd prefer leaving it as is. 
First of all we'll have to increase ctx->idx while adding ctx->offset[0] and 
more importantly, I don't think that's a 'special' case. 
It's still the same thing i.e the start of the 1st instruction (which happens 
to be the end of prologue), the next one will be the start of the second 
instruction etc etc. 

I don't mind changing if you feel strongly about it, but I think it makese sense
as-is.

Thanks
/Ilias
> 
> Will