netdev - RE: [PATCH v3 bpf-next 1/9] bpf: track spill/fill of constants

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Date:   Wed, 19 Jun 2019 17:24:32 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Alexei Starovoitov <ast@...nel.org>, davem@...emloft.net
Cc:     daniel@...earbox.net, netdev@...r.kernel.org, bpf@...r.kernel.org,
        kernel-team@...com
Subject: RE: [PATCH v3 bpf-next 1/9] bpf: track spill/fill of constants

Alexei Starovoitov wrote:
> Compilers often spill induction variables into the stack,
> hence it is necessary for the verifier to track scalar values
> of the registers through stack slots.
> 
> Also few bpf programs were incorrectly rejected in the past,
> since the verifier was not able to track such constants while
> they were used to compute offsets into packet headers.
> 
> Tracking constants through the stack significantly decreases
> the chances of state pruning, since two different constants
> are considered to be different by state equivalency.
> End result that cilium tests suffer serious degradation in the number
> of states processed and corresponding verification time increase.
> 
>                      before  after
> bpf_lb-DLB_L3.o      1838    6441
> bpf_lb-DLB_L4.o      3218    5908
> bpf_lb-DUNKNOWN.o    1064    1064
> bpf_lxc-DDROP_ALL.o  26935   93790
> bpf_lxc-DUNKNOWN.o   34439   123886
> bpf_netdev.o         9721    31413
> bpf_overlay.o        6184    18561
> bpf_lxc_jit.o        39389   359445
> 
> After further debugging turned out that cillium progs are
> getting hurt by clang due to the same constant tracking issue.
> Newer clang generates better code by spilling less to the stack.
> Instead it keeps more constants in the registers which
> hurts state pruning since the verifier already tracks constants
> in the registers:
>                   old clang  new clang
>                          (no spill/fill tracking introduced by this patch)
> bpf_lb-DLB_L3.o      1838    1923
> bpf_lb-DLB_L4.o      3218    3077
> bpf_lb-DUNKNOWN.o    1064    1062
> bpf_lxc-DDROP_ALL.o  26935   166729
> bpf_lxc-DUNKNOWN.o   34439   174607
                       ^^^^^^^^^^^^^^
Any idea what happened here? Going from 34439 -> 174607 on the new clang?

> bpf_netdev.o         9721    8407
> bpf_overlay.o        6184    5420
> bpf_lcx_jit.o        39389   39389
> 
> The final table is depressing:
>                   old clang  old clang    new clang  new clang
>                            const spill/fill        const spill/fill
> bpf_lb-DLB_L3.o      1838    6441          1923      8128
> bpf_lb-DLB_L4.o      3218    5908          3077      6707
> bpf_lb-DUNKNOWN.o    1064    1064          1062      1062
> bpf_lxc-DDROP_ALL.o  26935   93790         166729    380712
> bpf_lxc-DUNKNOWN.o   34439   123886        174607    440652
> bpf_netdev.o         9721    31413         8407      31904
> bpf_overlay.o        6184    18561         5420      23569
> bpf_lxc_jit.o        39389   359445        39389     359445
> 
> Tracking constants in the registers hurts state pruning already.
> Adding tracking of constants through stack hurts pruning even more.
> The later patch address this general constant tracking issue
> with coarse/precise logic.
> 
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> Acked-by: Andrii Nakryiko <andriin@...com>
> ---
>  kernel/bpf/verifier.c | 90 +++++++++++++++++++++++++++++++------------
>  1 file changed, 65 insertions(+), 25 deletions(-)

I know these are already in bpf-next sorry it took me awhile to get
time to review, but looks good to me. Thanks! We had something similar
in the earlier loop test branch from last year.