selftests/bpf: cover UTF-8 trace_printk output

Extend trace_printk coverage to verify that UTF-8 literal text is
emitted successfully and that '%' parsing still rejects non-ASCII
bytes once format parsing starts.

Use an explicitly invalid format string for the negative case so the
ASCII-only parser expectation is visible from the test code itself.

Signed-off-by: Yihan Ding <dingyihan@uniontech.com>
Acked-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/20260416120142.1420646-3-dingyihan@uniontech.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Yihan Ding

tools/testing/selftests/bpf/prog_tests/trace_printk.c

@@ -6,18 +6,21 @@
 #include "trace_printk.lskel.h"
 
 #define SEARCHMSG	"testing,testing"
+#define SEARCHMSG_UTF8	"中文,测试"
 
 static void trace_pipe_cb(const char *str, void *data)
 {
 	if (strstr(str, SEARCHMSG) != NULL)
-		(*(int *)data)++;
+		((int *)data)[0]++;
+	if (strstr(str, SEARCHMSG_UTF8))
+		((int *)data)[1]++;
 }
 
 void serial_test_trace_printk(void)
 {
 	struct trace_printk_lskel__bss *bss;
 	struct trace_printk_lskel *skel;
-	int err = 0, found = 0;
+	int err = 0, found[2] = {};
 
 	skel = trace_printk_lskel__open();
 	if (!ASSERT_OK_PTR(skel, "trace_printk__open"))
@@ -46,11 +49,24 @@ void serial_test_trace_printk(void)
 	if (!ASSERT_GT(bss->trace_printk_ret, 0, "bss->trace_printk_ret"))
 		goto cleanup;
 
-	/* verify our search string is in the trace buffer */
-	ASSERT_OK(read_trace_pipe_iter(trace_pipe_cb, &found, 1000),
-		 "read_trace_pipe_iter");
+	if (!ASSERT_GT(bss->trace_printk_utf8_ran, 0, "bss->trace_printk_utf8_ran"))
+		goto cleanup;
+
+	if (!ASSERT_GT(bss->trace_printk_utf8_ret, 0, "bss->trace_printk_utf8_ret"))
+		goto cleanup;
+
+	if (!ASSERT_LT(bss->trace_printk_invalid_spec_ret, 0,
+		       "bss->trace_printk_invalid_spec_ret"))
+		goto cleanup;
+
+	/* verify our search strings are in the trace buffer */
+	ASSERT_OK(read_trace_pipe_iter(trace_pipe_cb, found, 1000),
+		  "read_trace_pipe_iter");
+
+	if (!ASSERT_EQ(found[0], bss->trace_printk_ran, "found"))
+		goto cleanup;
 
-	if (!ASSERT_EQ(found, bss->trace_printk_ran, "found"))
+	if (!ASSERT_EQ(found[1], bss->trace_printk_utf8_ran, "found_utf8"))
 		goto cleanup;
 
 cleanup:

tools/testing/selftests/bpf/progs/trace_printk.c

@@ -10,13 +10,23 @@ char _license[] SEC("license") = "GPL";
 
 int trace_printk_ret = 0;
 int trace_printk_ran = 0;
+int trace_printk_invalid_spec_ret = 0;
+int trace_printk_utf8_ret = 0;
+int trace_printk_utf8_ran = 0;
 
 const char fmt[] = "Testing,testing %d\n";
+static const char utf8_fmt[] = "中文,测试 %d\n";
+/* Non-ASCII bytes after '%' must still be rejected. */
+static const char invalid_spec_fmt[] = "%\x80\n";
 
 SEC("fentry/" SYS_PREFIX "sys_nanosleep")
 int sys_enter(void *ctx)
 {
 	trace_printk_ret = bpf_trace_printk(fmt, sizeof(fmt),
 					    ++trace_printk_ran);
+	trace_printk_utf8_ret = bpf_trace_printk(utf8_fmt, sizeof(utf8_fmt),
+						 ++trace_printk_utf8_ran);
+	trace_printk_invalid_spec_ret = bpf_trace_printk(invalid_spec_fmt,
+							 sizeof(invalid_spec_fmt));
 	return 0;
 }