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From:   Cong Wang <xiyou.wangcong@gmail.com>
Date:   Tue, 5 Jan 2021 12:48:06 -0800
Message-ID: <CAM_iQpW5ajiTTW7HBZiK+n_F1MhGyzzD+OWExns1YbejHRsy5A@mail.gmail.com>
Subject: Re: [RFC PATCH bpf-next] ksnoop: kernel argument/return value
 tracing/display using BTF
To:     Alan Maguire <alan.maguire@oracle.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>, andrii@kernel.org,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, yhs@fb.com,
        John Fastabend <john.fastabend@gmail.com>, kpsingh@kernel.org,
        Nathan Chancellor <natechancellor@gmail.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@redhat.com>,
        jean-philippe@linaro.org, bpf <bpf@vger.kernel.org>,
        Linux Kernel Network Developers <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Jan 4, 2021 at 7:29 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>
> BPF Type Format (BTF) provides a description of kernel data structures
> and of the types kernel functions utilize as arguments and return values.
>
> A helper was recently added - bpf_snprintf_btf() - that uses that
> description to create a string representation of the data provided,
> using the BTF id of its type.  For example to create a string
> representation of a "struct sk_buff", the pointer to the skb
> is provided along with the type id of "struct sk_buff".
>
> Here that functionality is utilized to support tracing kernel
> function entry and return using k[ret]probes.  The "struct pt_regs"
> context can be used to derive arguments and return values, and
> when the user supplies a function name we
>
> - look it up in /proc/kallsyms to find its address/module
> - look it up in the BTF kernel data to get types of arguments
>   and return value
> - store a map representation of the trace information, keyed by
>   instruction pointer
> - on function entry/return we look up the map to retrieve the BTF
>   ids of the arguments/return values and can call bpf_snprintf_btf()
>   with these argument/return values along with the type ids to store
>   a string representation in the map.
> - this is then sent via perf event to userspace where it can be
>   displayed.
>
> ksnoop can be used to show function signatures; for example:

This is definitely quite useful!

Is it possible to integrate this with bpftrace? That would save people
from learning yet another tool. ;)

Thanks.