Subject: Re: [PATCHv3 bpf-next 6/7] libbpf: allow specification of "kprobe/function+offset"

From: Jiri Olsa <[email-masked]>
Date: Sun, 11 Jul 2021 16:48:42 +0200

On Wed, Jul 07, 2021 at 05:14:20PM -0700, Andrii Nakryiko wrote:
> On Wed, Jul 7, 2021 at 2:54 PM Jiri Olsa <[email-masked]> wrote:
> >
> > From: Alan Maguire <[email-masked]>
> >
> > kprobes can be placed on most instructions in a function, not
> > just entry, and ftrace and bpftrace support the function+offset
> > notification for probe placement.  Adding parsing of func_name
> > into func+offset to bpf_program__attach_kprobe() allows the
> > user to specify
> >
> > SEC("kprobe/bpf_fentry_test5+0x6")
> >
> > ...for example, and the offset can be passed to perf_event_open_probe()
> > to support kprobe attachment.
> >
> > Signed-off-by: Alan Maguire <[email-masked]>
> > Signed-off-by: Jiri Olsa <[email-masked]>
> > ---
> >  tools/lib/bpf/libbpf.c | 20 +++++++++++++++++---
> >  1 file changed, 17 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 1e04ce724240..60c9e3e77684 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -10309,11 +10309,25 @@ struct bpf_link *bpf_program__attach_kprobe(struct bpf_program *prog,
> >                                             const char *func_name)
> 
> I think we should add bpf_program__attach_kprobe_opts instead for the
> programmatic API instead of parsing it here from func_name. It's a
> cumbersome API.
> 
> Parsing SEC() is fine, of course, but then it has to call into
> bpf_program__attach_kprobe_opts() internally.

ok, Alan, will you make the change, or should I do that?

thanks,
jirka

> 
> >  {
> >         char errmsg[STRERR_BUFSIZE];
> > +       char func[BPF_OBJ_NAME_LEN];
> > +       unsigned long offset = 0;
> >         struct bpf_link *link;
> > -       int pfd, err;
> > +       int pfd, err, n;
> > +
> > +       n = sscanf(func_name, "%[a-zA-Z0-9_.]+%lx", func, &offset);
> > +       if (n < 1) {
> > +               err = -EINVAL;
> > +               pr_warn("kprobe name is invalid: %s\n", func_name);
> > +               return libbpf_err_ptr(err);
> > +       }
> > +       if (retprobe && offset != 0) {
> > +               err = -EINVAL;
> > +               pr_warn("kretprobes do not support offset specification\n");
> > +               return libbpf_err_ptr(err);
> > +       }
> >
> > -       pfd = perf_event_open_probe(false /* uprobe */, retprobe, func_name,
> > -                                   0 /* offset */, -1 /* pid */);
> > +       pfd = perf_event_open_probe(false /* uprobe */, retprobe, func,
> > +                                   offset, -1 /* pid */);
> >         if (pfd < 0) {
> >                 pr_warn("prog '%s': failed to create %s '%s' perf event: %s\n",
> >                         prog->name, retprobe ? "kretprobe" : "kprobe", func_name,
> > --
> > 2.31.1
> >
>