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

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Date:   Wed, 25 Apr 2018 17:34:12 +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:
> 
> bool flag = false
> ...
> /* A bunch of other code */
> ...
> if (flag)
>         do_something()
> 
> In some cases llvm is not able to remove at compile time the code for
> do_something(), so the generated BPF program ends up with a large amount
> of dead instructions. In one specific real life example, there are two
> series of ~500 and ~1000 dead instructions in the program. When the
> verifier replaces them with a series of ja-1 instructions, it causes an
> interesting behavior at JIT time.
[...]
> 
> Fixes: 2a5418a13fcf ("bpf: improve dead code sanitizing")
> Signed-off-by: Gianluca Borello <g.borello@...il.com>

I've applied this fix to bpf tree, thanks Gianluca!