netdev - Re: len = bpf_probe_read_str(); bpf_perf_event

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Message-ID: <CAJWsiu=psGT41fk6y9YE14UCwGd+Fq4_CBMOkiJ8fC-pMq1b5Q@mail.gmail.com>
Date:   Wed, 22 Nov 2017 10:42:22 -0800
From:   Gianluca Borello <g.borello@...il.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Yonghong Song <yhs@...com>, Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        David Miller <davem@...emloft.net>,
        Linux Networking Development Mailing List 
        <netdev@...r.kernel.org>
Subject: Re: len = bpf_probe_read_str(); bpf_perf_event_output(... len) == FAIL

On Tue, Nov 21, 2017 at 2:31 PM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> yeah sorry about this hack. Gianluca reported this issue as well.
> Yonghong fixed it for bpf_probe_read only. We will extend
> the fix to bpf_probe_read_str() and bpf_perf_event_output() asap.
> The above workaround gets too much into llvm and verifier details
> we should strive to make bpf program writing as easy as possible.
>

Hi Arnaldo

With the help of Alexei, Daniel and Yonghong I just submitted a new
series ("bpf: fix semantics issues with helpers receiving NULL
arguments") that includes a fix in bpf_perf_event_output. This should
simplify the way you write your bpf programs, so you shouldn't be
required to write those convoluted branches anymore (there are a few
usage examples in the commit log).

In my case it made writing the code much easier, after applying it I
haven't been surprised by the compiler output in a while, and I hope
your experience will be improved as well.

Thanks