Re: [RFC PATCH bpf-next 2/4] bpf: make BTF show support generic, apply to seq files/bpf_trace_printk

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Aug 18, 2020 at 10:12:05AM +0100, Alan Maguire wrote:
> 
> Fair enough. I'm thinking a helper like
> 
> long bpf_btf_snprintf(char *str, u32 str_size, struct btf_ptr *ptr,
> 		      u32 ptr_size, u64 flags);
> 
> Then the user can choose perf event or ringbuf interfaces
> to share the results with userspace.

makes sense.

> 
> > If the user happen to use bpf_trace_printk("%s", buf);
> > after that to print that string buffer to trace_pipe that's user's choice.
> > I can see such use case when program author wants to debug
> > their bpf program. That's fine. But for kernel debugging, on demand and
> > "always on" logging and tracing the documentation should point
> > to sustainable interfaces that don't interfere with each other,
> > can be run in parallel by multiple users, etc.
> > 
> 
> The problem with bpf_trace_printk() under this approach is
> that the string size for %s arguments is very limited;
> bpf_trace_printk() restricts these to 64 bytes in size.
> Looks like bpf_seq_printf() restricts a %s string to 128
> bytes also.  We could add an additional helper for the 
> bpf_seq case which calls bpf_seq_printf() for each component
> in the object, i.e.
> 
> long bpf_seq_btf_printf(struct seq_file *m, struct btf_ptr *ptr,
> 			u32 ptr_size, u64 flags);

yeah. this one is needed as well.
Please double check that it works out of bpf iterator too.
Especially the case when output is not power of 2.
bpf_iter.c keeps page sized buffer and will restart
iteration on overflow.
Could you please modify progs/bpf_iter_task.c to do
bpf_seq_btf_printf(seq, {task, }, 0);
and check that the output doesn't contain repeated/garbled lines.

Also I'm not sure why you see 64 limit of bpf_trace_printk as a blocker.
We could do:
if (in_nmi())
  use 64 byte on stack
else
  spin_lock_irqsave and use 1k static buffer.
and/or we can introduce bpf_trace_puts(const char *buf, u32 size_of_buf);
with
        .arg1_type      = ARG_PTR_TO_MEM,
        .arg2_type      = ARG_CONST_SIZE,
add null termination check and call trace_bpf_trace_printk() on that
buffer. That will avoid a bunch of copies.
But that still doesn't make bpf_trace_puts() a good interface for
dumping stuff to user space. It's still debug only feature.