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