netdev - Re: [RFC PATCH bpf-next] ksnoop: kernel argument/return value tracing/display using BTF

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Message-ID: <alpine.LRH.2.23.451.2101052209360.30305@localhost>
Date:   Tue, 5 Jan 2021 22:44:29 +0000 (GMT)
From:   Alan Maguire <alan.maguire@...cle.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
cc:     Alan Maguire <alan.maguire@...cle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, andrii@...nel.org,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, yhs@...com,
        John Fastabend <john.fastabend@...il.com>, kpsingh@...nel.org,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        jean-philippe@...aro.org, bpf <bpf@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH bpf-next] ksnoop: kernel argument/return value
 tracing/display using BTF



On Tue, 5 Jan 2021, Cong Wang wrote:

> On Mon, Jan 4, 2021 at 7:29 AM Alan Maguire <alan.maguire@...cle.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. ;)
> 

I'd imagine (and hope!) other tracing tools will do this, but right 
now the aim is to make the task of tracing kernel data structures simpler, 
so having a tool dedicated to just that can hopefully help those 
discussions.  There's a bit more work to be done to simplify that task, for
example  implementing Alexei's suggestion to support pretty-printing of 
data structures using BTF in libbpf.

My hope is that we can evolve this tool - or something like it - to the 
point where we can solve that one problem easily, and that other more 
general tracers can then make use of that solution.  I probably should
have made all of this clearer in the patch submission, sorry about that.

Alan