On Tue, Mar 31, 2026 at 8:25 AM Hoyeon Lee <[email protected]> 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 <[email protected]>
> ---
> 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"
> + 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
> 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
>
