Re: [PATCH bpf-next 2/5] bpf: add bpf_trace_vprintk helper

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On Tue, Aug 24, 2021 at 11:24 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Tue, Aug 24, 2021 at 11:17 AM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > On Tue, Aug 24, 2021 at 11:02 AM Andrii Nakryiko
> > <andrii.nakryiko@xxxxxxxxx> wrote:
> > >
> > > On Tue, Aug 24, 2021 at 10:57 AM Alexei Starovoitov
> > > <alexei.starovoitov@xxxxxxxxx> wrote:
> > > >
> > > > On Mon, Aug 23, 2021 at 9:50 PM Andrii Nakryiko
> > > > <andrii.nakryiko@xxxxxxxxx> wrote:
> > > > >
> > > > > On Fri, Aug 20, 2021 at 7:59 PM Dave Marchevsky <davemarchevsky@xxxxxx> 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.

I see. Naming is the hardest.
I think Dave's current choice of lower case bpf_vprintk() macro and
bpf_trace_vprintk()
helper fits the existing bpf_printk/bpf_trace_printk the best.
Yes, it's inconsistent with BPF_SEQ_PRINTF/BPF_SNPRINTF,
but consistent with trace_printk. Whichever way we go it will be inconsistent.
Stylistically I like the lower case macro, since it doesn't scream at me.



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