selftests/bpf: Add compare_text_to_expected() helper

Some selftests generate text (such as BTF dump) and check it against
an expectation committed nearby.

There is no shared way to do that. prog_tests/btf_dump.c assembles an
"awk ... | diff -u" pipeline and hands it to system(). Any other test
wanting the same behaviour needs to reproduce both the comparison and
the reporting of a mismatch.

test_progs captures per-subtest output by pointing the stdout and
stderr FILE * globals at a memstream. But a child process inherits
descriptors, not the globals. So the "| diff -u" goes to the console
instead of the subtest log and is absent from the failure report.

Add a helper that compares two strings and runs diff(1) on mismatch,
properly relaying the output to stdout. Add tests for the helper.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260828215207.3105313-2-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -179,3 +179,26 @@ void test_prog_tests_framework_expected_msgs(void)
 		}
 	}
 }
+
+void test_prog_tests_framework_compare_text(void)
+{
+	int err;
+
+	if (test__start_subtest("compare_text_match")) {
+		err = compare_text_to_expected("same\n", "same\n");
+		ASSERT_EQ(err, 0, "match_rc");
+		test__end_subtest();
+	}
+
+	if (test__start_subtest("compare_text_mismatch")) {
+		err = compare_text_to_expected("line two\n", "line one\n");
+		fflush(stdout);
+
+		ASSERT_EQ(err, -1, "mismatch_rc");
+		ASSERT_HAS_SUBSTR(env.subtest_state->log_buf, "-line one",
+				  "diff_has_expected");
+		ASSERT_HAS_SUBSTR(env.subtest_state->log_buf, "+line two",
+				  "diff_has_actual");
+		test__end_subtest();
+	}
+}

tools/testing/selftests/bpf/testing_helpers.c

@@ -534,3 +534,46 @@ int stack_mprotect(void)
 		       PROT_READ | PROT_WRITE | PROT_EXEC);
 	return ret;
 }
+
+int compare_text_to_expected(const char *actual, const char *expected)
+{
+	char exp_path[] = "/tmp/selftest_expected.XXXXXX";
+	char act_path[] = "/tmp/selftest_actual.XXXXXX";
+	char buf[512], cmd[128];
+	int exp_fd, act_fd;
+	FILE *p;
+
+	if (!strcmp(actual, expected))
+		return 0;
+
+	exp_fd = mkstemp(exp_path);
+	act_fd = mkstemp(act_path);
+	if (exp_fd < 0 || act_fd < 0) {
+		fprintf(stdout, "output differs, no temp file for a diff\n");
+		goto out;
+	}
+
+	dprintf(exp_fd, "%s", expected);
+	dprintf(act_fd, "%s", actual);
+
+	snprintf(cmd, sizeof(cmd), "diff -u '%s' '%s'", exp_path, act_path);
+	p = popen(cmd, "r");
+	if (!p) {
+		fprintf(stdout, "output differs, '%s' did not run\n", cmd);
+		goto out;
+	}
+	while (fgets(buf, sizeof(buf), p))
+		fputs(buf, stdout);
+	pclose(p);
+
+out:
+	if (exp_fd >= 0) {
+		close(exp_fd);
+		unlink(exp_path);
+	}
+	if (act_fd >= 0) {
+		close(act_fd);
+		unlink(act_path);
+	}
+	return -1;
+}

tools/testing/selftests/bpf/testing_helpers.h

@@ -61,4 +61,7 @@ int testing_prog_flags(void);
 bool is_jit_enabled(void);
 int stack_mprotect(void);
 
+/* Runs diff(1) on mismatch */
+int compare_text_to_expected(const char *actual, const char *expected);
+
 #endif /* __TESTING_HELPERS_H */