Re: [PATCH bpf-next v4 1/2] libbpf: clarify raw-address single kprobe attach behavior

[Hoyeon Lee]

On Wed, Apr 1, 2026 at 8:05 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Tue, Mar 31, 2026 at 8:25 AM Hoyeon Lee <hoyeon.lee@xxxxxxxx> wrote:
> >
> > bpf_program__attach_kprobe_opts() documents single-kprobe attach
> > through func_name, with an optional offset. For the PMU-based
> > non-legacy path, func_name = NULL with an absolute address in offset
> > already works as well, but that is not described in the API.
> >
> > This commit clarifies this existing non-legacy behavior. For PMU-based
> > attach, callers can use func_name = NULL with an absolute address in
> > offset as the raw-address form. For legacy tracefs/debugfs kprobes,
> > reject this form explicitly.
> >
> > Signed-off-by: Hoyeon Lee <hoyeon.lee@xxxxxxxx>
> > ---
> >  tools/lib/bpf/libbpf.c | 21 ++++++++++++---------
> >  tools/lib/bpf/libbpf.h |  3 ++-
> >  2 files changed, 14 insertions(+), 10 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 9ea41f40dc82..9083f542a3b0 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -11523,7 +11523,8 @@ static int determine_uprobe_retprobe_bit(void)
> >  #define PERF_UPROBE_REF_CTR_OFFSET_SHIFT 32
> >
> >  static int perf_event_open_probe(bool uprobe, bool retprobe, const char *name,
> > -                                uint64_t offset, int pid, size_t ref_ctr_off)
> > +                                uint64_t offset_or_addr, int pid,
>
> don't rename it, keep it as "offset"
>

Got it, I'll drop it in v5.

> > +                                size_t ref_ctr_off)
> >  {
> >         const size_t attr_sz = sizeof(struct perf_event_attr);
> >         struct perf_event_attr attr;
> > @@ -11558,7 +11559,7 @@ static int perf_event_open_probe(bool uprobe, bool retprobe, const char *name,
> >         attr.type = type;
> >         attr.config |= (__u64)ref_ctr_off << PERF_UPROBE_REF_CTR_OFFSET_SHIFT;
> >         attr.config1 = ptr_to_u64(name); /* kprobe_func or uprobe_path */
> > -       attr.config2 = offset;           /* kprobe_addr or probe_offset */
> > +       attr.config2 = offset_or_addr;   /* kprobe_addr or probe_offset */
> >
> >         /* pid filter is meaningful only for uprobes */
> >         pfd = syscall(__NR_perf_event_open, &attr,
> > @@ -11816,6 +11817,8 @@ bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
> >         default:
> >                 return libbpf_err_ptr(-EINVAL);
> >         }
> > +       if (!func_name && legacy)
> > +               return libbpf_err_ptr(-ENOTSUP);
>
> -EOPNOTSUPP
>
> pw-bot: cr
>
> >
> >         if (!legacy) {
> >                 pfd = perf_event_open_probe(false /* uprobe */, retprobe,
> > @@ -11835,21 +11838,21 @@ bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
> >                                                     offset, -1 /* pid */);
> >         }
> >         if (pfd < 0) {
> > -               err = -errno;
> > -               pr_warn("prog '%s': failed to create %s '%s+0x%zx' perf event: %s\n",
> > +               err = pfd;
> > +               pr_warn("prog '%s': failed to create %s '%s%s0x%zx' perf event: %s\n",
> >                         prog->name, retprobe ? "kretprobe" : "kprobe",
> > -                       func_name, offset,
> > -                       errstr(err));
> > +                       func_name ?: "", func_name ? "+" : "",
> > +                       offset, errstr(err));
> >                 goto err_out;
> >         }
> >         link = bpf_program__attach_perf_event_opts(prog, pfd, &pe_opts);
> >         err = libbpf_get_error(link);
> >         if (err) {
> >                 close(pfd);
> > -               pr_warn("prog '%s': failed to attach to %s '%s+0x%zx': %s\n",
> > +               pr_warn("prog '%s': failed to attach to %s '%s%s0x%zx': %s\n",
> >                         prog->name, retprobe ? "kretprobe" : "kprobe",
> > -                       func_name, offset,
> > -                       errstr(err));
> > +                       func_name ?: "", func_name ? "+" : "",
> > +                       offset, errstr(err));
> >                 goto err_clean_legacy;
> >         }
> >         if (legacy) {
> > diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> > index 0be34852350f..f75f3ab0f20c 100644
> > --- a/tools/lib/bpf/libbpf.h
> > +++ b/tools/lib/bpf/libbpf.h
> > @@ -557,7 +557,7 @@ struct bpf_kprobe_opts {
> >         size_t sz;
> >         /* custom user-provided value fetchable through bpf_get_attach_cookie() */
> >         __u64 bpf_cookie;
> > -       /* function's offset to install kprobe to */
> > +       /* function offset, or raw address if func_name == NULL (non-legacy) */
>
> legacy mode will be useful on very-very old kernels only, let's not
> distract with constant mentions for "non-legacy", please remove
> "(non-legacy)" here and other legacy mentions above
>
>
> but also, while you are at it, let's add a proper doc comment to
> bpf_program__attach_kprobe and bpf_program__attach_kprobe_opts APIs?
> That seems like a more obvious place to explain name+offset and offset
> as absolute address behavior, IMO
>

Right. I'll drop the "(non-legacy)" wording and add proper API
documentation to bpf_program__attach_kprobe() and
bpf_program__attach_kprobe_opts() in v5.

> >         size_t offset;
> >         /* kprobe is return probe */
> >         bool retprobe;
> > @@ -565,6 +565,7 @@ struct bpf_kprobe_opts {
> >         enum probe_attach_mode attach_mode;
> >         size_t :0;
> >  };
> > +
> >  #define bpf_kprobe_opts__last_field attach_mode
> >
> >  LIBBPF_API struct bpf_link *
> > --
> > 2.52.0
> >