[PATCH bpf-next 1/2] libbpf: Add unique_match option for multi kprobe

[Yonghong Song <yonghong.song@linux.dev>]

Jordan reported an issue in Meta production environment where func
try_to_wake_up() is renamed to try_to_wake_up.llvm.<hash>() by clang
compiler at lto mode. The original 'kprobe/try_to_wake_up' does not
work any more since try_to_wake_up() does not match the actual func
name in /proc/kallsyms.

There are a couple of ways to resolve this issue. For example, in
attach_kprobe(), we could do lookup in /proc/kallsyms so try_to_wake_up()
can be replaced by try_to_wake_up.llvm.<hach>(). Or we can force users
to use bpf_program__attach_kprobe() where they need to lookup
/proc/kallsyms to find out try_to_wake_up.llvm.<hach>(). But these two
approaches requires extra work by either libbpf or user.

Luckily, suggested by Andrii, multi kprobe already supports wildcard ('*')
for symbol matching. In the above example, 'try_to_wake_up*' can match
to try_to_wake_up() or try_to_wake_up.llvm.<hash>() and this allows
bpf prog works for different kernels as some kernels may have
try_to_wake_up() and some others may have try_to_wake_up.llvm.<hash>().

The original intention is to kprobe try_to_wake_up() only, so an optional
field unique_match is added to struct bpf_kprobe_multi_opts. If the
field is set to true, the number of matched functions must be one.
Otherwise, the attachment will fail. In the above case, multi kprobe
with 'try_to_wake_up*' and unique_match preserves user functionality.

Reported-by: Jordan Rome <linux@jordanrome.com>
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 tools/lib/bpf/libbpf.c | 10 +++++++++-
 tools/lib/bpf/libbpf.h |  4 +++-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 66173ddb5a2d..649c6e92972a 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -11522,7 +11522,7 @@ bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
 	struct bpf_link *link = NULL;
 	const unsigned long *addrs;
 	int err, link_fd, prog_fd;
-	bool retprobe, session;
+	bool retprobe, session, unique_match;
 	const __u64 *cookies;
 	const char **syms;
 	size_t cnt;
@@ -11558,6 +11558,14 @@ bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
 			err = libbpf_available_kallsyms_parse(&res);
 		if (err)
 			goto error;
+
+		unique_match = OPTS_GET(opts, unique_match, false);
+		if (unique_match && res.cnt != 1) {
+			pr_warn("prog '%s': failed to find unique match: cnt %lu\n",
+				prog->name, res.cnt);
+			return libbpf_err_ptr(-EINVAL);
+		}
+
 		addrs = res.addrs;
 		cnt = res.cnt;
 	}
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index d45807103565..3020ee45303a 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -552,10 +552,12 @@ struct bpf_kprobe_multi_opts {
 	bool retprobe;
 	/* create session kprobes */
 	bool session;
+	/* enforce unique match */
+	bool unique_match;
 	size_t :0;
 };
 
-#define bpf_kprobe_multi_opts__last_field session
+#define bpf_kprobe_multi_opts__last_field unique_match
 
 LIBBPF_API struct bpf_link *
 bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
-- 
2.43.5