[PATCH bpf-next v4 1/3] libbpf: Optimize kprobe.session attachment for exact function names

[Andrey Grodzovsky <andrey.grodzovsky@crowdstrike.com>]

Detect exact function names (no wildcards) in
bpf_program__attach_kprobe_multi_opts() and bypass kallsyms parsing,
passing the symbol directly to the kernel via syms[] array.  This
benefits all callers, not just kprobe.session.

When the pattern contains no '*' or '?' characters, set syms to point
directly at the pattern string and cnt to 1, skipping the expensive
/proc/kallsyms or available_filter_functions parsing (~150ms per
function).

Error code normalization: the fast path returns ESRCH from kernel's
ftrace_lookup_symbols(), while the slow path returns ENOENT from
userspace kallsyms parsing.  Convert ESRCH to ENOENT in the
bpf_link_create error path to maintain API consistency - both paths
now return identical error codes for "symbol not found".

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@crowdstrike.com>
---
 tools/lib/bpf/libbpf.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 0be7017800fe..0662d72bad20 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -12041,7 +12041,16 @@ bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
 	if (addrs && syms)
 		return libbpf_err_ptr(-EINVAL);
 
-	if (pattern) {
+	/*
+	 * Exact function name (no wildcards) without unique_match:
+	 * bypass kallsyms parsing and pass the symbol directly to the
+	 * kernel via syms[] array.  When unique_match is set, fall
+	 * through to the slow path which detects duplicate symbols.
+	 */
+	if (pattern && !strpbrk(pattern, "*?") && !unique_match) {
+		syms = &pattern;
+		cnt = 1;
+	} else if (pattern) {
 		if (has_available_filter_functions_addrs())
 			err = libbpf_available_kprobes_parse(&res);
 		else
@@ -12084,6 +12093,14 @@ bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
 	link_fd = bpf_link_create(prog_fd, 0, attach_type, &lopts);
 	if (link_fd < 0) {
 		err = -errno;
+		/*
+		 * Normalize error code: when exact name bypasses kallsyms
+		 * parsing, kernel returns ESRCH from ftrace_lookup_symbols().
+		 * Convert to ENOENT for API consistency with the pattern
+		 * matching path which returns ENOENT from userspace.
+		 */
+		if (err == -ESRCH)
+			err = -ENOENT;
 		pr_warn("prog '%s': failed to attach: %s\n",
 			prog->name, errstr(err));
 		goto error;
-- 
2.34.1