ftrace: Use kallsyms binary search for single-symbol lookup

When ftrace_lookup_symbols() is called with a single symbol (cnt == 1),
use kallsyms_lookup_name() for O(log N) binary search instead of the
full linear scan via kallsyms_on_each_symbol().

ftrace_lookup_symbols() was designed for batch resolution of many
symbols in a single pass.  For large cnt this is efficient: a single
O(N) walk over all symbols with O(log cnt) binary search into the
sorted input array.  But for cnt == 1 it still decompresses all ~200K
kernel symbols only to match one.

kallsyms_lookup_name() uses the sorted kallsyms index and needs only
~17 decompressions for a single lookup.

This is the common path for kprobe.session with exact function names,
where libbpf sends one symbol per BPF_LINK_CREATE syscall.

If binary lookup fails (duplicate symbol names where the first match
is not ftrace-instrumented), the function falls through to the existing
linear scan path.

Before (cnt=1, 50 kprobe.session programs):
  Attach: 858 ms  (kallsyms_expand_symbol 25% of CPU)

After:
  Attach:  52 ms  (16x faster)

Cc: <bpf@vger.kernel.org>
Link: https://patch.msgid.link/20260302200837.317907-3-andrey.grodzovsky@crowdstrike.com
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@crowdstrike.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: andrey-grodzovsky

kernel/trace/ftrace.c

@@ -9267,13 +9267,35 @@ static int kallsyms_callback(void *data, const char *name, unsigned long addr)
  * @addrs array, which needs to be big enough to store at least @cnt
  * addresses.
  *
+ * For a single symbol (cnt == 1), uses kallsyms_lookup_name() which
+ * performs an O(log N) binary search via the sorted kallsyms index.
+ * This avoids the full O(N) linear scan over all kernel symbols that
+ * the multi-symbol path requires.
+ *
+ * For multiple symbols, uses a single-pass linear scan via
+ * kallsyms_on_each_symbol() with binary search into the sorted input
+ * array.
+ *
  * Returns: 0 if all provided symbols are found, -ESRCH otherwise.
  */
 int ftrace_lookup_symbols(const char **sorted_syms, size_t cnt, unsigned long *addrs)
 {
 	struct kallsyms_data args;
 	int found_all;
 
+	/* Fast path: single symbol uses O(log N) binary search */
+	if (cnt == 1) {
+		addrs[0] = kallsyms_lookup_name(sorted_syms[0]);
+		if (addrs[0] && ftrace_location(addrs[0]))
+			return 0;
+		/*
+		 * Binary lookup can fail for duplicate symbol names
+		 * where the first match is not ftrace-instrumented.
+		 * Retry with linear scan.
+		 */
+	}
+
+	/* Batch path: single-pass O(N) linear scan */
 	memset(addrs, 0, sizeof(*addrs) * cnt);
 	args.addrs = addrs;
 	args.syms = sorted_syms;