netdev - Re: [RFC bpf] bpf, x64: fix JIT emission for dead code

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Message-ID: <04f6d13e-4912-f988-623c-fcf330ec6a35@iogearbox.net>
Date:   Wed, 25 Apr 2018 09:48:42 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Gianluca Borello <g.borello@...il.com>, netdev@...r.kernel.org
Cc:     ast@...nel.org
Subject: Re: [RFC bpf] bpf, x64: fix JIT emission for dead code

On 04/25/2018 07:42 AM, Gianluca Borello wrote:
> Commit 2a5418a13fcf ("bpf: improve dead code sanitizing") replaced dead
> code with a series of ja-1 instructions, for safety. That made JIT
> compilation much more complex for some BPF programs. One instance of such
> programs is, for example:
[...]
> A possible approach to mitigate this behavior consists into noticing that
> for ja-1 instructions we don't really need to rely on the estimated size
> of the previous and current instructions, we know that a -1 BPF jump
> offset can be safely translated into a 0xEB instruction with a jump offset
> of -2.
> 
> Such fix brings the BPF program in the previous example to complete again
> in ~9 passes.
> 
> Fixes: 2a5418a13fcf ("bpf: improve dead code sanitizing")
> Signed-off-by: Gianluca Borello <g.borello@...il.com>

Thanks for reporting, Gianluca. The approach your fix takes looks good to me!