Re: [RFC] libbpf,selftests: Question on btf_dump__emit_type_decl for BTF_KIND_FUNC

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Mar 03, 2020 at 09:09:38AM -0800, Andrii Nakryiko wrote:
> On Tue, Mar 3, 2020 at 6:12 AM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > hi,
> > for bpftrace I'd like to print BTF functions (BTF_KIND_FUNC)
> > declarations together with their names.
> >
> > I saw we have btf_dump__emit_type_decl and added BTF_KIND_FUNC,
> > where it seemed to be missing, so it prints out something now
> > (not sure it's the right fix though).
> >
> > Anyway, would you be ok with adding some flag/bool to struct
> > btf_dump_emit_type_decl_opts, so I could get output like:
> >
> >   kfunc:ksys_readahead(int fd, long long int offset, long unsigned int count) = ssize_t
> >   kfunc:ksys_read(unsigned int fd, char buf, long unsigned int count) = size_t
> >
> > ... to be able to the arguments and return type separated,
> > so I could easily get to something like above?
> >
> > Current interface is just vfprintf callback and I'm not sure
> > I can rely that it will allywas be called with same arguments,
> > like having separated calls for parsed atoms like 'return type',
> > '(', ')', '(', 'arg type', 'arg name', ...
> >
> > I'm open to any suggestion ;-)
> 
> Hey Jiri!
> 
> Can you please elaborate on the use case and problem you are trying to solve?
> 
> I think we can (and probably even should) add such option and support
> to dump functions, but whatever we do it should be a valid C syntax
> and should be compilable.
> Example above:
> 
> kfunc:ksys_read(unsigned int fd, char buf, long unsigned int count) = size_t
> 
> Is this really the syntax you need to get? I think btf_dump, when
> (optionally) emitting function declaration, will have to emit that
> particular one as:
> 
> size_t ksys_read(unsigned int fd, char buf, long unsigned int count);
> 
> But I'd like to hear the use case before we add this. Thanks!

the use case is just for the 'bpftrace -l' output, which displays
the probe names that could be used.. for kernel BTF kernel functions
it's 'kfunc:function(args)'

	software:task-clock:
	hardware:backend-stalls:
	hardware:branch-instructions:
	...
	tracepoint:kvmmmu:kvm_mmu_pagetable_walk
	tracepoint:kvmmmu:kvm_mmu_paging_element
	...
	kprobe:console_on_rootfs
	kprobe:trace_initcall_start_cb
	kprobe:run_init_process
	kprobe:try_to_run_init_process
	...
	kfunc:x86_reserve_hardware
	kfunc:hw_perf_lbr_event_destroy
	kfunc:x86_perf_event_update

I dont want to print the return type as is in C, because it would
mess up the whole output, hence the '= <return type>'

	kfunc:ksys_readahead(int fd, long long int offset, long unsigned int count) = ssize_t
	kfunc:ksys_read(unsigned int fd, char buf, long unsigned int count) = size_t

also possible only in verbose mode ;-)

the final shape of the format will be decided in a bpftrace review,
but in any case I think I'll need some way to get these bits:
  <args> <return type>


thanks,
jirka