netdev - Re: [PATCH bpf-next 2/5] bpf: add bpf_trace

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Message-ID: <CAEf4Bza30Rkg02AzmG7Mw5AyE1wykPBuH6f_fXAQXLu2qH2POA@mail.gmail.com>
Date:   Tue, 24 Aug 2021 11:02:03 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Dave Marchevsky <davemarchevsky@...com>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Yonghong Song <yhs@...com>,
        Florent Revest <revest@...omium.org>,
        Networking <netdev@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf-next 2/5] bpf: add bpf_trace_vprintk helper

On Tue, Aug 24, 2021 at 10:57 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Mon, Aug 23, 2021 at 9:50 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Fri, Aug 20, 2021 at 7:59 PM Dave Marchevsky <davemarchevsky@...com> wrote:
> > >
> > > This helper is meant to be "bpf_trace_printk, but with proper vararg
> >
> > We have bpf_snprintf() and bpf_seq_printf() names for other BPF
> > helpers using the same approach. How about we call this one simply
> > `bpf_printf`? It will be in line with other naming, it is logical BPF
> > equivalent of user-space printf (which outputs to stderr, which in BPF
> > land is /sys/kernel/debug/tracing/trace_pipe). And it will be logical
> > to have a nice and short BPF_PRINTF() convenience macro provided by
> > libbpf.
> >
> > > support". Follow bpf_snprintf's example and take a u64 pseudo-vararg
> > > array. Write to dmesg using the same mechanism as bpf_trace_printk.
> >
> > Are you sure about the dmesg part?... bpf_trace_printk is outputting
> > into /sys/kernel/debug/tracing/trace_pipe.
>
> Actually I like bpf_trace_vprintk() name, since it makes it obvious that

It's the inconsistency with bpf_snprintf() and bpf_seq_printf() that's
mildly annoying (it's f at the end, and no v- prefix). Maybe
bpf_trace_printf() then? Or is it too close to bpf_trace_printk()? But
either way you would be using BPF_PRINTF() macro for this. And we can
make that macro use bpf_trace_printk() transparently for <3 args, so
that new macro works on old kernels.

> it's a flavor of bpf_trace_printk() and its quirks that users learned
> to deal with.
> I would reserve bpf_printf() for the future. We might have standalone
> bpf programs in the future (without user space component) and a better
> equivalent
> of stdin/stdout. clang -target bpf hello_world.c -o a.out; ./a.out
> should print to a terminal. Such future hello world in bpf would be
> using bpf_printf()
> or bpf_dprintf().