Re: [RFC PATCH bpf-next 1/2] libbpf: Add libbpf_probe_bpf_kfunc API

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Jan 23, 2025 at 09:07:11PM +0100, Jiri Olsa wrote:
> On Thu, Jan 23, 2025 at 01:06:19AM +0800, Tao Chen wrote:
> > Similarly to libbpf_probe_bpf_helper, the libbpf_probe_bpf_kfunc
> > used to test the availability of the different eBPF kfuncs on the
> > current system.
> 
> hi,
> there's "bpf_kfunc" DECL_TAG for each kfunc in BTF data,
> I think that should do the same job? please check [1] and
> related commits for details

nah there's v2 with support for modules.. makes sense then, will check

sorry for noise ;-)

jirka

> 
> jirka
> 
> 
> [1] 770abbb5a25a bpftool: Support dumping kfunc prototypes from BTF
> 
> > 
> > Signed-off-by: Tao Chen <chen.dylane@gmail.com>
> > ---
> >  tools/lib/bpf/libbpf.h        | 16 +++++++++++++++-
> >  tools/lib/bpf/libbpf.map      |  1 +
> >  tools/lib/bpf/libbpf_probes.c | 36 +++++++++++++++++++++++++++++++++++
> >  3 files changed, 52 insertions(+), 1 deletion(-)
> > 
> > diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> > index 3020ee45303a..3b6d33578a16 100644
> > --- a/tools/lib/bpf/libbpf.h
> > +++ b/tools/lib/bpf/libbpf.h
> > @@ -1680,7 +1680,21 @@ LIBBPF_API int libbpf_probe_bpf_map_type(enum bpf_map_type map_type, const void
> >   */
> >  LIBBPF_API int libbpf_probe_bpf_helper(enum bpf_prog_type prog_type,
> >  				       enum bpf_func_id helper_id, const void *opts);
> > -
> > +/**
> > + * @brief **libbpf_probe_bpf_kfunc()** detects if host kernel supports the
> > + * use of a given BPF kfunc from specified BPF program type.
> > + * @param prog_type BPF program type used to check the support of BPF kfunc
> > + * @param kfunc_id The btf ID of BPF kfunc to check support for
> > + * @param opts reserved for future extensibility, should be NULL
> > + * @return 1, if given combination of program type and kfunc is supported; 0,
> > + * if the combination is not supported; negative error code if feature
> > + * detection for provided input arguments failed or can't be performed
> > + *
> > + * Make sure the process has required set of CAP_* permissions (or runs as
> > + * root) when performing feature checking.
> > + */
> > +LIBBPF_API int libbpf_probe_bpf_kfunc(enum bpf_prog_type prog_type,
> > +				      int kfunc_id, const void *opts);
> >  /**
> >   * @brief **libbpf_num_possible_cpus()** is a helper function to get the
> >   * number of possible CPUs that the host kernel supports and expects.
> > diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> > index a8b2936a1646..e93fae101efd 100644
> > --- a/tools/lib/bpf/libbpf.map
> > +++ b/tools/lib/bpf/libbpf.map
> > @@ -436,4 +436,5 @@ LIBBPF_1.6.0 {
> >  		bpf_linker__add_buf;
> >  		bpf_linker__add_fd;
> >  		bpf_linker__new_fd;
> > +		libbpf_probe_bpf_kfunc;
> >  } LIBBPF_1.5.0;
> > diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
> > index 9dfbe7750f56..bc1cf2afbe87 100644
> > --- a/tools/lib/bpf/libbpf_probes.c
> > +++ b/tools/lib/bpf/libbpf_probes.c
> > @@ -413,6 +413,42 @@ int libbpf_probe_bpf_map_type(enum bpf_map_type map_type, const void *opts)
> >  	return libbpf_err(ret);
> >  }
> >  
> > +int libbpf_probe_bpf_kfunc(enum bpf_prog_type prog_type, int kfunc_id,
> > +			   const void *opts)
> > +{
> > +	struct bpf_insn insns[] = {
> > +		BPF_EXIT_INSN(),
> > +		BPF_EXIT_INSN(),
> > +	};
> > +	const size_t insn_cnt = ARRAY_SIZE(insns);
> > +	int err;
> > +	char buf[4096];
> > +
> > +	if (opts)
> > +		return libbpf_err(-EINVAL);
> > +
> > +	insns[0].code = BPF_JMP | BPF_CALL;
> > +	insns[0].src_reg = BPF_PSEUDO_KFUNC_CALL;
> > +	insns[0].imm = kfunc_id;
> > +
> > +	/* Now only support kfunc from vmlinux */
> > +	insns[0].off = 0;
> > +
> > +	buf[0] = '\0';
> > +	err = probe_prog_load(prog_type, insns, insn_cnt, buf, sizeof(buf));
> > +	if (err < 0)
> > +		return libbpf_err(err);
> > +
> > +	/* If BPF verifier recognizes BPF kfunc but it's not supported for
> > +	 * given BPF program type, it will emit "calling kernel function
> > +	 * bpf_cpumask_create is not allowed"
> > +	 */
> > +	if (err == 0 && strstr(buf, "not allowed"))
> > +		return 0;
> > +
> > +	return 1; /* assume supported */
> > +}
> > +
> >  int libbpf_probe_bpf_helper(enum bpf_prog_type prog_type, enum bpf_func_id helper_id,
> >  			    const void *opts)
> >  {
> > -- 
> > 2.43.0
> >