Re: [RFC PATCH bpf-next v2 1/3] libbpf: Auto-upgrade uprobes to multi-uprobes when supported

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Mar 30, 2026 at 04:30:17PM +0530, Varun R Mallya wrote:
> This patch modifies libbpf to automatically "upgrade" standard
> SEC("uprobe") and SEC("uretprobe") programs to use the multi-uprobe
> infrastructure (BPF_TRACE_UPROBE_MULTI) at load time if the kernel
> supports it, making them compatible with BPF tokens.
> 
> To maintain backward compatibility and handle rare cases where singular
> uprobes are required, new SEC("uprobe.single") and SEC("uretprobe.single")
> section types are introduced. These force libbpf to use the legacy
> perf_event_open() attachment path.
> 
> tools/testing/selftests/bpf/progs/test_fill_link_info.c has been
> modified to use SEC("uprobe.single") as it asserts the program type to be
> `BPF_LINK_TYPE_PERF_EVENT` and checks properties related to uprobes that
> use perf.
> 
> Signed-off-by: Varun R Mallya <varunrmallya@gmail.com>
> ---
>  tools/lib/bpf/libbpf.c                        | 53 +++++++++++++++++--
>  .../selftests/bpf/progs/test_fill_link_info.c |  2 +-
>  2 files changed, 51 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 1eaa7527d4da..bd7b6f486430 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -8248,6 +8248,23 @@ static int bpf_object_prepare_progs(struct bpf_object *obj)
>  
>  	for (i = 0; i < obj->nr_programs; i++) {
>  		prog = &obj->programs[i];
> +
> +		if (kernel_supports(obj, FEAT_UPROBE_MULTI_LINK)) {
> +			const char *sec_name = prog->sec_name;
> +			/* Here, we filter out for u[ret]probe or "u[ret]probe/"
> +			 * but we leave out anything with an '@'
> +			 * in it as uprobe_multi does not support versioned
> +			 * symbols yet, so we don't upgrade.
> +			 */

nice, I missed that uprobe.multi does not support versioned symbols,
I guess we should fix that

> +			if (((strncmp(sec_name, "uprobe", 6) == 0 &&

str_has_pfx ?

> +			      (sec_name[6] == '/' || sec_name[6] == '\0')) ||
> +			     (strncmp(sec_name, "uretprobe", 9) == 0 &&
> +			      (sec_name[9] == '/' || sec_name[9] == '\0'))) &&
> +			    !strchr(sec_name, '@')) {
> +				prog->expected_attach_type = BPF_TRACE_UPROBE_MULTI;
> +			}
> +		}
> +
>  		err = bpf_object__sanitize_prog(obj, prog);
>  		if (err)
>  			return err;
> @@ -9909,9 +9926,11 @@ static const struct bpf_sec_def section_defs[] = {
>  	SEC_DEF("kprobe+",		KPROBE,	0, SEC_NONE, attach_kprobe),
>  	SEC_DEF("uprobe+",		KPROBE,	0, SEC_NONE, attach_uprobe),
>  	SEC_DEF("uprobe.s+",		KPROBE,	0, SEC_SLEEPABLE, attach_uprobe),
> +	SEC_DEF("uprobe.single+",	KPROBE,	0, SEC_NONE, attach_uprobe),

should we add sleepable counterparts?

>  	SEC_DEF("kretprobe+",		KPROBE, 0, SEC_NONE, attach_kprobe),
>  	SEC_DEF("uretprobe+",		KPROBE, 0, SEC_NONE, attach_uprobe),
>  	SEC_DEF("uretprobe.s+",		KPROBE, 0, SEC_SLEEPABLE, attach_uprobe),
> +	SEC_DEF("uretprobe.single+",	KPROBE,	0, SEC_NONE, attach_uprobe),

just an idea for discussion.. I wonder if it'd be better to add new uprobe
section that will upgrade itself to uprobe.multi if it's present, instead
of changing the existing (expected) type

but I guess we want existing uprobe programs to benefit from that and
there's not really a reason anyone would want perf based uprobe when
uprobe_multi is supported

ok I talked myself out of it ;-)

>  	SEC_DEF("kprobe.multi+",	KPROBE,	BPF_TRACE_KPROBE_MULTI, SEC_NONE, attach_kprobe_multi),
>  	SEC_DEF("kretprobe.multi+",	KPROBE,	BPF_TRACE_KPROBE_MULTI, SEC_NONE, attach_kprobe_multi),
>  	SEC_DEF("kprobe.session+",	KPROBE,	BPF_TRACE_KPROBE_SESSION, SEC_NONE, attach_kprobe_session),
> @@ -12737,6 +12756,32 @@ bpf_program__attach_uprobe_opts(const struct bpf_program *prog, pid_t pid,
>  		func_offset += sym_off;
>  	}
>  
> +	/* This provides backwards compatibility to programs using uprobe, but
> +	 * have been auto-upgraded to multi uprobe.
> +	 */
> +	if (prog->expected_attach_type == BPF_TRACE_UPROBE_MULTI) {
> +		LIBBPF_OPTS(bpf_uprobe_multi_opts, multi_opts);
> +		unsigned long offsets[1] = {func_offset};
> +		__u64 bpf_cookie;
> +
> +		multi_opts.retprobe = OPTS_GET(opts, retprobe, false);
> +		if (offsets[0] || func_name) {
> +			multi_opts.offsets = offsets;

could we do the same as for ref_ctr_off case and drop the offsets array?

     multi_opts.offsets = &func_offset;

> +			multi_opts.cnt = 1;
> +		}
> +		if (ref_ctr_off) {
> +			multi_opts.ref_ctr_offsets = &ref_ctr_off;
> +			multi_opts.cnt = 1;
> +		}
> +		bpf_cookie = OPTS_GET(opts, bpf_cookie, 0);
> +		if (bpf_cookie) {
> +			multi_opts.cookies = &bpf_cookie;
> +			multi_opts.cnt = 1;

I think it's better just to set multi_opts.cnt = 1 once outside those if conditions

> +		}
> +
> +		return bpf_program__attach_uprobe_multi(prog, pid, binary_path,
> +							NULL, &multi_opts);
> +	}
>  	legacy = determine_uprobe_perf_type() < 0;
>  	switch (attach_mode) {
>  	case PROBE_ATTACH_MODE_LEGACY:
> @@ -12830,6 +12875,7 @@ static int attach_uprobe(const struct bpf_program *prog, long cookie, struct bpf
>  	char *probe_type = NULL, *binary_path = NULL, *func_name = NULL, *func_off;
>  	int n, c, ret = -EINVAL;
>  	long offset = 0;
> +	bool is_retprobe;
>  
>  	*link = NULL;
>  
> @@ -12856,13 +12902,14 @@ static int attach_uprobe(const struct bpf_program *prog, long cookie, struct bpf
>  			else
>  				offset = 0;
>  		}
> -		opts.retprobe = strcmp(probe_type, "uretprobe") == 0 ||
> -				strcmp(probe_type, "uretprobe.s") == 0;
> -		if (opts.retprobe && offset != 0) {
> +		is_retprobe = strcmp(probe_type, "uretprobe") == 0 ||
> +			      strcmp(probe_type, "uretprobe.s") == 0;
> +		if (is_retprobe && offset != 0) {
>  			pr_warn("prog '%s': uretprobes do not support offset specification\n",
>  				prog->name);
>  			break;
>  		}
> +		opts.retprobe = is_retprobe;

is there any functional change above? looks like just opts.retprobe
is replaced with is_retprobe ?

jirka