netdev - Re: [PATCH net] bpf: fix liveness marking

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Message-ID: <759810f5-f6db-0439-1b2d-637cf83ac2f8@solarflare.com>
Date:   Fri, 6 Oct 2017 17:33:10 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Alexei Starovoitov <ast@...com>,
        "David S . Miller" <davem@...emloft.net>
CC:     Daniel Borkmann <daniel@...earbox.net>, <netdev@...r.kernel.org>,
        <kernel-team@...com>
Subject: Re: [PATCH net] bpf: fix liveness marking

On 06/10/17 00:20, Alexei Starovoitov wrote:
> while processing Rx = Ry instruction the verifier does
> regs[insn->dst_reg] = regs[insn->src_reg]
> which often clears write mark (when Ry doesn't have it)
> that was just set by check_reg_arg(Rx) prior to the assignment.
> That causes mark_reg_read() to keep marking Rx in this block as
> REG_LIVE_READ (since the logic incorrectly misses that it's
> screened by the write) and in many of its parents (until lucky
> write into the same Rx or beginning of the program).
> That causes is_state_visited() logic to miss many pruning opportunities.
Good catch!
> Furthermore mark_reg_read() logic propagates the read mark
> for BPF_REG_FP as well (though it's readonly) which causes
> harmless but unnecssary work during is_state_visited().
Surely it's unnecessary for is_state_visited() to even look at
 BPF_REG_FP anyway, so in addition to your change we could make
 states_equal just do `for (i = 0; i < BPF_REG_FP; i++)`?  That
 might save a bit more time.
> Note that do_propagate_liveness() skips FP correctly,
> so do the same in mark_reg_read() as well.
> It saves 0.2 seconds for the test below
>
> program               before  after
> bpf_lb-DLB_L3.o       2604    2304
> bpf_lb-DLB_L4.o       11159   3723
> bpf_lb-DUNKNOWN.o     1116    1110
> bpf_lxc-DDROP_ALL.o   34566   28004
> bpf_lxc-DUNKNOWN.o    53267   39026
> bpf_netdev.o          17843   16943
> bpf_overlay.o         8672    7929
> time                  ~11 sec  ~4 sec
>
> Fixes: dc503a8ad984 ("bpf/verifier: track liveness for pruning")
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
Very nice numbers!
Acked-by: Edward Cree <ecree@...arflare.com>