On Tue, Aug 24, 2021 at 2:24 PM Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > On Tue, Aug 24, 2021 at 2:00 PM Alexei Starovoitov > <alexei.starovoitov@xxxxxxxxx> wrote: > > > > 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. > > Ok, it's fine. Even more so because we don't need a new macro, we can > just extend the existing bpf_printk() macro to automatically pick > bpf_trace_printk() if more than 3 arguments is provided. > > Dave, you'll have to solve a bit of a puzzle macro-wise, but it's > possible to use either bpf_trace_printk() or bpf_trace_vprintk() > transparently for the user. > > The only downside is that for <3 args, for backwards compatibility, > we'd have to stick to > > char ___fmt[] = fmt; > > vs more efficient > > static const char ___fmt[] = fmt; > > But I'm thinking it might be time to finally make this improvement. We > can also allow users to fallback to less efficient ways for really old > kernels with some extra flag, like so > > #ifdef BPF_NO_GLOBAL_DATA > char ___fmt[] = fmt; > #else > static const char ___fmt[] = fmt; > #end > > Thoughts? +1 from me for the latter assuming macro magic is possible.
