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Date:   Fri, 25 Sep 2020 21:36:06 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        John Fastabend <john.fastabend@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [bpf-next PATCH 1/2] bpf, verifier: Remove redundant
 var_off.value ops in scalar known reg cases

Alexei Starovoitov wrote:
> On Thu, Sep 24, 2020 at 11:45 AM John Fastabend
> <john.fastabend@...il.com> wrote:
> >
> > In BPF_AND and BPF_OR alu cases we have this pattern when the src and dst
> > tnum is a constant.
> >
> >  1 dst_reg->var_off = tnum_[op](dst_reg->var_off, src_reg.var_off)
> >  2 scalar32_min_max_[op]
> >  3       if (known) return
> >  4 scalar_min_max_[op]
> >  5       if (known)
> >  6          __mark_reg_known(dst_reg,
> >                    dst_reg->var_off.value [op] src_reg.var_off.value)
> >
> > The result is in 1 we calculate the var_off value and store it in the
> > dst_reg. Then in 6 we duplicate this logic doing the op again on the
> > value.
> >
> > The duplication comes from the the tnum_[op] handlers because they have
> > already done the value calcuation. For example this is tnum_and().
> >
> >  struct tnum tnum_and(struct tnum a, struct tnum b)
> >  {
> >         u64 alpha, beta, v;
> >
> >         alpha = a.value | a.mask;
> >         beta = b.value | b.mask;
> >         v = a.value & b.value;
> >         return TNUM(v, alpha & beta & ~v);
> >  }
> >
> > So lets remove the redundant op calculation. Its confusing for readers
> > and unnecessary. Its also not harmful because those ops have the
> > property, r1 & r1 = r1 and r1 | r1 = r1.
> >
> > Signed-off-by: John Fastabend <john.fastabend@...il.com>
> 
> Applied. Thanks for the follow up.
> In the future please always cc bpf@...r for two reasons:
> - to get proper 'Link:' integrated in git commit
> - to get them into a new instance of
> https://patchwork.kernel.org/project/bpf/list

+1

>   which we will start using soon to send automatic 'applied' emails.


Apologies, I updated some scripts and unfortunately typo dropped a '-'
and cut off bpf@...r from the CC list. Also I just used it to land
two more patches without bpf@...r happy to resend with CC included
if folks want. Sorry for the extra work/noise.

Thanks.

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