netdev - Re: [PATCH bpf-next 06/29] bpf: Add bpf_arg/bpf_ret

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Date:   Wed, 1 Dec 2021 09:37:08 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Jiri Olsa <jolsa@...hat.com>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH bpf-next 06/29] bpf: Add bpf_arg/bpf_ret_value helpers for
 tracing programs

On Tue, Nov 30, 2021 at 11:13 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> Hm... I'd actually try to keep kprobe BTF-free. We have fentry for
> cases where BTF is present and the function is simple enough (like <=6
> args, etc). Kprobe is an escape hatch mechanism when all the BTF
> fanciness just gets in the way (retsnoop being a primary example from
> my side). What I meant here was that bpf_get_arg(int n) would read
> correct fields from pt_regs that map to first N arguments passed in
> the registers. What we currently have with PT_REGS_PARM macros in
> bpf_tracing.h, but with a proper unified BPF helper.

and these macros are arch specific.
which means that it won't be a trivial patch to add bpf_get_arg()
support for kprobes.
Plenty of things to consider. Like should it return an error
at run-time or verification time when a particular arch is not supported.
Or argument 6 might be available on one arch, but not on the other.
32-bit CPU regs vs 64-bit regs of BPF, etc.
I wouldn't attempt to mix this work with current patches.