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
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
>
> >
> > > 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
