On Thu, Apr 13, 2023 at 2:37 PM andrea terzolo <andreaterzolo3@xxxxxxxxx> wrote:
>
> On Wed, 12 Apr 2023 at 20:48, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Sat, Apr 8, 2023 at 6:34 AM andrea terzolo <andreaterzolo3@xxxxxxxxx> wrote:
> > >
> > > Il giorno mer 5 apr 2023 alle ore 00:32 Andrii Nakryiko
> > > <andrii.nakryiko@xxxxxxxxx> ha scritto:
> > > >
> > > > On Thu, Mar 30, 2023 at 10:21 AM andrea terzolo
> > > > <andreaterzolo3@xxxxxxxxx> wrote:
> > > > >
> > > > > Hi all!
> > > > >
> > > > > If I can I would like to ask one question about the
> > > > > `libbpf_probe_bpf_prog_type` API. The idea is to use `fentry/fexit`
> > > > > bpf progs only if they are available and fall back to simple `kprobes`
> > > > > when they are not. Is there a way to probe `BPF_TRACE_FENTRY` support
> > > > > with `libbpf` APIs? I was looking at `libbpf_probe_bpf_prog_type` API
> > > > > but it seems to check the `prog_type` rather than the `attach_type`,
> > > > > when I call it `libbpf_probe_bpf_prog_type(BPF_PROG_TYPE_TRACING,
> > > > > NULL);` it returns `1` even if `fentry/fexit` progs are not supported
> > > > > on my machine. Is there a way to probe this feature with other
> > > > > `libbpf` APIs?
> > > > >
> > > >
> > > > looking at libbpf probing code, for BPF_PROG_TYPE_TRACING we choose
> > > > BPF_TRACE_FENTRY attach type automatically (because it doesn't really
> > > > matter whether its BPF_TRACE_FEXIT or BPF_MODIFY_RETURN, they all are
> > > > either supported or none is). We then expect that verifier will
> > > > complain with "attach_btf_id 1 is not a function" error. If we do see
> > > > that error, we know that verifier supports fentry/fexit programs *in
> > > > principle*, which is what we are checking with
> > > > libbpf_probe_bpf_prog_type().
> > >
> > > Ok got it, thank you. My issue is that in my project I need to use
> > > BPF_TRACE_RAW_TP programs that AFAIK don't require the support for bpf
> > > trampoline, so they could be supported even if
> > > BPF_TRACE_FENTRY/BPF_MODIFY_RETURN are not supported. This is what
> > > happens on arm64 kernels where we have BPF_TRACE_RAW_TP but
> > > BPF_TRACE_FENTRY/BPF_MODIFY_RETURN are still not supported... Right
> > > now I'm using libbpf_probe_bpf_prog_type() to check the support for
> > > BPF_TRACE_RAW_TP but this is just an approximation, probably the best
> > > way to do that is to inject a small
> > > BPF_TRACE_RAW_TP prog and check that it is correctly loaded. It seems
> > > that libbpf doesn't provide APIs to do that, is it right?
> >
> > BPF_TRACE_RAW_TP is not a program type, so not sure what you are checking.
> >
> > fentry/fexit is BPF_PROG_TYPE_TRACING, while raw tracepoint is
> > BPF_PROG_TYPE_RAW_TRACEPOINT, there shouldn't be any problem for you.
>
> If I understood correctly BPF_PROG_TYPE_TRACING has several attach types:
> * BPF_MODIFY_RETURN
> * BPF_TRACE_FENTRY
> * BPF_TRACE_FEXIT
> * BPF_TRACE_ITER
> * BPF_TRACE_RAW_TP
>
> I'm using the last one so BPF_TRACE_RAW_TP ('tp_btf' elf section
ah, so we were talking about BTF-aware raw tracepoint, sorry, I missed
that. Was thinking about plain raw tracepoint all along the way.
> name). libbpf_probe_bpf_prog_type API allows checking the support for
> a specific prog_type like BPF_PROG_TYPE_TRACING but indeed I need a
> check on the attach_type support. BPF_TRACE_FENTRY/FEXIT have
> different requirements with respect to BPF_TRACE_RAW_TP so I would
> like to know if libbpf provides out-of-the-box an API to check the
> attach_type support. libbpf_probe_bpf_prog_type doesn't tell if
> BPF_TRACE_FENTRY/FEXIT are effectively supported it just tells if
> BPF_PROG_TYPE_TRACING is supported, so I was just asking what is the
> best way to probe the support for a specific attach_type
ok, this sounds like a perfect use case to use that opts argument and
allow programs to optionally specify expected_attach_type. see
LIBBPF_OPTS, OPTS_GET and other cases of using `struct xxx_opts` in
libbpf code base.
Would you be interested in contributing such a patch?
>
> >
> > >
> > > > If kernel doesn't support fentry/fexit attachment for some specific
> > > > function you'd like to attach to, that's a different matter. This
> > > > would be equivalent to BPF_PROG_TYPE_KPROBE check -- we check if
> > > > kprobes in general are supported, but not whether kprobing specific
> > > > kernel function works.
> > > >
> > > > I assume by "not supported on my machine" you mean that you can't
> > > > attach fentry/fexit to some function? If not, let me know, and we'd
> > > > have to debug this further.
> > >
> > > Sorry, probably I was not so clear, with this statement I mean that
> > > libbpf_probe_bpf_prog_type() returns 1 even if BPF_TRACE_FENTRY progs
> > > cannot be attached into the kernel. [0] is an example of what I'm
> > > doing.
> > > 1. Check fentry support with libbpf_probe_bpf_prog_type
> > > 2. Check fentry support with an approach similar to libbpf-tools (as
> > > you suggested)
> > > 3. Try to inject my real BPF programs.
> > >
> > > (2) (libbpf-tool check) is correctly able to detect that
> > > BPF_TRACE_FENTRY progs are not supported, when we call
> > > `bpf_raw_tracepoint_open` to attach the fentry prog, `524` is returned
> > > so we understand that this program is not supported. On the other
> > > side, (1) is not able to detect that programs are not supported, the
> > > API returns `1` as if they were supported. Now I have to highlight
> > > that this API is called libbpf_probe_bpf_prog_type and not
> > > libbpf_probe_bpf_attach_type, so 1 could be the right return value
> > > since BPF_PROG_TYPE_TRACING progs are effectively supported, for
> > > example, attach_type BPF_TRACE_RAW_TP is supported, but some other
> > > attach types like BPF_TRACE_FENTRY/BPF_MODIFY_RETURN are not. If this
> > > API just checks for BPF_PROG_TYPE_TRACING support, probably the best
> > > way I have to check if a specific attach type is supported is to
> > > directly inject a small prog of this type, as libbpf-tool does. WDYT?
> >
> > Could it be that you are mixing up enum bpf_prog_type with enum
> > bpf_attach_type when calling libbpf_probe_bpf_prog_type()?
> >
> > >
> > > [0]: https://github.com/Andreagit97/BPF-perf-tests/blob/main/templates/fentry_attach.c
> > >
> > > > If you want to know if some function can be traced with fentry/fexit,
> > > > check below helper function from libbpf-tools ([0])
> > > >
> > > > bool fentry_can_attach(const char *name, const char *mod)
> > > >
> > > >
> > > > [0] https://github.com/iovisor/bcc/blob/master/libbpf-tools/trace_helpers.c#LL1043-L1043C58
> > > >
> > > Thank you for the pointer!
> > > >
> > > > > Thank you in advance for your time,
> > > > > Andrea
