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

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Message-ID: <CAEf4BzbU6xt49+VYSDGoXonOMdB3SPDdh_sr2pTeUC66sT3kPw@mail.gmail.com>
Date:   Tue, 24 Aug 2021 11:24:34 -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 11:17 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Aug 24, 2021 at 11:02 AM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > 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()?
>
> bpf_trace_printf could be ok, but see below.
>
> > 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.
>
> Cannot we change the existing bpf_printk() macro to work on old and new kernels?

Only if we break backwards compatibility. And I only know how to
detect the presence of new helper with CO-RE, which automatically
makes any BPF program using this macro CO-RE-dependent, which might
not be what users want (vmlinux BTF is still not universally
available). If I could do something like that without breaking change
and without CO-RE, I'd update bpf_printk() to use `const char *fmt`
for format string a long time ago. But adding CO-RE dependency for
bpf_printk() seems like a no-go.

> So bpf_printk() would use bpf_trace_printf() on new and
> bpf_trace_printk() on old?
> I think bpf_trace_vprintk() looks cleaner in this context if we reuse
> bpf_printk() macro.