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

On Tue, Aug 24, 2021 at 11:02 AM Andrii Nakryiko
[off-list ref] wrote:

On Tue, Aug 24, 2021 at 10:57 AM Alexei Starovoitov
[off-list ref] wrote:

quoted

On Mon, Aug 23, 2021 at 9:50 PM Andrii Nakryiko
[off-list ref] wrote:

quoted

On Fri, Aug 20, 2021 at 7:59 PM Dave Marchevsky [off-list ref] wrote:

quoted

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.

quoted

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?
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.